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Major and trace element geochemistry of basalts from the Explorer area, Northeast Pacific Ocean 1982

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MAJOR AND TRACE ELEMENT GEOCHEMISTRY:OF BASALTS FROM THE EXPLORER AREA, NORTHEAST PACIFIC OCEAN by BRIAN LLOYD COUSENS B. Sc., McGill University, 1979 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of Geological Sciences) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l 1982 @ c B r i a n L l o y d C o u s e n s , 1982 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by h i s or her representatives. It i s understood that copying or p u b l i c a t i o n of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of Geological Sciences. The University of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 D a t e A p r i l 2nd, 1982 np-fi O /7Q) ABSTRACT i i . F i f t y fragments of young, fresh basalts from the Explorer Ridge, Paul Revere Ridge (Fracture Zone), Dellwood Knolls, and the J. Tuzo Wilson Knolls have been analysed for 12 major and minor elements, as well as 11 t r a c e . e l e - ments, by X-ray fluorescence spectrometry. Rare earth element concentrations i n 25 of the samples have been determined by instrumental neutron a c t i v a t i o n , and S r ^ / S r * ^ r a t i o s have been obtained for 11 of the basalts. The Explorer Ridge basalts have.-jnajor element compositions s i m i l a r to most mid-ocean ridge basalts (MORB), and can be c l a s s i f i e d as fer r o b a s a l t s , s i m i l a r to those of the southern Juan de Fuca Ridge. The incompatible minor and trace elements K, T i , Rb, Zr, and Nb are weakly to strongly enriched i n the Explorer samples, with respect to MORB, part of which i s the r e s u l t of ' c r y s t a l f r a c t i o n a t i o n . The observed trace element and l i g h t rare earth e l e - ment (LREE) enrichment of many of the samples, p a r t i c u l a r l y those from Explorer Deep, suggest that a weak hptspot may exist beneath the Explorer Deep. The adjacent ridge segments, Explorer R i f t and the Southern Explorer Ridge, are erupting basalts both enriched and depleted i n incompatible elements, which could be an ind i c a t o r of a chemically heterogenous mantle source, or may be the r e s u l t of intermittent i n j e c t i o n of enriched magmas from the postulated hotspot beneath Explorer Deep into areas producing normal MORB. The enrich- ed basalts do not have s i g n i f i c a n t l y d i f f e r e n t S r ^ / S r ^ r a t i o s from the de- pleted basalts. A l l the samples f a l l within the range of values t y p i c a l for Juan de Fuca and Gorda Ridge basalts, and East P a c i f i c Rise t h o l e i i t e s i n gen- e r a l . Thus, although the source areas for the 2 basalt types may d i f f e r chem- i c a l l y , they are s i m i l a r r a d i o g e n i c a l l y , unlike-other hypothetically plume-in- fluenced areas such as the Mid-Atlantic Ridge at 45°N and the FAMOUS area. The basalts from the northwest and southeast Dellwood Knolls appear to be r e l a t e d by c r y s t a l f r a c t i o n a t i o n , based on major element a n a l y s i s . However, the very d i f f e r e n t REE patterns and Sr87/Sr86 r a t i o s exhibited by the two knolls suggest that they have d i f f e r e n t mantle sources, one t y p i c a l l y depleted (northwest knoll) and one chemically and r a d i o g e n i c a l l y enriched (southeast k n o l l ) . In terms of t h e i r major and trace element chemistry, the J. Tuzo Wilson Knolls basalts are t y p i c a l of late-stage volcanism on ocean islands associated with mantle plumes. The hawaiites strongly resemble a l k a l i basalts dredged from several seamounts i i i the Pratt-Welker Chain, which are c o - l a t i t u d i n a l 7 . with the J. Tuzo Wilson Knolls on a small c i r c l e about the Pacific-Hotspot pole of r o t a t i o n . Geochronological evidence questions the hypothesis that the mantle plume responsible for Pratt-Welker volcanism i s also the source for the J. Tuzo Wilson basalts. The existence of a second mantle plume, 300 km south- east of the f i r s t , would explain minor chemical and physiographical d i f f e r - ences between the Knolls and the other Pratt-Welker seamounts, as well as the evidence for two' phases of volcanism on the southeastern seamounts of the chain. A second plume also explains the coeval volcanism of Bowie Seamount and the J . Tuzo Wilson Kn o l l s . Recent geophysical evidence suggests that the J . Tuzo Wilson Knolls are also part of the Explorer-Dellwood spreading system. Although the JTW basalts are plume-type basalts chemically, the s i t u a t i o n appears to be somewhat analagous to other ridge segments where plumes . are coincident with the ridge i t s e l f . i v . TABLE OF CONTENTS Page ABSTRACT i i . LIST OF TABLES v i . LIST OF PLATES AND FIGURES v i i . ACKNOWLEDGEMENTS i x . INTRODUCTION 1. Physiography and Tectonic History 3. Previous Work 7. Sample C o l l e c t i o n 8. BASALT PETROGRAPHY 12. MAJOR ELEMENTS 18. A n a l y t i c a l Procedure 18. Pre c i s i o n and Accuracy 18. Results 19. TRACE ELEMENTS 33. A n a l y t i c a l Procedure 33. Pr e c i s i o n and Accuracy 33. Results 34. STRONTIUM ISOTOPES 44. A n a l y t i c a l Procedure 44. Pr e c i s i o n and Accuracy 44. Results 44. DISCUSSION 47. Explorer Ridge and Propagating R i f t s 47. Explorer Deep, Explorer R i f t , and Southern Explorer Ridge 49. Dellwood Knolls 52. J. Tuzo Wilson Knolls 53. CONCLUSIONS 68. BIBLIOGRAPHY 70. Table of Contents, continued. Page APPENDICES _ 75'.'.- 1. Petrographic Descriptions 75. 2. Major Element, Trace Element, Strontium Isotope, Normative Compositions, and Pre c i s i o n Data Tables. 79. 3. Howarth and Thompson P r e c i s i o n Plots 92. 4. XRF Operating Conditions for Major and Trace Element Analysis. 95. v i . LIST OF TABLES Table Page 1. K-Ar A n a l y t i c a l Data and Age Determination r J. Tuzo Wilson Knolls. 9. 2. Locations and Depths of Dredge Hauls 11. 3. Duplicate Analyses of Explorer Area Basalts by Different A n a l y t i c a l Methods. 20. 4. Trace Element Concentrations in Oceanic Basalts. 40. 5. A l k a l i Basalt Composition of Pratt-Welker Seamounts, J. Yuzo Wilson Knolls, and "Average" Ocean Island Basalt. 55. 6. P o s i t i o n and Age Data for Pratt-Welker Seamounts. 59. v i i . LIST OF PLATES AND FIGURES Plate Page 1. Photograph of Thin-section 15. 2. Photographs of Thin-seactions 16. 3. Photograph of Thin-section 17. Figure 1. Explorer Ridge Area Dredge Haul Locations pocket \0 3 2. Tectonic Elements of the Explorer Area 2. 3. Magnetic Anomaly Map of the Juan de Fuca Area 4. 4. Tectonic History of Explorer Area 6. 5. AFM Diagram for Explorer Area Basalts 21. 6. S i l i c a V a r i a t i o n Diagram 22. 7. MgO V a r i a t i o n Diagram for FeOt and AI2O3 24. 8. MgO V a r i a t i o n Diagram for CaO and Ti02 25. 9. CIPW Normative Triangle of the Pl a g i o c l a s e - O l i v i n e - Pyroxene System. 26. 10. Plots of T i 0 2 vs. K2O and Na 20 28. 11. Magnesium Ratio V a r i a t i o n Diagram for TIO2 and K 20 29. 12. Frequency Diagram of Magnesium Ratios 31. 13. CIPW Normative Triangle for'An-Ne-01-Hy-Q System 32. 14. Ba vs. Sr _ 35. 15. Magnesium Ratio V a r i a t i o n Diagram for Sr, Zr, Y, Nb, and Rb 36. 16. Magnesium V a r i a t i o n Diagram for Ni 37. 17. Magnesium V a r i a t i o n Diagram f o r Cr and V 38. 18. V a r i a t i o n i n La/Sm ef With Latitude 41. v i i i . L i s t of Plates and Figures, continued. Figure Page 19. Zr/Nb Diagram 43. 20. La/Sm e f vs. 8 7 S r / 8 6 S r 45. 21. Magnetic R e l i e f and FeOt Content Along the Juan de Fuca System 48. 22. Normalized Incompatible Element Patterns for Explorer Basalts 50. 23. Basement Map of the Pratt-Welker Chain 54. 24. S i l i c a V a r i a t i o n Diagram and Normalized Rare Earth Element Patterns for Pratt-Welker Seamount Basalts 56. 25. Geochronology of Pratt-Welker Chain 61. 26. Microseismicity of Explorer Area 63. 27. CSP P r o f i l e Across the Continental Slope From the NW Dellwood Knoll 64. 28. Magnetic Anomaly Map of JTW and Dellwood Knolls Area 65. i x . ACKNOWLEDGEMENTS I f i r s t thank R.L. Chase and R.L. Armstrong f o r guidance i n t h i s project. Stanya Horsky, Rob Berman, and Graham Nixon a s s i s t e d with a n a l y t i c a l proce- dures and computer programming, as well as discussion of r e s u l t s . Discussions with W.K. Fletcher, R.P. Riddihough, R.G. Currie, and R.D. Hyndman were also h e l p f u l . E. Montgomery c a r e f u l l y prepared photographs for the manuscript. R.L. Chase, Arnie Thomlinson, Guy Beland, Ken Hansen, and others involved with sea-floor study at the University of B r i t i s h Columbia, created the UBC basalt c o l l e c t i o n , from which samples for t h i s study were selected. The co- operation of the o f f i c e r s and crews of the CSS PARIZEAU, CNAV ENDEAVOUR, and, CSS HUDSON, i s appreciated by all''concerned. R.L. Chase, R.L. Armstrong, and K.C. Mc Taggart c r i t i c a l l y reviewed the thesis manuscript. C o l l e c t i o n of the basalts studied i n t h i s paper has involved grants from the following agencies to R.L. Chase, and j o i n t l y to R.L. Chase, J.W. Murray, and E.V. G r i l l : the Defense Research Board of Canada; the National Research Council and the Natural Science and Engineering Research Council of Canada; Energy, Mines, and Resources Canada; the Minis t r y of Energy, Mines, and Pet- roleum Resources of B r i t i s h . Columbia; Placer Development Ltd.; Cominco Ltd; and the University of B r i t i s h Columbia. Funding f o r t h i s project was i n the form of a u n i v e r s i t y grant to R.L. Chase, as well as grants from EMR and NSERC. 1. INTRODUCTION The Juan de Fuca-Gorda Ridge system, the northern extension of the East P a c i f i c Rise i n the northeast P a c i f i c , i s one of the most extensively studied ocean ridge systems i n the world (Kay et a l . , 1970; Moore, 1970; Barr and Chase, 1974; Vogt and Byerly, 1976; Clague and Bunch, 1976; Wakeham, 1977; L i i a s et a l . , 1982), but the basalt geochemistry of the spreading segments north of the Sovanco Fracture Zone (Figure 2) has received r e l a t i v e l y l i t t l e a t t e n t i o n . This i s the f i r s t study of basalt geochemistry on the Explorer Ridge, and p a r t i a l l y completes a project i n i t i a t e d i n 1970 by A.G. Thomlinson, but not f i n i s h e d . This paper also presents trace element data for the D e l l - wood Knolls spreading segment, and the J . Tuzo Wilson K n o l l s . Several questions concerning basalt chemistry of the Explorer area can be addressed: ( i ) Are Explorer basalts chemically s i m i l a r to other mid-ocean ridge basalts (MORB)? Where do they f i t in.the Melson et a l . (1976) c l a s s i f i c a t i o n of ocean ridge t h o l e i i t e s ? Do the basalts resemble those from the northern Juan de Fuca (Barr and Chase, 1974) or the southern Juan de Fuca Ridge (Wakeham, .1977) , or are they chemically d i s t i n c t from both? ( i i ) Do any f a m i l i a r geochemical patterns e x i s t within the Explorer- Dellwood system? I f so, what process(es) can explain them? ( i i i ) I f the J . Tuzo Wilson Knolls are the surface expression of a man- t l e plume or hotspot, i s there evidence for magma mixing of the plume basalts with ocean ridge basalts, i n the form of chemical gradients, as has been obe served on the Reykjanes Ridge ('Schilling, 1973) or on the Azores Platform ( S c h i l l i n g , 1975)? (iv) Is there any chemical evidence to suggest that the J. Tuzo Wilson Knolls are part of a spreading ridge, as i s indicated by geophysical obser- vations? Figure 2: Tectonic elements of the Explorer spreading area. 3. Physiography and Tectonic History The spreading segments of the Explorer area are physiographically v a r i - able. The southern Explorer Ridge (Figure 2) does not possess a well-devel- oped a x i a l trough, s i m i l a r to most fast-spreading ridges, although seismic r e f l e c t i o n p r o f i l e s (CSP) do show f a u l t i n g i n the cen t r a l ridge area. The southern end of the ridge i s very poorly expressed bathymetrically. In con- t r a s t , the Explorer Deep and Explorer R i f t are well-developed, graben-like r i f t zones, that l a c k high flanking walls. To the northeast, they are abrupt- l y terminated by the Revere-Dellwood-. Fault Zone. The Dellwood Knolls are two hea v i l y - f a u l t e d volcanic ridges with peaks, 400 to 500 meters above the sea- f l o o r , separated by the disturbed, s e d i m e n t - f i l l e d , Dellwood Spreading Zone (Bertrand, 1972). The K n o l l s terminate to the southwest at the Revere-Dellwood transform. CSP data reveals that the K n o l l s terminate before the continental r i s e to the east, and that the intervening sedimentary p i l e i s heavily f a u l t - ed. The J . Tuzo Wilson Knolls (JTW) are two irregularly-shaped, 500 meter high, NE-SW elongate peaks, aligned northeast-southwest. CSP p r o f i l e s (Chase, 1977) i n d i c a t e that the steep-sided k n o l l s penetrate a t h i c k sedimentary blanket. The magnetic anomaly pattern of the Juan de Fuca Ridge area (Raff and Mason, 1961) was used by Wilson (1965) and Vine and Wilson (1965) to demon- s t r a t e sea-floor spreading. A magnetic-anomaly-based tectonic model f or r e - cent plate i n t e r a c t i o n s i n the Juan de Fuca-Explorer area was f i r s t developed by Riddihough (1977), and has been revised by Davis and Riddihough (1982). Figure 4 i l l u s t r a t e s sequence of events i n the evolution of the ridge: 4 Ma BP The Pacific-America-Explorer t r i p l e j u n c t i o n l i e s near the Brooks Peninsula. Predominantly r i g h t l a t e r a l transform motion with a small component of convergence occurs between the P a c i f i c and America plates along the margin northwest of the junct i o n . Nearly normal convergence 4. Figure 3 : Raff and Mason (1961) magnetic anomaly map of the Juan de Fuca area. Straight lines are pseudofaults as proposed by Vine (1968), and are interpreted to be a result of ridge propagation (Hey, 1977). Brunhes anomaly in black. 5. occurs along the margin to the southeast of the j u n c t i o n . Rapid sed- imentation near the base of the continental margin causes magnetization of the crust to be reduced. A small l e f t l a t e r a l transform i s i n i t i a t e d on the ridge ( t h i s w i l l become the Paul Revere transform). 3 Ma BP The lengthening l e f t l a t e r a l f r a c t u r e zone migrates northwards, and along with the ridge system, acts as a sediment b a r r i e r , possibly short- ening the length of the section of ridge which produces poorly magnet- ized crust. 1.5-2.0 Ma BP Flexural stresses associated with the i n t e r a c t i o n of the P a c i f i c p l a t e at the margin (including convergence and sediment loading) cause normal f a u l t i n g near and along the i n a c t i v e trace of the f r a c t u r e zone, which i s now within 80 km of, and s u b p a r a l l e l to, the margin. A Winona Basin l i t h o s p h e r i c block becomes p a r t l y decoupled from the P a c i f i c Plate; assymmetric subsidence of the Winona Basin and u p l i f t of the Paul Revere Ridge begins. 1 Ma BP Spreading on the section of the ridge o f f northern Vancouver Island stops and transfers to a new postion i n o l d crust near the western end of the f r a c t u r e zone (Dellwood K n o l l s ) . A Winona l i t h o s p h e r i c block i s thus f u l l y i s o l a t e d and ceases to move with the P a c i f i c Plate. This c r i t i c a l change i n configuration may have been caused by the resistance to the 5-6 cm/yr P a c i f i c Plate p a r t i a l s t r i k e - s l i p i n t e r a c t i o n between the small, p a r t l y decoupled Winona Basin block, and the continent. 1 Ma BP to present Spreading on the Explorer Ridge migrates northwestwards by assym- metric spreading, eventually s p l i t t i n g and jumping to i t s most recent p o s i t i o n (Explorer R i f t ) . The Winona block continues to t i l t and sub^i. side and the thickening sediment f i l l continues to deform as a r e s u l t of convergence with the continent. (from Davis and Riddihough, 1982) The History of the Explorer spreading area has been one of gradual breakup of a si n g l e ridge into several smaller segments. This has been Figure 4 : Tectonic analysis and history of the Explorer area. Davis & Riddihough (1982). 7. accompanied by clockwise r o t a t i o n of the d i r e c t i o n of spreading, such that i t i s presently p a r a l l e l to the margin. The Explorer Plate now moves independ- ently of the Juan de Fuca Plate (Riddihough, 1977) . The reason for the r o t a - t i o n of the ridge segments i s probably re l a t e d to the r e l a t i v e l y unstable subduction regime east of the ridge. R e l a t i v e l y young crust i s being subduct- ed,, and as such, i s s t i l l hot and bouyant, r e s u l t i n g i n increased resistance to subduction. By r o t a t i n g , the subduction rate i s lowered to a minimum, and les s work i s required. Even though spreading "jumped" from Explorer Deep to Explorer R i f t during the l a s t m i l l i o n years, both r i f t s are se i s m i c a l l y a c t i v e , and fre s h basalts have been dredged from each one i n more than one l o c a t i o n . Previous Work Bertrand (1972) completed the i n i t i a l p e trologic and tectonic a n a l y s i s of the Dellwood Knolls, although h i s chemical and petrologic data applied to only two successful dredge hauls. The Dellwood basalts are chemically i n t e r - mediate between t h o l e i i t i c and a l k a l i basalt. Thick manganese encrustations on the basalt fragments suggest that the basalts from the southeast k n o l l are older than basalts from the northwest k n o l l . The more d i f f e r e n t i a t e d nature of the southeast k n o l l basalts i n d i c a t e that they were erupted further away from the spreading center. It appears that the southeast k n o l l has ceased a c t i v i t y , while the northwest k n o l l i s s t i l l a c t i v e . Suggested ages f o r the k n o l l s are 0.2-1 Ma for the northwest k n o l l , and 1-2 Ma for the southeast k n o l l (Bertrand, 1972) . Trace element contents and Sr isotope data f o r Dellwood" Knolls sample 70T25-2D-8 were obtained by Armstrong and Nixon ( 1 9 8 0 ) . The values were si m i l a r to those of normal MORB. Chase (1977) published major element data on the J. Tuzo Wilson Knolls 8. basalts, based on two dredge hauls from the southwest peak. The basalts are hawaiites, s i m i l a r to late-stage a l k a l i c v o l c anic rocks found on ocean islands associated with mantle plumes, or "hotspots". The JTW Knolls l i e on the same Pacific-Hotspot c o l a t i t u d e as the seamounts i n the Pratt-Welker Seamount Chain, which includes Bowie Seamount and Kodiak Seamount. Assuming the rate of r o t a t i o n about the PCFC-HSPT pole of Minster et al.(1974), Chase concluded that the hotspot responsible for the Pratt-Welker Chain i s presently beneath the JTW area. • A K-Ar date of 54,000 yrs was obtained for one of the basalt f r a g - ments (Table;1). A recent geochronological study of the Pratt-Welker Chain (Turner et a l . , 1980) suggests, however, that the hotspot l i e s 250-300 km northwest of the JTW area, based on a K-Ar date from Bowie Seamount of 74,000 y r s . I n t e r e s t i n g l y , two phases of volcanism, spaced 10 m i l l i o n years apart, have been i d e n t i f i e d on the southeastern seamounts of the chain. Geophysical evidence indicates that the JTW Knolls may be the newest segment of spreading ridge on the Explorer-Dellwood system (Keen and Hyndman, 1979; R.D. Hyndman, personal communication). Seismicity suggests that a trans- form f a u l t e x i s t s between the Dellwood and JTW K n o l l s - Several geophysical surveys have been performed over the Explorer area. These include magnetic, bathymetric, gravity, heat flow, and seismic r e r a c t i o n studies (Srivastava et al.,1971; T i f f i n and Seeman, 1975; Malacek and Clowes, ly76; Hyndman et al.,1978; Riddihough et al.,1980). Sample C o l l e c t i o n Basalts from the Explorer Ridge and the Paul Revere Fracture Zone ( F i g - ure 1) were c o l l e c t e d by A.G. Thomlinson and R.L. Chase between 1970 and 1972, as part of a Phi D. program that was not completed. Fresh basalts from Explor- er R i f t (1977) and Explorer Deep (1979) were c o l l e c t e d by G. Beland and K. Hansen, along with R.L. Chase, i n the course of sedimentological and bathymetric 9. TABLE 1 K-Ar A n a l y t i c a l Data and Age Determination f o r J . Tuzo Wilson Knolls SAMPLE: 73-26-2-1C POSITION: 51° 24'30''N l a t i t u d e , 131° 02'W longitude. MATERIAL ANALYSED: Basalt- whole rock POTASSIUM CONTENT: (%K): 2.025±0.023% (average of 2 analyses) A r 4 0 r / t o t a l A r 4 0 : 0.0079 Ar^Or (10" 5cc STP/g): 4.366 x IO" 4 A r 4 0 r / K 4 0 : 3.161 x 10"& APPARENT AGE: 55,000±100% y r s . Constants used: X £ = 0.581 x 1 0 " 1 0 y r _ 1 Ag= 4.962 x io-lOyr-l K 4 0/K= 1.167 x 10~ 4 Ar40r_ radiogenic A r 4 ^ ANALYST: J . Harakel, for R.L. Chase. 10. studies of the two r i f t zones. In addition to the samples from the southwest J . Tuzo Wilson K n o l l , dredged by R.L. Chase i n 1973, fresh p i l l o w basalts were c o l l e c t e d by D.L. T i f f i n of the Geological Survey of Canada (sample "73"). Figure 1 ( i n pocket) shows the locati o n s of a l l dredge hauls from which samples for t h i s study were selected. The l a t i t u d e s , longitudes, and depths of recovery of the dredge stations are l i s t e d i n Table 2;  11. TABLE 2 Locations and Depths of Dredge Hauls Each UBC dredge haul has a s i x - d i g i t descriptor, of which the f i r s t two d i g i t s i n d i c a t e the year of c o l l e c t i o n , the t h i r d and fourth d i g i t s the c r u i s e number, and the f i f t h and s i x t h d i g i t s the s t a t i o n number of the dredge. Other d i g i t s appended to t h i s are fragment numbers. TOPOGRAPHIC NAVIGATION DREDGE HAUL FEATURE SYSTEM LATITUDE (°N) LONGITUDE ( W) DEPTH RANGE (meters) 67.-6-12 Bowie Smt radar transponder 53° 19' 135° 38' 100-120 73-26-2 JTW Knolls ' Radar 51° 24' 30* 130° 02' 1883-1682 73-26-5 JTW Knolls Radar 51° 25' 30" 131° 01' ? "73" JTW Knolls ? 51° 28' 30" 130° 51' ? 70-25-2D D e l l . Kn. Satnav 50° 53' 40" 130° 33' • 1940-1554 70-25-3D D e l l . Kn. Satnav 50° 46' 38* 130° 25'12" 1875-1509 70-25-8D D e l l . Smt. Satnav 50° 27' 12" 130° 32'30" 1475-1300 70.-25-9D D e l l . Smt. Satnav 50° 36' 130° 45' 30" 1800-1500 71-15-77 Ex-y R i f t 1 50° 18' 24" 130° 17' 18" 2460-2300 70-25-4 Ex. R i f t Satnav 50° 13' 54" 130° 15'06* 2500 70-25-16 Ex. R i f t 1 50° 13' 130° 14 2100-1900 77-14-33 Ex R i f t Loran A 50° 04' 42" 130° 17' .48* 2675 70-25-11 P. Revere R. Satnav 50° 14' 18" 129° 54'42" 2300-2200 71-15-92 P. Revere R. Loran A 50° 12'48" 129° 54' 2600-2420 71-15-91 P. Revere R. Satnav 50° 12' 29" 129° 48'42" 2050-1975 72-22-7 P. Revere R. Satnav 50° 00'25" 129° 31' 30" 1800 70-25-17 Ex. Deep Satnav 50° 05'30" 129" 44' 30" 3200-2400 79-6-32 Ex. Deep Loran C 49° 59' 24" 129° 53' 06" 2465-2375 77-14-36 S. Ex. Ridge Loran A 49° 55' 12" 130° 10'48" 2450-2130 73-26-13 S. Ex. Ridge ? 49° 46'30" 130° 30' 2200-2148 70-25-15 S. Ex. Ridge 1 49° 46' 130° 18' 2100-2000 71-15-70 S. Ex. Ridge 1 49° 07' 130°36 /30" 2540 12. BASALT PETROGRAPHY The basalt samples studied are fragments of glassy p i l l o w lavas, and other flows, dredged from the sea f l o o r . Fragments were chosen for analysis based on apparent freshness. The i n i t i a l assessment of freshness was confirmed under the microscope. With few exceptions, a l l the samples are very fresh, with unaltered o l i v i n e and pl a g i d c l a s e phenocrysts, and no apparent a l t e r a t i o n minerals. Four samples show red st a i n i n g , apparently due to oxidation of iron-bearing minerals, most notably s p i n e l . Four others display minor o l i v i n e a l t e r a t i o n , at phenocryst edges and along f r a c t u r e s . As was expected, the somewhat older rocks of the Paul Revere Ridge show a s l i g h t l y higher degree of a l t e r a t i o n than do the bas- a l t s of the ac t i v e volcanic areas. The dominant phenocryst phases are o l i v i n e and pl a g i o c l a s e . Pyroxene - only r a r e l y occurs as phenocrysts, which i s a common feature of oceanic bas- a l t s (Bryan, 1972). Increasing c r y s t a l l i z a t i o n of magnesian o l i v i n e increases the FeO/MgO r a t i o of the re s i d u a l l i q u i d , u n t i l the o l i v i n e - p l a g i o c l a s e - p y r - oxene eutectic i s reached. In oceanic basalts, the high rate of cooling gen- e r a l l y does not allow phenocryst pyroxene to c r y s t a l l i z e . Phenocrysts are not abundant i n the Explorer, Dellwood, or JTW Knolls basalts, r a r e l y exceeding 10% by volume of the samples. Plagioclase microphenocrysts are common to a l l areas except the JTW Knolls, suggesting that the ridge magmas had a somewhat longer period of c r y s t a l l i z a t i o n a f t e r extrusion than did the seamount ba s a l t s . Commonly, the phenocrysts are glomeroporphyritic. The phenocrysts exhibit excellent quench and fast-growth textures, as described by Bryan (1972). Olivines are i n places euhedral or anhedral, but most commonly are subhedral and s k e l e t a l (Plate 2-A,B,C). Plagioclase pheno- crysts have glass inclusions (Plate 2),. and are generally normally zoned. The Dellwood basalts have pl a g i o c l a s e phenocrysts with An contents exceeding 80%, 13. and o l i v i n e s with Fo contents of 90%, which are out of equilibrium with the bulk composition of the rock (Bertrand, 1972). The Explorer basalts show the same petrographic features; the phenocrysts are probably also more a n o r t h i t i c and f o r s t e r i t i c than would be predicted from the bulk chemistry of the basalts. Texturally, the majority of the basalt fragments studied are h y a l o p i l i t i c or h y a l o p h i t i c , c o n s i s t i n g of p l a g i o c l a s e , o l i v i n e , and" rare pyroxene pheno- cry s t s , i n a glassy to f a n - s h e r u l i t i c groundmass (Plate 2-A,B,C). The miner- a l o g i c a l composition of the fan-spherulites i s not d i s t i n g u i s h a b l e o p t i c a l l y , but i s probably a combination of plagioclase and pyroxene (Bryan, 1972) . Less commonly, the basalts exhibit intergranular or i n t e f s e r t a l texture, and i n 3 cases, the basalts are h o l o c r y s t a l l i n e (Plate 3). These fragments may be from the centers of pillows, or from more massive lava flows (Bryan, 1972). One sample (Plate 2-D) shows f a n - s p e r u l i t i c texture: r a d i a l c l u s t e r s of elongate p l a g i o c l a s e and pyroxene c r y s t a l s , often with cross - c u t t i n g "lantern s t r i n g " (Bryan, 1972) o l i v i n e c r y s t a l s , and only minor amounts of f a n - s p h e r u l i t i c matrix. This texture probably r e f l e c t s a slower rate of cooling than that ex- perienced by the h y a l o p i l i t i c b asalts. The Explorer basalts studied are r a r e l y v e s i c u l a r . In some samples, a v e s i c u l a r zone i s found about 1 cm below the p i l l o w surface. The v e s i c u l a r i t y decreases quickly i n both d i r e c t i o n s away from t h i s zone. Vesicles r a r e l y occupy more than 5% of the rock by volume, r e f l e c t i n g the low v o l a t i l e content and depth of extrusion of the basalts. In contrast, the JTW and Dellwood Knolls samples are very v e s i c u l a r . Some JTW basalts have elongated v e s i c l e s up to 5 cm i n length and 1 cm in width, and have high dissolved v o l a t i l e con- tents ( H 2 0 ~ l . l % , CO2~0.6%). The Dellwood basalts do not have s i m i l a r high v o l a t i l e contents. In summary, the Explorer, Dellwood, and JTW basalts are petrographically very s i m i l a r , with uniform texture and phenocryst assemblages. The dominant pheno- 14. cryst phases are o l i v i n e and pl a g i o c l a s e , but these r a r e l y occupy more than 10-15% by volume of the rock. They commonly show glomeroporphyritic texture. Most of the samples selected are h y a l o p i l i t i c basalts, and a l t e r a t i o n i s gen- e r a l l y minor. Appendix 1 contains petrographic d e t a i l s f o r the J . Tuzo Wilson Knolls, Dellwood Knolls and Seamounts, and Explorer Ridge basalts. PLATE 1 15. Intergranular ba s a l t . O l i v i n e altered along f r a c t u r e s , groundmass a l t e r a t i o n extensive. Mag. 35x. 71-15-91-1. A. S k e l e t a l , quenched o l i v i n e and B. Quenched o l i v i n e phenocrysts i n a plag i o c l a s e phenocrysts i n a h y a l o p i l i t i c matrix. Mag. 25x. h y a l o p i l i t i c matrix. Mag 25x. 77_14_36-G. 77-14-36-X. C. Zoned pl a g i o c l a s e phenocryst with D. Fan-spherulitic b a s a l t . Mag. lOOx. glass i n c l u s i o n s . Mag. 35x. 77_14_36-36. 77-14-33-B. H o l o c r y s t a l l i n e b a s a l t . Radial pyroxene and p l a g i o c l a s e c r y s t a l s with o l i v i n e phenocrysts. Mag. 30x. 77-14-36-35. 18. MAJOR ELEMENTS (i ) A n a l y t i c a l Procedure Each basalt fragment selected for analysis was f i r s t crushed i n a cus- tom b u i l t hydraulic rock s p l i t t e r , then ground to a f i n e powder i n a Rock- land C r - s t e e l r i n g m i l l . Ten grams of powder were then formed into a 3.1 cm- diameter, 1.2 cm-thick p e l l e t , with a boric a c i d backing. Major element oxide concentrations were determined by X-ray fluorescence spectroscopy, on a P h i l l i p s PW-1410 spectrometer, using the pressed powder method of Brown et a l . (1973). This method has been refined by Peter van der Heyden, Stanya Horsky, and W.K. Fletcher, of the Department of Geological S c i - ences at the University of B r i t i s h Columbia. This procedure uses mass absorp- t i o n c o e f f i c i e n t s from the Handbook of Spectroscopy (1974) to correct for matrix e f f e c t s i n both standards and unknowns. Also, instrument d r i f t and sample backgrounds are monitored and corrected f o r . XRF operating conditions for the major element analysis are l i s t e d i n Appendix 4, along with a b r i e f d e s c r i p t i o n of the computer program used for the data reduction. For a more complete des c r i p t i o n of the program, see van der Heyden (1982). The major element composition of a l l the samples studied i s l i s t e d i n Appendix 2. ( i i ) P r e c i s i o n and Accuracy Appendix 2 contains p r e c i s i o n data f o r the seven.staridards .used "for the c a l i b r a t i o n curves, along with p r e c i s i o n estimates for the unknowns. The mean (average of the seven standards) percent deviation for each element (the d i f - ference between the "recommended values" (Abbey, 1980) and the values c a l c u l a - ted using the working curves) i s nearly equal to, or i s le s s than, the p r e c i - sion of data used to generate the "recommended values" (Flanagan, 1973; 19. S. Berman, "personal communication to S. Horsky). P r e c i s i o n estimates for the unknowns were also calculated, using the method of Howarth and Thompson (1976). Duplicate analyses of several of the unknowns were c a r r i e d out, a f t e r which the d i f f e r e n c e between the two runs for each un- known was plotted against the mean of the two runs. P r e c i s i o n l i n e s indicate the 90 and 99% p r o b a b i l i t y l i m i t s of any r e p l i c a t e point f a l l i n g below the l i n e s , given a s p e c i f i e d p r e c i s i o n (95% confidence l i m i t s ) i n the data. These precisions are maximum values, assuming there are no extraordinary points. An example of two of the p r e c i s i o n plots (Fe 203 and MgO) are shown i n Appendix 3. Table 3 presents duplicate major element analyses , using d i f f e r e n t anal- y t i c a l procedures, f o r 3 b a s a l t s : 70-25-2D-8 from the Dellwood Knolls; 73-26-2-1 and 73-26-5-1, both from the JTW K n o l l s . It i s evident that the pressed p e l l e t analyses y i e l d somewhat lower S i 0 2 and A12C>3 values, ,'and higher values for Fe2C>3, MgO, and CaO. The most obvious r e s u l t of t h i s i s that the samples appear to be more s i l i c a undersaturated using the pressed powder a n a l y s i s . ( i i i ) Results The Explorer basalts, with the exception of the JTW samples, plot within the f i e l d of abyssal t h o l e i i t e s (Miyashiro et a l . , 1970) on an AFM diagram, as shown i n Figure 5. The c l u s t e r of points f a l l s between the Hawaiian t h o l e i i t e and Hawaiian a l k a l i basalt d i f f e r e n t i a t i o n trends. The t r a n s i t i o n a l to t h o l - e i i t i c nature of the basalts i s further emphasized i n a s i l i c a v a r i a t i o n d i a - gram (Figure 6). The Explorer basalts appear to be s l i g h t l y enriched i n a l k a l i metals compared to the Gorda and Juan de Fuca Ridges. However, t h i s may•be*due to d ifferences i n a n a l y t i c a l procedure rather than to true chemical d i f f e r e n c e s . The JTW hawaiites plot as a d i s t i n c t group on both diagrams, due to t h e i r s i g - n i f i c a n t l y higher a l k a l i metal contents. They do not o u t l i n e any d i f f e r e n t i a - t i o n trend on the AFM diagram, but l i e along an S i 0 2 and a l k a l i enrichment path. Figures 7 and 8 compare the concentrations of the major element oxides TABLE 3, Duplicate Analyses of Explorer Area Basalts by Different A n a l y t i c a l Methods SAMPLE: 70-25--2D-8 73-26--2-1B 73-26 -5-1B METHOD: Atom. Absor. Fused Disk Fused Disk Press. P e l . Fused Disk Press. P e l . Fused Disk Press. P e l . ANALYST: W. Bertrand (1972) R.L. Chase Armstrong & Nixon,1980 B.: Cousens (1982) R.L. Chase B.- -Cousens. (1982) R.L. Chase B. Cousens (1982) S i 0 2 46.10 47.77 •47.87 47.33 * 49.63 48.75 51.11 50.12 T i 0 2 1.24 1.21 1.19 1.30 2.27 2.40 1.52 1.73 A1 20 3 16.30 16.81 17.38 16.9.1 ^ 17.30 15.87 17.87 16.16 F e 2 0 3 * 9.20 9.48 9.24 10.23 9.03 9.28 6.90 7.62 MnO 0.12 0.15 0.16 0.16 0.15 0.16 0.14 0.17 MgO 8.76 8.57 8.76 9.50 x 1 4.53 5.06 1 5.08 6.78 CaO 11.00 11.79 11.89 12.36 I 8.03 8.00 , 8.15 8.57 Na 20 3.20 2.46 2.39 2.29 | 4.52 5.18 1 4.82 4.95 K 20 o ; i 9 0.20 0.14 0.22 1 2.57 2.46 • 2.20 2.02 P2°5 no data 0.08 0.13 0.Q9 | - 0.91 0.66 I 0.79 0.58 H20++ C0 2 0.80 >1.03 1.00 0.80 ] >1.44 2.77 ] >0.66 1.79 TOTAL 96.90 98.75 99.61 101.19 i 99.86 100.60 1 99.03 100.48 A J. T. Wilson Knolls Fig. 5: AFM diagram for Explorer area basalts. Differentiation trends from MacDonald and Katsura (1964). Abyssal tholeilte field from Mlyashiro et al.(1970). 43 i 44 45 46 — i — 47 48 i 49 "so" — i — 51 52 % Si02 Figure 6 : Silica variation diagram for Explorer area basalts. TholeiKe/alkali basalt boundaryfrom MacDonald and Katsura (1964). Juan de Fuca basalts: x- Barr & Chase (1974). f and g- Wakeham (1977). NJ 23. AI2O3, FeC^, CaO and TiC^, r e l a t i v e to MgO, i n Explorer basalts, with those of other MORB. In a l l cases, the Explorer rocks plot within the f i e l d of MORB whole rock analyses, and generally plot near or within the f i e l d of MORB glass analyses. This suggests that phenocryst composition i s not influencing the whole rock chemistry s i g n i f i c a n t l y . It i s notable that although Explorer CaO and AI2O3 contents plot i n the middle of the MORB range, FeOt and T i 0 2 p l o t i n the high range, s i g n i f i c a n t l y higher than the northern Juan de Fuca Ridge anal- yses. The Explorer t h o l e i i t e s somewhat resemble the "average" chemical compos- i t i o n of basalts from the southern Juan de Fuca Ridge (Melson et a l . , 1976). Similar high values for FeOt, Ti02, K2O and P2O5 are encountered, but MgO con- tents are s i g n i f i c a n t l y higher than the southern Juan de Fuca "average". Using a Melson et a l . (1976) c l a s s i f i c a t i o n , the Explorer basalts most resemble low titanium members of the FETI group, more accurately termed f e r r o b a s a l t s . P i c r i t i c basalts, s i m i l a r to those found on Gorda Ridge (Wakeham, 1977) and on northern Juan de Fuca Ridge (Barr and Chase, 1974), are present i n the Explorer R i f t and along the Paul Revere Ridge. The JTW Knolls basalts do not f i t into any catagory of Melson et a l . (1976), and__they do not report any analysis of ocean ridge volcanic rocks with s i m i l a r chemistry. Figure 9 i s a ternary diagram of the system plagioclase-pyroxene-olivine, with phase boundaries from the simpler system a n o r t h i t e - d i o p s i d e - f o r s t e r i t e superimposed on i t . As has been previously noted for most MORB (Thompson et a l ., 1980; B a s a l t i c Vol canism Study Project, 1981), the Explorer basalts c l u s t e r along the p l a g i o c l a s e - o l i v i n e c o t e c t i c . This c o r r e l a t e s well with the occur-' rence of o l i v i n e and plag i o c l a s e phenocrysts i n the rocks, and with the lack of pyroxene phenocrysts. It i s notable that i n Figures 7 to 9, the JTW basalts plot within the f i e l d of MORB whole rock analyses, although they more resemble Bowie Seamount a l k a l i 24. 244 5" 16-1 < 12 8 \MORB whole rock Haw>v ak. bas. Y VMORB glass * b"'•• ocean island basalts s. A T • o x b J. T. Wilson Knolls DeRwood Seamounts Dellwood Knolls Explorer Rift Explorer Deep South Explorer Ridge Paul Revere Ridge Juan de Fuca Ridge Bowie Seamount 10 MgO (%) —r~ 15 20 —1— 25 20- 16- 5 12J. 8 ocean island basalt \ Haw. a Ik. bas.\ / / /'* .••-• / x ? MORB glass MORB whole rock » 1 / - 1 — 10 —1— 15 20 —1— 25 MgO (%) Figure 7 : MgO-variation diagram for FeO and A l 2 0 3 i n Explorer basalts. MORB glass, MORB whole rock, ocean island basalt, and Hawaiian alkali basalt fields from Basaltic Volcanism Study Project (1981). Juan de Fuca basalts: Barr and Chase (1974). 25. o co O 204 16H 12 8-I \\ MORB whole rock \ V /••"'wlaft 1'—"• / ocean island basalt MORB gW--rgfcfr'' x ^ / \ LS 1 -V / >Haw. alk. bas. —i— 10 — r - 15 — i — 20 6* 4H 3 H MgO (%) i « -i-Haw. ah. bas. • O x b J. T. Wilson Knolls OeBwood Seamounts Dellwood Knolls Explorer Rift Explorer Deep South Explorer Ridge Paul Revere Ridge Juan de Fuca Ridge Bowie Seamount ; ocean island basalt MORB whole rock Il2> l 5 - T - 10 15 MgO(%) — i — 20 —i— 2 5 Figure 8 : MgO-variation diagram for CaO and Ti02 in Explorer basalts. Basalt fields and data sources as in Figure 7 . 26. J. T. Wilson Knolls Dellwood Seamounts Dellwood Knolls Explorer Rift Explorer Deep South Explorer Ridge Paul Revere Ridge Juan de Fuca Ridge Bowie Seamount MORB glass PL OL Figure 9 : CIPW-normative triangle diagram of the plagioclase-oiivine-pyroxene system. Basalt fields and data sources as in Figure 7. Phase boundaries after Osborn and Talt (1952) for simplified system An-Di-Fo. 27. basalts (part of the Pratt-Welker Seamount Chain) than Explorer t h o l e i i t e s . The incompatible major elements, titanium and potassium, are present i n weakly to strongly enriched concentrations i n Explorer basalts, compared to MORB (Figure 10). Average IOJO i s e s p e c i a l l y high i n the Explorer Deep samples. In these elements, the Explorer rocks are s i m i l a r to basalts from the FAMOUS area. The l e v e l s of Na20, K2O, and Ti02 i n the Explorer basalts are higher than those found on the northern Juan de Fuca Ridge (Barr and Chase, 1974) , and K2O contents .are generally higher than those of the southern Juan de Fuca and Gorda Ridges (Kay et a l . , 1970; Wakeham, 1977). K 20 and Na 20 are highly enriched i n the JTW hawaiites, s u b s t a n t i a l l y above l e v e l s t y p i c a l of MORB. The high concentrations of incompatible major elements could be the r e s u l t of c r y s t a l f r a c t i o n a t i o n , minor "plume source" influence, smaller degrees of p a r t i a l melting of the mantle source, or differences i n the chemistry of the mantle source. To test the influence of f r a c t i o n a t i o n , K2O and Ti02 are p l o t - ted against the magnesium r a t i o , 100 (Mg/Mg+Fe2+), as i l l u s t r a t e d i n Figure 11. The observed negative c o r r e l a t i o n between the two oxides and the magnesium r a t i o i s due, at l e a s t i n part, to f r a c t i o n a t i o n . In several cases, basalts from the same dredge haul (e.g. 70-25-16, 79-6-32, 71-15-92, and 70-25-4) appear to be d i r e c t l y r e l a t e d by t h i s process. However, the general scatter of points i s appreciable. There i s substantial v a r i a b i l i t y i n the K2O content of basalts from the same ridge segment, with s i m i l a r magnesium r a t i o s , notably from Explor- er R i f t and the Southern Explorer Ridge. It i s also notable that whereas the highly d i f f e r e n t i a t e d FETI basalts from the Galapagos Rise (Byerly, 1980) and the southern Juan de Fuca Ridge r a r e l y have K2O greater than 0.3%, several Ex- plor e r area samples exceed t h i s value. Thus, f r a c t i o n a t i o n alone icannot explain the v a r i a t i o n i n K2O i n the basalts. The occurrence of r e l a t i v e l y unfractionated basalts (magnesium r a t i o of 68 to 72) i n the Explorer R i f t i s somewhat puzzling. This spreading segment i s ' the r e s u l t of a ridge "jump" from Explorer Deep within the l a s t lMa BP. It i s 4 O # 2 3H et 2 1 • b b b MORB • O X b 28. J T. Wilson Knolls DeBwood Seamounts Dellwood Knolls Explorer Rift Explorer Deep South Explorer Ridge Paul Revere Ridge Juan de Fuca Ridge Bowie Seamount F A M 0 W / V JTW —r— 2 3 4 % Na 20 MORB . JTW • Bowie FAMOUS 0.1 0.2 ~o!i~ 0.4 0.5 0.6 ~0J Figure 10: Plots of TiOjVS. Nagp and K̂ O. JTW basalts plot off-scale on KaO diagram. MORB and'FAMOUS fields from compilation in Thompson et aL (1980). Juan de Fuca basalts: x-Barr & Chase (1974). k-Kay et a1.(1970). f and g- Wakeham (1977). 2.0- 1XM f o o • • x x • X x •b go A ^ A g g A • • ** Jta i _ f _ 0.3- 0.24 0.1 JTwf Bowie A J. T. Wilson Knolls T Dellwood Seamounts A Dellwood Knolls • Explorer Rift • Explorer Deep • South Explorer Ridge O Paul Revere Ridge x Juan de Fuca Ridge b Bowie Seamount o f • JTWj 0 ° T • • • o _p • • l-J x • x g • • o g o fl g g xo x g f 70 50 6 5 100 (Mg/Mg*Fe2*) Fig. 11: Magnesium ratio variation diagram for Kjp and TIOj. J. Tuzo Wilson samples plot off the KjO diagram (>1.0%) Juan de Fuca basalts: x- Barr (1974) and Chase , f- Wakeham (1977). Qorda basalts: g- Wakeham (1977). t o VO 30. r i f t i n g r e l a t i v e l y older and cooler crust (1.3-2.1 Ma). Thus, a high degree of low-pressure c r y s t a l f r a c t i o n a t i o n would be expected, s i m i l a r to that seen at propagating ridge t i p s (Clague and Bunch, 1976; Byerly, 1980), such as the Gal- apagos Rise. It appears that the greater age and consequent coolness of the r i f t e d crust has l i t t l e e f f e c t on the chemistry of the Explorer R i f t rocks, contrary to what was expected. In general, the abundance of r e l a t i v e l y unfractionated basalts i n the Explorer area i s greater than the ocean ridge average, as shown i n the frequency histogram i n Figure 12. Almost ha l f of the Explorer basalts studied have mag- nesium r a t i o s of 62 or greater, while only one quarter of MORB l i e i n t h i s :: range. The Explorer d i s t r i b u t i o n i s wieghted by the numerous unfractionated samples from Explorer R i f t , but nevertheless,.the"Explorer system produces l e s s fractionated basalts than the average ocean ridge. CIPW normative compositions for a l l the Explorer area samples studied are l i s t e d i n Appendix 2. It i s immediately apparent that the JTW, Dellwood Knolls, and Explorer R i f t basalts are nepheline normative, while the Southern Explorer Ridge, Ex- plorer Deep, and Paul Revere Ridge basalts are olivine-hypersthene normative. The JTW Knolls samples, are hawaiites and mug ear i t es, with r e l a t i v e l y l a r ge amounts of nepheline and orthoclase i n the norm. Figure 13 i s a plot of the system An-Ne-01-Hy-Q, and i t demonstrates a general trend of increasing s i l i c a saturation, progressing southward along the ridge system. This c o r r e l a t e s well with the age of the spreading segments i n - volved. The youngest segments, including the Dellwood Knolls and Explorer R i f t , are l a r g e l y nepheline normative, while the older segments progress through o l - i v i n e t h o l e i i t e to quartz t h o l e i i t e i n composition. This i s probably a" r e f l e c - t i o n of the lower temperature gradient, and consequent higher pressure of magma generation, experienced at a newly i n i t i a t e d r i f t . This r e s u l t s i n a more a l k a l i c magma (Presnall et aL. } 1979). 31. 25 100 (Mg/Mg+Fe2+) Figure 12: Frequency histogram of Mg ratios in ocean floor basalts (solid line) and Explorer Area basalts (stipled area). Ocean floor data from Basaltic Volcanism Study Project (1981). Figure 13: Normative triangle for the system An-Ne-Oi-Hy-Q. Juan de Fuca basalt fields: dashed line: Barr & Chase (1974). dotted line: Kay et al.(1970). "x": Moore (1970). dot-dashed line: Wakeham (1977). ts3 33. TRACE ELEMENTS (i ) A n a l y t i c a l Procedure The concentrations of barium, cerium, chromium, niobium, neodymium, n i c k e l , rubidium, strontium, vanadium, yttrium, and zirconium were determined by X-ray fluorescence analysis of pressed powder p e l l e t s . The p e l l e t s used i n the major element a n a l y s i s were also used f o r the trace element determinations. Rb and Sr data were reduced by the method of Feather and W i l l i s (1976). Ba, Ce, Cr, Nb, Nd, Ni, V, Y, abd Zr data were reduced by the t r a d i t i o n a l peak measurement- background subtraction method, including interference correction and mass ab-* sorption adjustment, using computer programs written by R. G. Berman of the University of B r i t i s h Columbia. La/Sm ef r a t i o s were determined by neutron ac- t i v a t i o n a n a l y s i s , performed by J.-G. S c h i l l i n g at the University of Rhode Island. XRF operating conditions f or a l l trace element analyses are l i s t e d i n Appendix 4. The trace element data for a l l the samples studied are l i s t e d i n Appendix 2. ( i i ) P r e c i s i o n and Accuracy With the exception of Ba and Ce, the observed precisions for analyses of standards used to create the working curves i s better than 5 ppm (one standard d e v i a t i o n ) . In view of the uncertainty of the "recommended values" (Abbey, 1980), t h i s l e v e l of p r e c i s i o n i s acceptable. The precision, f o r analyses of unknowns was estimated using the method of Howarth and Thompson (1976), following the same procedure discussed i n the pre- vious chapter. An example of a p r e c i s i o n plot i s presented i n Appendix 3, and the precisions of both standards and unknowns are l i s t e d i n Appendix 2. Ba P r e c i s i o n i s poor due to low i n t e n s i t i e s , while Ce p r e c i s i o n i s poor due to the interference of Nd, for which no c o r r e c t i o n p e l l e t was a v a i l a b l e . 34. ( i i i ) Results Compared to other ocean ridge systems, the Explorer basalts have high concentrations of Ba ( F i g . 14), Rb, Nb, Sr, and Zr (Pearce and Cann, 1973; Sun, 1980; Engel et a l . , 1965). Figure 15 i s a magnesium r a t i o v a r i a t i o n diagram for these f i v e elements. A l l except Sr are highly incompatible, and exhibit nega- t i v e c o r r e l a t i o n s with the magnesium r a t i o , although, as with K2O, a considerable scatter of points i s evident. This indicates that, as previously suggested, c r y s t a l f r a c t i o n a t i o n can account for much of the chemical v a r i a t i o n seen i n the Explorer basalts, but another process must be i n f l u e n c i n g the chemistry to produce the observed s c a t t e r . Explorer Deep shows abnormally high concentra- tions of Rb, Nb, and Zr, which cannot be explained by f r a c t i o n a t i o n . The ferromagnesian elements, Ni and Cr, c o r r e l a t e p o s i t i v e l y with the mag- nesium r a t i o , as depicted i n Figures 16 and 17. Nickel follows the predicted pattern due to i t s removal from the magma by o l i v i n e (Sato, 1977). Cr also shows signs of removal by chromian s p i n e l , although only a few samples have been analysed for t h i s element. None of the basalts, except the h o l o c r y s t a l l i n e rocks, have l a r g e numbers of phenocrysts, and t h i s should not be an influence on the data. The n i c k e l diagram shows some sc a t t e r , s i m i l a r to that seen i n previous magnesium r a t i o diagrams. Figure 17 also shows v a r i a t i o n s i n V concentration with increasing f r a c - ' t i o n a t i o n . V acts as an incompatible element u n t i l titanomagnetite or c l i n o - pyroxene begin to c r y s t a l l i z e , whereupon i t r e a d i l y enters the c r y s t a l l a t t i c e s of these two phases. The pattern i n f i g u r e 17 shows a steady l i n e a r increase as the magnesium r a t i o decreases, s i m i l a r to the behavior of TiC^. This i n d i - cates that neither magnetite nor clinopyroxene are c r y s t a l l i z i n g phases i n the Explorer area, as was noted i n t h i n - s e c t i o n (Clague et a l , 1981). It i s evident from Figures 15 to 17 that the l e a s t fractionated basalts are found i n the Dellwood Knolls and the Explorer R i f t . Ni contents and the magnes- £ a a E 3 * k (S m 110 100 90 80 70 60 50 40 30 20 10H Bowie JTW V FAMOUS / / / B ' / a a T / ° o n f / MORB Jir̂  n-^f • / / A T • O x b 4*0 BO ' 120 160 260 240 Strontium (ppm) J. T. Wilson Knolls Dellwood Seamounts Dellwood Knolls Explorer Rift Explorer Deep South Explorer Ridge Paul Revere Ridge Juan de Fuca Ridge Bowie Seamount Figure 14: Plot of Ba vs. Zr . MORB and FAMOUS fields from Thompson et al.(1980). Juan de Fuca basalts from Armstrong and Nixon (1980). J. Tuzo Wilson and Bowie basalts plot above the scale. Co i ' E 3 S i 10 5 H 36. • D DO 55 T 1 1——r- 1 1 1 1——JZ 1 1 1 1 -T— 60 65 70 E a a E Z o E a a 24 20 16 12 8 4 55 40- 35- 30- g 25- 20 _ 55 o n * . o no f o • A B T • • X 1 1 i — — i — — i 1 1 1 1 i 1 — i — i — _ i 60 65 70 A A • o T i 1 1 — — i — — I 1 i 1 — — r - — i 1 1 r •••• • E 150-a a 130 E 3 110- C O 90- o N 70- 60 o Dm • • • o • T • • 65 70 55 ~« ' 1 r——T— 60 -% 1 1 1 i 1 r 65 •c 220-c a a. 200 E 180 s ? 160- c o m. 140- CO 120- O • • • o 55 1 1 1 1 65 60 100 (Mg/Mg+ Fe24) T 1 r 70 A J T. Wilson Knolls T Dellwood Seamounts • Dellwood Knolls • Explorer Rift • Explorer Deep • South Explorer Ridge o Paul Revere Ridge X /o - Juan de Fuca Ridge Figure 15: Magnesium ratio variation diagram for Rb, Nb, Y, Zr, and Sr. JTW basalts plot off the diagrams, except for Y. 280 260 240 220 200 180 E 160 a a «^ 140 © 120 o Z 100 SO 60 40 20 A J. T. Wilson Knolis T DeOwood Seamounts A Dellwood Knolls • Explorer Rift • Explorer Deep • South Explorer Ridge O Paul Revere Ridge 50 # 55 60 100 (Mg/Mg*Fe2*) Figure 16: Magnesium variation diagram for o 3&24 A J. T. Wilson Knolls T ' Dellwood Seamounts A Dellwood Knolls • Explorer Rift • Explorer Deep • South Explorer Ridge O Paul Revere Ridge 38.24 O o • • Figure 17: Magnesium ratio variation diagram for V and Cr. • JTW - i r • • JTW i 70 50 "6T 100 (Mg/Mg+Fe*+) 65 LO co 39. ium r a t i o are high, while incompatible element l e v e l s are low. The most "evol- ved" (fractionated) basalts are from the Southern Explorer Ridge, which i s the oldest of the ridge segments. This suggests that the newer segments have not yet developed a large magma chamber, i n which magma can re s i d e for a period of time, to allow f r a c t i o n a t i o n to occur. The JTW basalts are highly enriched i n every incompatible trace element with respect to t y p i c a l MORB, and strongly resemble basalts dredged from the co-li n e a r Pratt-Welker Chain (Engel et aL, 1965; Table 4). The observed trace element l e v e l s are si m i l a r to those of an "average" ocean i s l a n d a l k a l i basalt. In terms of rare earth element patterns (Figure 18), the Explorer area i s anomalous, i n that two-thirds of the samples analysed show l i g h t rare earth element (LREE) enrichment. This i s not c h a r a c t e r i s t i c of normal ocean ridge t h o l e i i t e s , which generally exhibit LREE depletion ( S c h i l l i n g , -1971; Sun et a l . , 1979). The observed La/Smef r a t i o s resemble those of plume-influenced MORB, t y p i c a l of the FAMOUS area and the Mid-Atlantic Ridge at 45°N (Sun et al.,1979; White et al.,1976). Kay et a l . (1970) and Wakeham (1977) report l i g h t REE depletion i n a l l samples from the Gorda and southern Juan de Fuca Ridges, with only one exception. Basalts from Explorer Deep, with high, l e v e l s of other incompatible elements, are LREE enriched. Explorer R i f t and the Southern Explorer Ridge again show a range of La/Sm e£ values, from LREE depleted (0.59) to LREE enriched (1.73). It i s notable that the northwest Dellwood Knoll has a La/Sm e£ r a t i o of 0.81, but the southeast k n o l l has a r a t i o of 1.49. The JTW Knolls exhibit extreme enrichment i n the LREE , above the l e v e l s f o r major mantle plumes (e.g. Iceland, Azores, Jan Mayen). Such a high l e v e l of enrichment has been encountered i n an o l i v i n e t h o l e i i t e dredged from a single volcanic cone on a short segment of spreading ridge i n the Tadjura Trough, at the west end of the Gulf of Aden ( S c h i l l i n g , personal communication). Erlank and Kable (1976) use the Zr/Nb r a t i o to measure the degree of de- 40. TABLE 4 Trace Element Concentrations i n Oceanic Basalts Ba Cr Nb Ni Rb Sr V Y Zr FeC-t/MgO MORB AVERAGE* 14 ±7 297 ±73 3 97 ±19 1.2 ^ 130 ±25 292 ±57 43 ±10 95 ±35 1.20 OCEAN ISLAND AVERAGE* 498 ±136 67 ±57 72 ±9 51 ±33 33 ± ? 815 ±375 252 ±32 54 ±7 333 ±48 1.99 BOWIE SEAMOUNT J. TUZO WILSON PV-50* 67-6-12+ 73-26-2-1+ '420 170 82 76 33 1100 260 48 350 1.72 335 86 41 670 338 1.80** 361 25 87 38 36 590 232 31 396 1.63 Sources: * - Engel et a l . (1965), except Nb and Rb, which are from Sun (1980). **••- Herzer (197-1) + - t h i s study T — i — i — i — j — i — i — i — — i — " — r EXPLORER RIDGE SYSTEM o E 3f CO cd • o J T. Wilson Knolls Dellwood Seamounts Dellwood Knolls Explorer Rift Explorer Deep South Explorer Ridge Paul Revere Ridge • • o " • cib • « Tadjura Trough A Azores H Jan Mayen Iceland Morb 49 5 0 LATITUDE N 5 1 52 Figure 18 : Latitudinal variation In the La/Sm ratio, normalized to chondrites, for the Explorer system. Average values for MORB and typical ocean island basalts indicated on right side of diagram. 42. p l e t i o n of the magma-generating mantle source. The Explorer area basalts have low to t r a n s i t i o n a l Zr/Nb r a t i o s compared to MORB, varying from 6 to 30 ( F i g - ure 19). Explorer Deep and the JTW Knolls have the lowest r a t i o s , which are t y p i c a l of ocean i s l a n d plume ba s a l t s . The same patterns noted for the REE i n Explorer rocks are evident i n the Zr/Nb r a t i o s . L i i a s et a l . (1981) -report Zr/Nb r a t i o s of approximately 25 for one hundred and twenty four samples, from fift y - t w o dredge hauls, from the Juan de Fuca Ridge. This i s thought to be the f i r s t occurrence of intermediate Zr/Nb r a t i o s along such an extended ridge seg- ment (400 km). Thus, the Explorer basalts have somewhat a t y p i c a l Zr/Nb r a t i o s for MORB, but are s i m i l a r to, or lower than, those of Juan de Fuca Ridge ba- s a l t s . The mantle source of Explorer basalts appears to be undepleted r e l a t i v e to most ocean ridge systems. 500- 300- .o 1(XH 50-̂ 3W AVERAGE MORB • • A3. • O x b J. T. Wilson Knolls Deftwood Seamounts Dellwood Knolls Explorer Rift Explorer Deep South Explorer Ridge Paul Revere Ridge Juan de Fuca Ridge Bowie Seamount If " ~ i — i — i — i — ' M I — 30 50 100 Zr/Nb Figure 19: Zr/Nb diagram (Erlank & Kable, 1976 ) for Explorer area basalts. Closed star Average MORB, Erlank and Kable, 1976. Open star Average MORB, Sun, 1980. 44. STRONTIUM ISOTOPES (i ) A n a l y t i c a l Procedure Seven basalts from the Explorer-Dellwood system, three JTW Knolls hawai- i t e s , and one a l k a l i basalt from Bowie Seamount have been analysed for 8 7 S r / 8 % r r a t i o s , using a VG Isomass 54R mass spectrometer. Data a c q u i s i t i o n i s automated using a Hewlett-Packard HP-85 computer. Experimental data has been adjusted so that the NBS standard SrC03 (SRM987) gives a 8 7 S r / 8 6 S r r a t i o of .7102012. Samples were unspiked, and were prepared using standard i o n - exchange techniques. 8 7 S r / 8 f % r r a t i o s for the basalts are l i s t e d i n Appendix 2. ( i i ) P r e c i s i o n and Accuracy For each sample, between 6 and 15 separate data blocks were completed, and machine l a precisions range from ±.00002 to ±.00015. The average l a p r e c i s i o n i s .00006, although true reproducability i s probably ±.00010 (R.L. Armstrong, personal communication). Three of four duplicate analyses do f a l l within .00010 of each other. ( i i i ) Results The spreading ridge basalts have Sr/ Sr r a t i o s ranging from .70232 to .70254. They f a l l within the range of analyses from the Juan de Fuca arid .̂ Gorda Ridges (Figure 20), which i s between .7023 and .7027 (Sun et a l . , 1979). Three other analyses from the northwest Dellwood K n o l l , northern Juan de Fuca Ridge (71-23-3), and Explorer Seamount (69-6-4), also f a l l within the range of the Explorer analyses (Nixon and Armstrong, 1980). It i s notable that the Explorer samples enriched in incompatible elements (79-6-32-39, 77-14-33-A, 70-25-15-3) have s i m i l a r i s o t o p i c r a t i o s to the i n - compatible-element-depleted basalts (70-25-4-62, 77-14-36-X), and thus t h e i r mantle sources appear to be s i m i l a r i s o t o p i c a l l y . However, the northwest Dellwood Knoll basalt i s s i g n i f i c a n t l y l e s s rad- iogenic than the southeast Knoll basalt. This, combined with the LREE enrich- 5H 4 i E CO cd — o — -TTAT i . . . • • • . : : : ; • .^iH23r.3;:\::\:^:::!::.::i: Juan de Fuca & Gorda N-MORB 1 T " T .7023 .7025 .7027 8 7 S r / 8 6 S r A J. T. Wilson Knolls r Dellwood Seamounts A Dellwood Knolls • Explorer Rift • Explorer Deep • South Explorer Ridge O Paul Revere Ridge x Juan de Fuca Ridge b Bowie Seamount FAMOUS P-MORB .7029 r .7031 Figure 20: La/Sm Q f vs. 87Sr/86Sr for Explorer.Dellwood, JTW, and Bowie basalts. Juan de Fuca and NW Dellwood Knoll analyses from Nixon and Armstrong (1980). Error bars are precisions determined from within-run variation of the measured ratio for each sample. Reference fields from Sun et aL, (1979). Asterisk: leached sample 70-25-3D-1. merit of the southeast Kno l l , suggests that the mantle sources for the basalts from the two adjacent knolls are d i f f e r e n t . To ensure that the observed difference: i n ^ S r / ^ S r r a t i o s i s not due to a l t e r a t i o n by seawater of sam- ple 3D-1, a leached subsample (following an HC1-leaching procedure described i n Zhou and Armstrong, 1982, i n press) was run through the mass spectrometer, y i e l d i n g a s l i g h t l y lower r a t i o of .70267. This i s s t i l l s i g n i f i c a n t l y d i f - ferent from the northwest Knoll value of .70240, which i f subjected to the leaching process, would probably also be reduced (R.L. Armstrong, personal communication). The JTW and Bowie Seamount a l k a l i basalts appear to be d i s t i n c t from the Explorer basalts, as indicated i n Figure 20. They f a l l within the range of ^ S r / ^ S r values obtained from the other Pratt-Welker seamounts, although the observed r a t i o s are somewhat lower than the Pratt-Welker average (.7032-.7039) and the ocean i s l a n d basalt average (Forbes et a l . , 1982; Sun et a l . , 1979). 47. DISCUSSION Explorer Ridge and Propagating R i f t s In terms of major element chemistry, the Explorer area ridge basalts can be c l a s s i f i e d as f e r r o b a s a l t s , or as MORB bordering on ferrobasalt (Melson et a l . , 1976). This i s c h a r a c t e r i s t i c of many other segments of the fast-spreading East P a c i f i c Rise, and i s considered to be the r e s u l t of low-pressure c r y s t a l f r a c t i o n a t i o n (Clague and Bunch, 1976). The r e s u l t s of t h i s study are c o n s i s - tent with the findings of Vogt and Byerly (1976), who suggest that the observ- ed high magnetic r e l i e f over the Explorer area (Figure 21) could be due to high Fe-Ti basalts, s i m i l a r to those dredged from the southern Juan de Fuca Ridge. The FeO1- and Ti02 contents of the Explorer basalts are very s i m i l a r to those of the Juan de Fuca and Galapagos Ridges, and one sample (72-22-7-1) from the Paul Revere Ridge resembles the extreme FETI basalts that are associated with both areas. FETI basalts are associated with propagating r i f t s (Hey et al.,1980; Byerly, 1980), an example of which i s the southern Juan de Fuca Ridge. Figure 3 i s a magnetic anomaly map for the Juan de Fuca area, showing several magnetic "pseudofaults" which are interpreted to be features of major ridge propagation sequences (Hey, 1977). If the V-shaped pseudofault marked "1" i s the r e s u l t of ridge propagation, i t implies that the Explorer Ridge has propagated southwest to i t s present southwestern termination at the Sovanco Fracture Zone i n the recent past. As i s c h a r a c t e r i s t i c of other propagating r i f t s , the ridge segment i s oriented obliquely to the Sovanco F.Z. (Hyndman et a l . , 1978). Another fea-^ ture of propagators i s that the highest FeOt and Ti02 contents are found at the t i p of the propagating segment. Samples 71-15-70-1/8, from the extreme south- west end of the Southern Explorer Ridge, have among the highest FeOt (12.0%) •• and Ti02 (2.1%) contents of a l l the samples studied. At the northeast end of 48. 49*N 45'N 40* N 8-3 • DELLWOODCiT EXPLORER R.9 1 X MAGNETIC RELEF >1200 * ED 1000-1200 O800-10007 MENDOCINO FZ. 130'W 125*W Figure 21: Map of magnetic relief and FeO content along the Juan de Fuca system. Total iron expressed as FeO. Dots mark dredge sites. Magnetic relief and Juan de Fuca FeO data from Vogt and Byerly (1976). 49. Explorer Deep, Paul Revere Ridge ( f r a c t u r e zone) samples 71-15-92-8 and 72-22- 7-1 have very high 7eOt (11.8-17.3%) and T i 0 2 (1.95-3.53%) contents, which could be interpreted as evidence for an older, northeasterly propagator (Figure 20). However, sampling i s at present i n s u f f i c i e n t to make a d e f i n i t i v e conclusion concerning the propagating r i f t theory as i t applies to the Explorer area. A deta i l e d magnetic and bathymetric survey of the Southern Explorer Ridge i s also lacking, the south end of which i s extremely i n t e r e s t i n g i n terms of t h i s theo- ry- Explorer Deep, Explorer R i f t , and the Southern Explorer Ridge Much of the chemical v a r i a t i o n within the Explorer-Dellwood system can be explained by low-pressure c r y s t a l f r a c t i o n a t i o n (Figures 11,'15-17). However, a l l the plot s show considerable scatter, suggesting that some other process i s infl u e n c i n g the basalt chemistry. As well, the LREE enrichment measured i n many of the samples cannot.be explained by f r a c t i o n a t i o n ( S c h i l l i n g , 1971). Explorer Deep Basalts have exceptionally high concentrations of K2O, Ba, Nb., and Rb, as well as an anomalous LREE-^enriched rare earth pattern. Figure 22, a f t e r Sun (1980)., i l l u s t r a t e s the normalized pattern of incompatible trace elements. Explorer Deep basalts, and some of the Explorer R i f t rocks, have the c h a r a c t e r i s t i c s of plumeT-influenced MORB, or P-MORB (Sun et a l . , 1979), where ocean xidge basalt magmas aire augmented by an incompatible-element-rich phase, such. as. a plume magma. The. Azores Platform and the Reykjanes Hidge are areas where t h i s process: has been documented (White et al.1976; S c h i l l i n g , 1973). Explorer Deep i s presently a 40.0-meter deep grahen however, with no ob- vious topographic expression of a plume nearby. There are many small seamount chains trending away from the ridge, such-as the Dellwood Seamounts, which probably originated at Explorer Deep between 2.8 and 4.5 Ma BP. In the past, therefore, we have evidence for above-average volumes of magma generation i n the Explorer Deep area. In addition, the average sea f l o o r depth, i n the Ex- 50. 400 10CH (0 ID ZD < > Q LL! N or o 60H 10 14 • • o x J. T. Wilson Knolls Dellwood Seamounts Dellwood Knolls Explorer Rift Explorer Deep South Explorer Ridge Paul Revere Ridge Juan de Fuca Ridge 73-26-2 A ^ J / OCEAN ISL*AND 70-25-17»— f7-14-33 P—MORB/ I70-25-2DA—f 70-25-4 N-MORB- y v "Nb K La Ce S> Nd ' Sm Ti P Zr Rb Ba Figure 22: 'Sun diagram" (Sun, 1980) showing variations in incompatible element concentrations relative to chondrites for Explorer area basalts, average normal MORBXN-MORB), plume-enriched MORB (P-MORB), and ocean island basalts. 51. plorer Deep i s s l i g h t l y shallower than the average depth of the northern Juan de Fuca Ridge C 2 2 0 0 meters and 2400 meters, r e s p e c t i v e l y ) . For the Explorer Ridge as a whole, the average depth increases from 2200 meters near Explorer Deep, to 2800 meters at the Sovanco Fracture Zone. The shallower depth of the Explorer Deep area may r e f l e c t the presence of a broad hotspot zone centered beneath i t . It should be noted that a plume model i s not required to explain the anom- alous chemistry of Explorer Deep. A smaller degree of p a r t i a l melting of the mantle source would also explain the high l e v e l s of incompatible elements with- out e f f e c t i n g the major element chemistry (O'Hara, 1977). The S r 8 7 / S r 8 6 r a t i o s for the Explorer Deep basalts are not d i f f e r e n t from the other more t y p i c a l MORB from the Explorer system, and suggests that the above explanation may be more sui t a b l e for t h i s case. Other plume-influenced areas, such as the FAMOUS area and the Mid-Atlantic Ridge at 45 N, exhibit anomalously high S r 0 / / S r o a r a t i o s compared to A t l a n t i c MORB (White et a l . , 1978). An alternate.explanation for the shallower depths of Explorer Deep i s "" flexure of the crust caused by oblique subduction of the P a c i f i c and Explorer Plates beneath the American Plate. This process i s probably responsible for the t i l t i n g of the Paul Revere Ridge and compression i n the Winona Basin (Davis and Riddihough, 1982). The Oshawa Rise, an elongate basement high extending south- east from Bowie Seamount (Figure 23), i s thought by S i l v e r et a l . (1974) to mark the passage of the P a c i f i c Plate over a hotspot. However, t h i s feature may also represent flexure due to subduction (C. Yorath, personal communication) . The three other ridge segments display a large degree of chemical v a r i - a t i o n . Three dredge hauls from Explorer R i f t , within 15 km of each other, recovered three d i f f e r e n t basalt types. Type 1 i s a d i f f e r e n t i a t e d , incompat- i b l e element enriched basalt (.71-15-77). Type 2 i s a r e l a t i v e l y u n d i f f e r e n t - iated, incompatible element depleted basalt (70-25-4 and 70-25-16-1). Type 3 i s also r e l a t i v e l y u n d i f f e r e n t i a t e d , but i s weakly enriched in: incompatible 52. elements (77-14-33). The incompatible element patterns of Type 2 and Type 3 are shown i n Figure 22. Even within a s i n g l e dredge haul, 70-26-16, basalts of Types 1 and 2 were recovered. This v a r i a b i l i t y cannot be rela t e d to f r a c t i o n a - t i o n . It could be an indicato r of a heterogenous mantle source beneath Explorer R i f t , e s p e c i a l l y since no appreciable d i f f e r e n c e i n Sr isotopes i s observed between the Type 2 and 3 bas a l t s . A l t e r n a t e l y , i f a hotspot does exist beneath Explorer Deep, feeder dykes could extend northwards to Explorer R i f t , and at times produce basalts enriched i n incompatible elements with respect to other magmas erupted i n the same r i f t . Note that i n Figure 22, the pattern of sample 77-14-33 l i e s between the Explor- er Deep pattern and 70-25-4-62, as i f mixing of the l a t t e r two magmas could produce the pattern of 77-14-33. Basalts from the Southern Explorer Ridge show the same v a r i a b i l i t y i n i n - compatible element concentrations without much change i n the degree of f r a c t i o n - a t i o n . Although a l l the samples have moderate to high l e v e l s of K£0 and trace elements, and s i m i l a r magnesium r a t i o s , t h e i r La/Sm ef r a t i o s vary from 0.85 to 1.35. Thus, although the Southern Explorer Ridge t h o l e i i t e s appear to be more fractionated than the Explorer R i f t basalts, the same heterogeneity a f f e c t s both ridge segments. Unfortunately, sample coverage of the Southern Explorer Ridge i s i r r e g u l a r , mostly from the northern end; only one sample i s from the southern end. More samples are needed to give even coverage, but these do not exist at present. Dellwood Knolls The geology of the Dellwood Knolls has been studied i n d e t a i l by Bertrand (1972) and Riddihough. et al.(1977). Both studies i n d i c a t e that the northwest k n o l l (sample 70-25-2D-8) i s presently a c t i v e s e i s m i c a l l y and v o l c a n i c a l l y , while the southeast k n o l l (sample 70-25-3D-1) ceased a c t i v i t y approximately 1 Ma 53. BP. Chemically, the k n o l l s are very d i f f e r e n t . Based on major element data, sample 3'D-l. i s a more fractionated basalt, that could have had a parent magma of s i m i l a r composition to sample 2D-8 , (Figure 5, 11). The only new data presented i n t h i s study from the Dellwood Knolls are selected trace element contents, La/Sm ef r a t i o s , and S r ^ / S r ^ r a t i o s for both 3D-1 and 2D-8. The rare earth and Sr isotope data suggest that 2D-8 could not be a parent magma for 3D-1. The more radiogenic nature and LRHE enrichment of 3D-1 indi c a t e that i t had a very d i f f e r e n t mantle source from 2D-8. As with Explorer Deep, a plume influence may be responsible for the chemical v a r i a t i o n i n the Dellwood area. J. Tuzo Wilson Knolls The c o n f l i c t i n g evidence as to the o r i g i n of the J. Tuzo Wilson Knolls makes t h i s an i n t e r e s t i n g part of t h i s study. In the o r i g i o n a l study of the Knolls , Chase (1977) considered them to be the expression of the mantle plume which had probably created the Pratt-Welker Seamount Chain (also c a l l e d the Bowie-Kodiak Seamount Chain). As shown i n Figure 23, the JTW Knolls l i e on the trend (colatitude) of the Pratt-Welker Chain, and l i e only 60 km northwest of the Dellwood spreading segment. One aim of t h i s study i s to determine i f , as with other ridges with nearby hotspots, magma mixing of the two types of basalt i s occurring, r e s u l t i n g i n a chemical gradient from the JTW Knolls south through the Dellwood-Explorer system. Examples of areas where t h i s gradation from ocean i s l a n d basalt composition to MORB i s observed include the Reykjanes Ridge, the Galapagos Rise, and the Azores Platform ( S c h i l l i n g , 1973; S c h i l l i n g et a l . , 1976; White et al.., 19781. It i s immediately obvious that no smooth chemical gradient exists that could he due to simple magma mixing. In terms of a l k a l i metals, incompatible elements, and e s p e c i a l l y the La/Sm ef data, the JTW Knolls are d i s t i n c t l y d i f - ferent from the Dellwood and Explorer segments. It i s possible that the magma Figure 23: Simplified bathymetrlc map of the Gulf of Alaska, showing locations of the Pratt-Welker Seamounts. Dashed line describes a small circle about the Paclflc-Hotspot pole of rotation. Map from Turner £ et al.(1980). 55. TABLE 5 A l k a l i Basalt Composition of Pratt-Welker Seamounts, J. Tuzo Wilson Knolls, and "Average" Ocean Isalnd Basalt. Kodiak Giacomini Hodgekins Bowie JTW "Average" S i 0 2 44.08 47.62 45.33 45.40 50.12 47.41 T i 0 2 3.16 2.42 3.53 2.60 1.78 2.87 A1 20 3 16.60 16.42 15.50 18.50 16.16 18.02 F e 2 0 3 * 13.25 13.82 13.72 11.86 '7.62 10.67 MnO 0.16 0.14 0.23 0.19 0.17 0.16 MgO 2.43 1.65 6.77 5.90 6.78 4.79 CaO 7.59 8.13 8.50 8.50 8.57 8.65 Na 20 4.01 4.79 4.30 3.80 4.95 3.99 K 20 1.69 1.81 2.36 1.90 2.02 1.66 ?2°5 1.14 1.84 0.73 0.70 0.58 0.92 H20+ 1.83 1.51 0.20 0.40 1.17 0.79 SUM 98.11 99.89 99.91 98.59 100.48 100.54 FeOt/MgO 4.86 7.48 1.81 1.80 1.42 1.99 La/Sm ef 2.4-3.5 2.0 1.2-2.0 4.5-5.1 2.5 Source: Forbes and Ho skin, 1969. Forbes et a l . , 1969. Engel and Engel, 1964. Herzer, 1971. t h i s study Engel et a l . , 1965. K-Ar age : 23.4 Ma 20.8 Ma 2.8 Ma >75,000 yr s . 55,000 yrs. ( a l l age data from Turner et al.(1980), except JTW from t h i s study.) 56. Figure 24: Silica variation diagram and chondrite-normalized rare-earth patterns for the Pratt-Welker Seamount magma types. Data from Forbes et al.(1982), and Bowie Seamount data from Herzer,(1971), Engel & Engel (1964). 57. source for the JTW Knolls i s not very big, and that the spreading segments are too f a r away for mixing to occur. Iceland, the Galapagos Islands, and the Azores a l l r i s e above sea l e v e l , and must have higher magma production rates. A l l l i e c l o s e r to or on the ridge segment whose chemistry they influence. An- other factor to consider i s the eff e c t of f r a c t u r e zones on magma movement. These f a u l t s truncate ridge segments and form an e f f e c t i v e b a r r i e r to magmas migrating along the ridge magma chamber. This i s exemplified at the t i p of a propagating r i f t , which i s separated from the dying r i f t by a transform f a u l t . Sharp changes i n basalt chemistry are seen from one end of the f a u l t to the other ( S c h i l l i n g et a l . , 1976). Thus, due to the smaller rate of magma pro- duction, young age, the 60 km separation, and poor magma conductivity along transform f a u l t s , magma from JTW Knolls i s not observed at the Dellwood Kn o l l s . In terms of major and trace element chemistry, the JTW hawaiites c l o s e l y resemble the "average" composition of ocean island basalts (Engel et a l . , 1965) as shown i n Tables 4 and 5. As wel l , the JTW basalts compare c l o s e l y with the major element analyses from seamounts i n the Pratt-Welker chain, such as Bowie, Kodiak, Giacomini and Hodgkins (Herzer, 1971; Forbes et a l . , 1969; Forbes and Hoskin, -1969; Engel and Engel, 1964), data from which are included i n Table 5. However, there are minor differences between the JTW Knolls and the other sea- mounts, i l l u s t r a t e d i n Figure 24. The JTW a l k a l i basalts are enriched i n s i l - i c a and t o t a l a l k a l i e s with respect to the Pratt-Welker a l k a l i basalts, as i s evident i n the s i l i c a v a r i a t i o n diagram. JTW hawaiites also have lower FeO1-/ MgO r a t i o s , suggesting that the enrichment i s not simply the r e s u l t of c r y s t a l f r a c t i o n a t i o n . The La/Sm ef r a t i o s are also d i f f e r e n t . Ratios f o r the P r a t t - Welker Chain seamounts range from 1.3 ( t r a n s i t i o n a l basalt) to 3.5 (trachyte), although most are between 1.8 and 2.4 (Forbes et a l . , 1982). The values are consistent throughout the chain. The JTW basalts exhibit a higher degree of LREE enrichment, with La/Sm ef r a t i o s of 4.5 to 5.1 . Another d i f f e r e n c e between the JTW Knolls and the Pratt-Welker seamounts 58. i s physiography. Except for Kodiak and JTW, a l l the seamounts are guyots that extended above sea l e v e l at some time i n t h e i r h i s t o r y . The JTW Knolls have only 400-500 meters of r e l i e f above the t h i c k sedimentary p i l e they are pene- t r a t i n g . ;• . . .' Chase (1977) calculated that the c o l a t i t u d e of the JTW Knolls, r e l a t i v e . to the Pacific-Hotspot pole of r o t a t i o n , l i e s within the range of c o l a t i t u d e s for the Pratt-Welker seamounts. The JTW area i s connected to the seamount chain by a broad topographic r i s e , the Oshawa Rise, which S i l v e r et a l . (.1974) a t t r i b - uted to the passage of the P a c i f i c Plate over a mantle plume. From t h i s , and the s i m i l a r chemistries, Chase concluded that the JTW Knolls represented the present l o c a t i o n of the Pratt-Welker mantle plume. The rate of r o t a t i o n of the P a c i f i c Plate about the PCFC-HSPT pole, 0.83°/Ma, of Minster et al.(1974) was then used to estimate the ages of the Pratt-Welker seamounts, assuming a zero age for JTW, as shown i n Table 6. The K-Ar dates from Kodiak, nsDP Hole 178, and Giacomini Seamount match the estimated ages w e l l . The southeastern sea- mounts a l l have s i g n i f i c a n t l y younger K-Ar dates than Chase's estimates, however. A recent geochronological and bathymetric study of the Pratt-Welker Chain disputes the idea that the mantle plume that created the chain presently l i e s near the JTW Knolls (Turner et a l . , 1980). Bowie Seamount has a magnetic age of 0.72 Ma or l e s s , and tephra from a small, late-stage pinnacle y i e l d s a whole-rock age of 75,000 ±100,00.0 yrs. Morphological considerations suggest that the l a t e s t stage of volcanism on Bowie must have occurred within the l a s t 18,000. y r s . Turner et a l . (1980.) therefore conclude that the mantle plume presently s i t s only 40-130 km southeast of Bowie Seamount (Figure 23). Yet sample 73-26-2-1C from the JTW k n o l l s has a K-Ar age of ~54,000 yrs, and has a d i s t i n c t ocean is l a n d chemical a f f i n i t y . Consequently, we must explain two seamounts of s i m i l a r age and chemistry that are 300 km apart. The existence of a second hotspot, l y i n g on the same co l a t i t u d e as the TABLE 6' Posi t i o n and Age Data for the Pratt-Welker Seamounts* J.T. Wilson Bowie Hodgekins Dickens Giacomini DSDP 178 Kodiak Co-latitude about PCFC-HSPT Pole (deg): '" 37.1 37.4 37.4 36.9 38.8 38.6 39.4 Angular Distance from JTW about PCFC-HSPT Pole (deg) 0 5.5 6.1 7.8 16.6 17.6 18.9 Calculated Age (Myr) 0 6.7 7.4 9.4 20.0 21.2 22.8 K/Ar Age (Myr) <0.1 0.1±0.1 2.65±0.2 3.7±0.2 19.9±1.0 22^23 22.6±1.1 13.2±2.0t F i s s i o n Track Age (Myr) - - - 4.2±1.4t 19.3±3.8 - 25.3+4.3 19.8±1.9t 21.6±2.2t 30.1±2.2t Age of Underlying Crust from Magnetic Anomaly I d e n t i f i c a t i o n <10 18 19 20 46 47 50 * - Table taken from Chase (1977). t - Data from Turner et a l . (1980). vo 60. f i r s t , but 300 km southeast of i t , more LREE-enriched and le s s p r o l i f i c i n magma generation, could explain the differences between the JTW and P r a t t - Welker seamounts. Turner et a l . (1980) suggest that the southeastern seamounts have experienced two phases of volcanism, one near-ridge, and one plume. As shown i n Figure 25, the near-ridge phase from Denson, Davidson, and Hodgekins seamounts have K-Ar ages approximately 4 Ma younger than the crust they are penetrating. The JTW Knolls are presently intruding crust of 4.6 to 5.4 Ma i n age, and thus could represent a hotspot responsible for the near-ridge phase. It must be noted, however, that a progression l i n e drawn through the JTW and '.'near-ridge'' seamount basalts i n Figure 25 would have a steeper slope, and smaller r o t a t i o n rate, than the progression l i n e for the a l k a l i b a s a l t s . This would imply that the proposed JTW plume i s not f i x e d , and i s moving i n a di-, r e c t i o n s i m i l a r to the r o t a t i o n of the P a c i f i c Plate about the PCFC-HSPT pole of r o t a t i o n . The near-ridge phase basalts from Denson, Davidson, and Hodgkins Seamounts are chemically d i f f e r e n t from JTW bas a l t s . The former are t r a n s i t i o n a l be- tween a l k a l i basalt and t h o l e i i t e (K20>.25%, La/Smef=1.3, Type 2 i n Figure 24). The l a t t e r are hawaiites (Type 4, Figure 24). This probably r e f l e c t s the normal ocean i s l a n d chemical c y c l e . Many ocean islands are l a r g e l y made up of t r a n s i t i o n a l to t h o l e i i t i c basalts (e.g. Hawaii), which i s the dominant magma type through most of the ac t i v e l i f e i f a seamount. A l k a l i basalts are t y p i c a l of the l a t e stage of a c t i v i t y , and vo l u m e t r i c a l l y form only a small part of an ocean i s l a n d . It could be argued that a second hotspot i s not necessary to explain the simultaneous volcanism of the two seamounts, Bowie and JTW, 300 km apart, c i t i n g the Hawaiian hotspot as an example. Coeval t h o l e i i t i c volcanism has .occurred on Waianae and Niihau, and Nihoa i s not much older than Kauai. Thus, the length of crust affected by the plume at any one time i s i n the order of 200-400 km (Dalrymple et a l . , 1973). However, the average rate of hotspot pro- Age of Underlying Crust I GEOCHRONOLOGY OF Distance from Kodiak Seamount (km) Figure 25: K-Ar and fission track ages of the Pratt-Welker seamounts plotted against distance from Kodiak Seamount. Solid line indicates trend of alkali basalts. Dashed line indicates age of oceanic crust. From Turner et al.(1980). Long dashed line is age progression of chain from Chase (1977, Table 6). Transitional basalts marked by solid triangles. * — ON I—' 62. gression on the Hawaiian chain i s 12.5 cm/yr (Dalrymple et a l . , 1973), which i s much faster than the 4.4 cm/yr rate calculated f or the Pratt-Welker Chain (Turner et a l . , 1980). It would be expected, therefore, that the length of crust effected by the hotspot at one time would be longer i n the Hawaiian chain than i n the Pratt-Welker chain. Thus, the existence of a second plume better explains the data. Other theories, besides the plume or hotspot, have been used to explain seamount chains, such as the propagating crack (Turcotte and Oxburgh, 1973), and the l o n g i t u d i n a l r o l l (Richter, 1973). Neither hypothesis requires that there be an age progression along the chain, nor do they explain the chemical cycle exhibited by the seamounts. Geophysical evidence suggests that the JTW Knolls are the topographic ex- pression of a new spreading segment, i n i t i a t e d l e s s than 1 Ma BP=(Hyndman et all, 1978"; Riddihough et a l . , 1980; R. Hyndman, personal communication). Ocean- bottom seismometers have recorded seismic a c t i v i t y along a proposed transform f a u l t between the Dellwood and JTW Knolls, l y i n g p a r a l l e l to, and 25 km southwest of, the continental margin (Figure 26). As w e l l , seismic r e f l e c - t i o n p r o f i l e s from the NW Dellwood Knoll to the continental margin show a 20 to 25 km gap between the fault-truncated Knoll and the fault-bounded c o n t i - nental slope (Figure 27). CSP P r o f i l e s from the JTW area reveal that the pre- e x i s t i n g sedimentary p i l e has been u p l i f t e d and pushed 20-25 km away from the new spreading center. Heat flow values from the surrounding sediments range from 4.7 to 8.3 peal em'^sec"^, which are t y p i c a l of other spreading centers. Magnetic data i s not very good close to the continental slope due to thermal blanketing by t h i c k sediments, but a broad, elongate p o s i t i v e anomaly i s present over the proposed spreading center (Figure 28). The Knolls are intruding crust between 4.5 and 5.3 Ma i n age, as indicated by extrapolated magnetic anomalies (Riddihough et a l . , 1980). The proposed rate of spreading i s 5.5 cm/yr, which i s the measured rate of r e l a t i v e motion along the Queen 63. Figure 26: Microseismicity of the Explorer area from three ten-day ocean bottom seismometer surveys. Filled triangles mark OBS sites. Epicentral uncertainty approximately 10 km. From Keen and Hyndman (1979). Figure 27: Southwest-northeast CSP profile across the continental rise from the NW Dellwood Knoll. Heavy line is oceanic basement, light lines are sedimentary reflectors, dashed lines are inferred faults (Bertrand, 1972). 0> Figure 28: Magnetic anomaly map of JTW and Dellwood area. From Currie, R.G., and Seeman, D, 1980, Marine magnetic anomaly map-west coast of British Columbia, GSC Open File 724 (revised). 66. Charlotte transform f a u l t . It i s p o s s i b l e that the JTW microplate, along with the Dellwood segment and the Winona Basin, are now locked to the American Plate, and are moving with i t (Davis and Riddihough, 1982). Bathymetrically, the JTW Knolls do not have the t y p i c a l c o n i c a l shape of seamounts (Can. Hydrographic Service Map 19410-A, 2nd Ed., 1980), which i s associated with most ocean i s l a n d s . Instead, the southwest Knoll i s more t r i - angular i n shape, with a steep, l i n e a r northwest face, while the northeast Knoll i s elongated i n a northeast-southwest d i r e c t i o n . The d i r e c t i o n of .'.; elongation of the seamounts i s p a r a l l e l to the proposed spreading a x i s . However, chemically there i s no evidence of t y p i c a l ocean ridge volcanism at the JTW Knolls. . The a l k a l i basalts are d i s t i n c t l y non-MORB, and are too extremely a l k a l i n e to be the product of mixing of a plume magma with MORB, as i s seen on the Reykjanes Ridge. It i s l i k e l y that spreading has "jumped" to the JTW area because the plume created a weak spot i n the crust, and r e a d i l y tapped the magma source. The proposed ridge segment i s only 25-30 km long, much smaller than the presumed siz e of the plume i t s e l f (Figure 23). This s i t - uation may be si m i l a r to that of Iceland, where the plume on the ridge pro- duces a l l the magma necessary f o r spreading, and no t y p i c a l MORB i s produced. Other newly i n i t i a t e d spreading centers i n s i m i l a r tectonic environments, such as the Gulf of C a l i f o r n i a and the Gulf of Aden, are not s i m i l a r chemical- l y to the JTW Knolls ( T e r r e l l et a l . , 1979; Barberi and Varet, 1977; Barberi et al . , 1980). Both areas of spreading are characterized by a l k a l i - r i c h t h o l e i i t e K20= 0.3-0.6%), but also exhibit LREE depletion (La/Sm e f= 0.60-0.75). J.-G. S c h i l l i n g (personal communication) reports a fresh o l i v i n e t h o l e i i t e dredged from a s i n g l e volcanic cone on the spreading r i d g e i n the Gulf of Tadjura (Afar area) with a La/Sm ef r a t i o of 4.5. However, t h i s cone i s anomalous, and other t h o l e i i t e s from the ridge exhibit LREE depletion. S c h i l l i n g a t t r i b u t e s the anomalous enrichment of the cone basalt to a smaller degree of p a r t i a l melting, since i t has a s i m i l a r ^ S r / ^ S r r a t i o to the rest of the ridge. 67. Thus, due to the l a c k of s i m i l a r analyses from other spreading ridges, i t seems u n l i k e l y that the JTW Knolls are only the r e s u l t of a spreading ridge. The chemical evidence points to a mantle plume o r i g i n for the JTW basalts, and i t i s l i k e l y that these two tectonic features are simultaneously i n f l u e n c i n g the morphology of the JTW area. Speculatively, there i s a l s o the p o s s i b i l i t y of contamination of JTW magmas i f they come i n contact with the continental crust at depth. No seismic r e - f r a c t i o n studies have been done i n the area to a s c e r t a i n the fashion i n which the continental crust extends beyond the Queen Charlotte Fault, or even i f i t does. At present, no geophysical or petrographic evidence exists to suggest that such contamination i s occurring, such as xenoliths of continental mat- e r i a l . 68. CONCLUSIONS The f i v e ridge segments of the Explorer spreading area, from the Southern Explorer Ridge north to the Dellwood Knolls and the J. Tuzo Wilson Knolls, show s i g n i f i c a n t v a r i a t i o n s i n basalt chemistry which cannot be explained by c r y s t a l f r a c t i o n a t i o n alone. Explorer Deep basalts are enriched i n a l l the incompatible t r a c e elements, such as K, Rb, Zr, Nb, and the LREE, which may r e f l e c t the presence of a weak hotspot beneath Explorer Deep. The nearby seg- ments, Explorer R i f t and the Southern Explorer Ridge, are erupting both incom- p a t i b l e element enriched andadepleted basalts, which could r e s u l t from a het- erogeneous mantle source, or from intermittent i n j e c t i o n of magma from the postulated hotspot beneath Explorer Deep into areas producing normal MORB. Sr isotope data d o n o t ind i c a t e that two ra d i o g e n i c a l l y d i s t i n c t mantle sources e x i s t (one hotspot, one t y p i c a l ocean r i d g e ) . The Dellwood Knolls d i s p l a y a considerable chemical d i f f e r e n c e between the two k n o l l s , which has previously been a t t r i b u t e d to f r a c t i o n a t i o n . However, new rare earth element and Sr isotope data suggest that the hi s t o r y of the Knolls i s more complex,:: and that the southeast Knoll had a more radiogenic and trace-element-enriched mantle source than does the presently a c t i v e northwest K n o l l . The three Explorer segments produce basalts with r e l a t i v e l y high iron contents (Fe203*= 11 to 14%), which are c l a s s i f i e d as fe r r o b a s a l t s . This basalt type occurs i n areas with high amplitude magnetic anomalies. The mag- neti c traces of "pseudofaults" i n the Explorer area, and the occurrance of high Fe-Ti basalts at the ends of the ridge, suggest that propagating r i f t s , s i m i l a r to those c u r r e n t l y a c t i v e on the Juan de Fuca and Galapagos Ridges, e x i s t along the Explorer system. The J. Tuzo Wilson Knolls were thought to be the present-day expression of the Pratt-Welker plume. In terms of major elements, trace elements, and rare earth element patterns, the Knolls are chemically s i m i l a r to other sea- 69. mounts i n the chain. The JTW basalts are even more large ion l i t h o p h i l e e l e - ment -'enriched. Geochronology disputes the hypothesis that the plume responsi- ble for the l a t e s t stage of volcanism on Bowie Seamount i s also the source of the JTW basalts. The existence of a second mantle plume, 300 km southeast of Bowie Seamount at the JTW Knolls, would explain the minor chemical and phys- io g r a p h i c a l d i f f e r e n c e s between the JTW Knolls and the other Pratt-Welker Seamounts, as well as the observed two-phase volcanic h i s t o r y of the southeast- ern part of the chain. Recent geophysical evidence suggests that the JTW Knolls are the newest, most northerly segment of the Explorer-Dellwood system. Although JTW hawaiites are a t y p i c a l for ocean ridge magmas, the s i t u a t i o n appears to be s i m i l a r to other ocean ridges where a ridge l i e s on a hotspot. 70. BIBLIOGRAPHY Abbey, S., 1980, Studies i n "standard samples" for use i n the general anal- s i s of s i l i c a t e rocks and minerals, Geological Survey of Canada Paper 80-14, 26 p. 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Tuzo Wilson Knolls 73-26-2-1 73-26-5-1 „ 7 3 „ glassy pillow glassy p i l l o w glassy p i l l o w i n t e r s e r t a l h yalophitic Dellwood Knolls and Seamounts 70-25-2D-8 glassy pillow hyalophitic 70-25-3D-1 glassy pillow h y a l o p i l i t i c 70-25-8D-121 glassy p i l l o w h y a l o p i l i t i c 70-25-9D-1 glassy pillow h y a l o p i l i t i c Explorer R i f t 71-15-77-5 71-15-77-6 glassy p i l l o w glassy p i l l o w hyalophitic h y a l o p i l i t i c 70-25-4-49 glassy p i l l o w h y a l o p i l i t i c 70-25-4-62/64 glassy lava tubes h y a l o p i l i t i c 70-25-16-1 glassy p i l l o w hyalophitic MATRIX ALTERATION- REMARKS glass & f-s none v e s i c , op common glass & f - s none v e s i c . none glomeroporphyritic glass, f - s , op, p i micro. glass, f - s , p i micro. glass, f - s , p i micro. glass, f - s , p i micro. 01 rims v e s i c , glomero. high Fe3 +/Fe2 + v e s i c , Mn crust minor none oxide crust magnetic f - s , plag micro none f- s , op, plag micro. glass, f - s , plag micro. glass, f - s , plag micro f-s none none none fr a c s Fe- stained glomero, s k e l . o l s k e l . o l . sk e l . phenos, glomero. SAMPLE NO, FRAGMENT TYPE TEXTURE PHENOCRYSTS 70-25-16-4 70-25-16-7 77-14-33A 77-14-33B block lava block lava i n t e r s e r t a l glassy p i l l o w hyalophitic glassy p i l l o w i n t e r s e r t a l Plag, 01, t r . intergranular Flag, Fx, 01 Plag, 01 Plag, 01 Paul Revere Ridge Fracture Zone 70-25-11-2 block lava h y a l o p i l i t i c 71-15-92-8 71-15-92-10 71- 15-91-1 72- 22-7-1 70-25-6-14 block lava block lava hyalophitic glassy p i l l o w hyalophitic hyalophitic none Plag, Px Plag, 01 block lava h o l o c r y s t a l l i n e glassy p i l l o w h y a l o p i l i t i c 01, Plag Plag, 01 Explorer Deep 70-25-17-1 block lava 79-6-32-39 79-6-32-41 79-6-32-42 glassy p i l l o w glassy p i l l o w glassy pillow i n t e r s e r t a l , 01. hyalophitic areas hyalophitic 01. hyalo p i l i t i c 01. h y a l o p i l i t i c 01, MATRIX ALTERATION REMARKS f - s , op. none v e s i c , magnetic, s k e l . o l . f - s , Px, plag 3mm Fe-stained micro rim f- s , plag micro none s k e l . o l . , p l a g resorbed on rims f - s , op, m. granular Px 01 frac s s k e l . phenos. f - s , minor plag micro ; f - s , op., plag micro glass, f - s , op. weathered rim weathered rim none skel phenos, 01 rims and f r a c s . glass, m. f-s none glass, f - s , plag micro 01 serp. s k e l . o l . ; dark and magnetic plag micros, none glomero. f - s , glass plag micro, f-s none glass, f-s none glomero, glass, f - s , none glomero, plag micro SAMPLE NO. FRAGMENT TYPE TEXTURE Southern Explorer Ridge 77-14-36-X block lava hyalophitic 77-14-36-G block lava hyalophitic 77-14-36-8 glassy p i l l o w hyalophitic 77-14-36-13 77-14-36-32 block lava block lava 77-14-36-35 block lava 77_14_36-36 block lava 73-26-13-3 block lava h y a l o p i l i t i c h y a l o p i l i t i c intergranular- h o l o c r y s t a l l i n e f a n - s p h e r u l i t i c i n t e r s e r t a l 73-26-13-4 block lava i n t e r s e r t a l 70-25-15-3 glassy p i l l o w i n t e r s e r t a l 70-25-15-8 glassy p i l l o w hyalophitic 70- 25-15-29 glassy p i l l o w hyalophitic to i n t e r s e r t a l 71- 15-70-8 block lava h y a l o p i l i t i c PHENOCRYSTS MATRIX ALTERATION REMARKS 01, Plag 01, Plag, r.Px. Plag, 01 Plag, 01, Px Plag, 01 f- s , plag none micro f - s , glass, none plag micro f-s none glass, f-s none glass, f - s none minor glass none s k e l . o l ; v e s i c ; glomero. skel o l ; glomero, v e s i c ; m i c r o l i t e s r a d i a l c l u s t e r s glomero; skel o l . v e s i c ; s k e l o l ; glomero. large o l . crystals none none none Plag, 01 r. Plag 01. opaques none plag micro, none glass, Px plag micro, none glass, Px plag and o l none micro, f - s , glass glass, f - s , op. none plag micro, none f - s , glass f - s , plag weathered micro. rim phenos small APPENDIX 2 Major element, trace element, precisions, and CIPW norm- a t i v e compositions for the J . Tuzo Wilson Knolls, D e l l - wood Knolls and Seamounts, and the Explorer Spreading Area. (i) Total i r o n i s reported as Fe20 3 . ( i i ) Normative compositions c a l c u l a t e d assuming an F e 3 + / ( F e 3 + + F e 2 + ) r a t i o of 0.16 . The magnesium r a t i o i s c alculated on the same basis. ( i i i ) Volcanic rock c l a s s i f i c a t i o n (Class.) i s a f t e r the method of Irvine & Baragar, (1971). (iv) A blank indicates that no data was c o l l e c t e d . A dash indicates a value.of zero. w*-* • j . Tuzo Wilson Knolls Seamount 67-6-12t 73-26-2-1A 73-26-2-1B 73-26-2-1C 73-26-5-1A S i 0 2 45.4 48.41 48.75 49.66 51.24 T i 0 2 2.60 2.51 2.40 2.43 1.78 AI2O3 18.50 15.22 15.87 16.03 16.08 Fe20j* 11.90 9.77 9.28 9.29 7.83 MnO 0.19 0.17 0.16 0.16 0.19 MgO 5.90 6.04 5.06 5.17 6.82 CaO 8.5 8.18 8.00 8.17 8.76 Na 20 3.8 4.87 5.18 5.02 4.99 K 20 1.9 2.22 '2.46 2.41 2.08 P 2 0 5 0.70 0.67 0.66 0.71 0.62 H 20+ 0.30 1.37 0.93 0.31 0.13 C0 2 etc. 0.1 1.13 .1.84 1.12 Ba 361 Ce 172 Cr 335 25 Nb 86 87 87 74 Nd 60 Ni 86 52 ' 38 33 101 Rb 41 55 36 . 38 48 Sr 679 593 590 599 567 V 232 Y 32 30 31 31 27 Zr 338 376 396 372 410 La/Sm ef 4.61 4.59 Zr/Nb 4.4 4.8 4.3 5.5 S r 8 7 / S r 8 6 .70272 " .70255 .70258 • .70250 Qtz - - - - - Ne 7.38 7.31 6.86 6.57 8.42 Or 11.12 13.35 14.71 14.34 13.33 Ab 18.86 29.00 31.96 30.85 26.98 An 28.08 13.21 12.79 14.07 15.29 Di 8.26 . 13.31 9.18 12.19 19.57 Hy - - - - - 01 17.59 13.03 12.27 11.29 11.04 Ilm 5.02 4.85 4.61 4.65 3.39 Mag 1.62 1.93 1.82 1.81 1.53 Ap 1.68 1.58 1.55 1.66 1.44 100(Mg/ Mg+Fe 2 +)52.43 * 57.63 54.54 55.05 65.71 Class. A l k a l i Hawaiite Mugearite Hawaiite Hawaiit Basalt t - Major element analysis by Geological Survey of Canada (Herzer, 1971). 81 J. Tuzo Wilson Knolls 73-26-5-lB 73-26-5-1C V73" Si02 Ti02 AI2O3 Fe 203* MnO MgO CaO Na20 K2O P2O5 H 20+ C0 2 etc 50.12 1.73 16.16 7.62 0.17 6.78 8.57 4.95 2.02 0.58 1.17 0.62 50.46 1.74 16.00 . 7773 0.16 6.55 8.54 4.92 2.03 0.57 1.18 0.51 50.00 1.79 15.62 9.02 0.16 6.37 9.12 4.30 1.78 0.53 1.04 0.80 Ba 274 291 42 Ce 180 156 Cr 105 85 342 177 Nb 75 75 63 4 Nd 68 56 11 Ni 90 91 76 206 66 Rb 48 48 41 4 7 Sr 575 565 576 182 345 V 193 201 212 Y 26 27 30 22 34 Zr 402 378 323 101 164 La/Sm ef 5.13 4.50 0.81 1.49 Zr/Nb 5.4 5.1 5.1 25.2 Sr87/sr 8 6 .70267*** ,.70256 .70240 .70290 .70262: .70279 Qtz - - - - — Ne 7.52 6.84 3.14 0.19 - Or 12.10 12.17 10.66 1.31 3.43 Ab 28.79 29.81 31.28 19.38 27.92 An 16.01 15.69 18.20 35.28 26.07 Di 15.55 16.35 15.46 20.86 25.82 Hy - - - - 8.98 Ol 12.39 11.76 12.94 18.29 0.66 Ilm 3.33 3.35 3.45 2.48 4.43 Mag 1.50 1.52 3.14 1.99 2.03 Ap 1.36 1.34 1.24 0.21 0.65 100 (Mg/ Mg+Fe2+) 66.19 65.09 60.85 67.14 51.25 Class. Hawaiite Hawaiite Hawaiite A l k a l i O l i v i n e Basalt T h o l e i i t e Dellwood Knolls 70-25-2D-8 70-25-3D-1** 47.33 1.30 16.91 10.23 0.16 9.50 12.36 2.29 0.22 0.09 0.80 51.25 2.33 15.60 10.47 0.16 5.00 11.95 3.24 0.58 0.28 0.84 **- Major element analysis by Japan A n a l y t i c a l Research Chemistry I n s t i t u t e (Bertrand, 1972). ***- Leached sample. 70 Dellwood Seamounts 71-15-77-5 Explorer R i f t -25- 4-4 -25--8D-121 70-25- 9D-1 . 71-15--77-6 70 Si02 49. 51 48. 59 48. 43 48. 33 46. 07 T i 0 2 AI2O3 1. 62 1. 64 1. 70 1. 69 1. 07 15. 68 14. 18 13. 32 13. 33 15. 45 Fe 203* 10. 14 11. 80 12. 49 12. 56 12. 25 MnO 0. 18 0. 16 0. 20 0. 20 0. 18 MgO 6. 86 8. 15 9. 02 9. 02 11. 71 CaO 12. 54 11. 94 12 30 12 34 12. 00 Na 20 3. 29 2. 59 2. 57 2 47 1. 93 K 20 0. 27 0. 33 0 21 0 21 0. 07 P 2 0 5 0. 17 0. 16 0 15 0 16 0. 08 H20+ 0. 40 1. 30 0 25 0 40 0. 36 C0 2 etc _ 0 17 0 22 0. 09 Ba 36 37 63 Ce 2 Cr 284 436 Nb 7 6 8 9 3 Nd 8 7 20 Ni 70 73 65 65 265 Rb 6 8 5 5 3 Sr 154 179 146 150 143 V 267 271 Y 29 28 29 29 20 Zr 103 113 106 109 72 La/Sm ef 1 .04 1 .11 1 .13 Zr/Nb 14 .8 18 .8 11 .8 13 .6 24 .0 S r 8 7 / S r 8 6 Qtz Ne 1 .34 0 .14 Or 1 .61 1 .98 1 .25 1 .25 0 .41 Ab 25 .79 22 .46 22 .06 21 .24 16 .33 An 27 .28 26 .41 24 .23 24 .75 33 .26 Di 27 .92 26 .68 28 .32 28 .34 20 .51 Hy 5 .42 3 .97 4 .95 01 10 .57 11 .18 13 .64 13 .03 24 .54 Ilm 3 .10 3 .16 3 .25 3 .23 2 .04 Mag 1 .98 2 .32 2 .43 2 .45 2 .34 Ap 0 .40 0 .38 0 .35 0 .37 0 .19 100 (Mg/ Mg+Fe2+)_ 59 .28 60 .31 61 .38 61 .24 67 .78 Class. A l k a l i O l i v i n e K-poor K-poor Picrit< Basalt T h o l e i i t e " 01. Bas. 01. Bas. Basalt 83. Explorer R i f t 70--25--4-62G 70-25-•4-62W 70-25-•4-64 70-25--4-104 70-25--4-119 S i 0 2 46. .03 46. ,03 46. 16 46. .41 46. 20 T i 0 2 1. .09 1. ,06 1. ,06 1. ,06 1. ,08 AI2O3 15, .19 15. ,65 15. ,62 16. .23 15. .37 Fe 203* 12, .27 12. .23 11. ,99 11. ,88 12. ,33 MnO 0, .19 0. ,19 6. ,19 0. ,18 0. ,19 MgO 12 .20 11, .74 11. ,92 10. ,98 11. ,76 CaO 12, .00 11. ,94 11. .96 11. .92 12. .07 Na 20 1 .87 1, .97 1. ,95 2. .19 1. ,92 K 20 0, .06 0. ,07 0. ,07 0. .12 0. ,07 P 2 0 5 0 .08 0. .09 0. ,08 0. .08 0. ,08 H20+ 0 .25 0. .18 0. ,13 0. .18 0. .12 C0 2 etc 0 .03 0. .14 0. ,12 0. .05 0. .08 Ba 8 Ce Cr 500 Nb 4 3 3 3 3 Nd 6 Ni 270 275 265 248 261 Rb 3 3 3 3 3 Sr 144 146 149 142 145 V 183 Y 20 13 19 20 20 Zr 70 73 74 72 72 La/Sm ef 0 .59 0 .61 Zr/Nb 17 .5 24 .3 24 .7 24 .0 24 .0 S r 8 7 / S r 8 6 .70232 Qtz — Ne 0 .25 0 .28 0 .17 1 .06 0 .12 Or 0 .36 0 .41 0 .41 0 .71 0 .41 Ab 15 .59 16 .37 16 .39 16 .83 16 .23 An 32 .82 33 .55 33 .55 33 .98 33 .01 Di 21 .10 19 .62 19 .82 19 .71 20 .93 Hy - 01 25 .18 24 .84 24 .85 23 .10 24 .48 Ilm 2 .07 2 .01 2 .01 2 .01 2 .05 Mag 2 .38 2 .37 2 .32 1 .06 2 .38 Ap 0 .19 0 .21 0 .19 0 .19 0 .19 100 (Mg/ Mg+Fe 2 +) 68 .63 67 .87 68 .63 67 .07 67 .73 Class. P i c r i t e Basalt P i c r i t e Basalt P i c r i t e Ankaramite P i c r i t e Basalt Basalt Explorer R i f t 70-25-16-1 70-25-16-4 70-25-16-7 77-14-33-A 77-14-33-B S102 46.85 48.79 T i 0 2 1.31 1.79 AI2O3 14.70 15.29 Fe203* 11.53 10.89 MnO 0.22 0.18 MgO 11.69 7.88 CaO 11.95 11.82 Na 20 2.02 3.46 K 20 0.18 0.28 P2O5 0.11 0.21 H 20+ - 0.47 0.12 C0 2 etc 0.09 0.16 Ba Ce Cr 374 Nb 7 8 Nd Ni 275 84 Rb 4 6 Sr 160 210 V 257 Y 21 23 Zr 95 114 La/Smef 0.81 1.19 Zr/Nb 13.6 14.3 S r 8 7 / S r 8 6 Qtz Ne - 2.43 Or 1.07 1.66 Ab 17.36 25.14 An 30.54 25.29 Di 22.38 25.31 Hy 1.71 01 21.74 13.79 Ilm 2.50 3.41 Mag 2.24 2.11 Ap 0.26 0.49 100 (Mg/ Mg+Fe 2 +) 69.05 61.42 Class. Average A l k a l i T h o l e i i t e Basalt 48.73 47.99 47.87 1.82 1.26 1.28 15.06 15.18 15.32 10.92 10.65 10.46 0.18 0.18 0.17 7.89 10.17 10.06 11.92 12.91 13.13 3.52 1.79 1.88 0.32 0.27 0.27 0.21 0.13 0.14 0.11 0.21 0.27 0.19 0.30 0.22 44 575 8 11 12 12 72 204 150 6 8 6 212 150 153 229 26 22 20 121 84 83 1.69 1.73 15.1 7.6 6.9 .70246 3.09 1.90 1.54 1.60 24.42 15.33 16.11 24.27 32.59 32.53 26.39 23.31 24.58 10.00 6.24 13.42 11.78 13.63 3.47 2.40 2.44 2.17 2.07 2.03 0.49 0.30 0.32 61.39 67.76 67.91 A l k a l i K-rich K-rich Basalt T h o l e i i t e T h o l e i i t e 85. Paul Revere Ridge (F.Z.) 70-25-11-2 71-15-92-8W 71-15-92-8G 71-15-92-10 71-15-91-iy Si02 46.70 48.43 T i 0 2 1.28 1.95 AI2O3 16.37 13.94 Fe 203* 11.21 13.00 MnO 0.15 0.22 MgO 9.12 7.69 CaO 10.68 11.61 Na 20 2.41 3.10 K 20 0.18 0.26 P2O5 0.10 0.21 H20+ 0.77 0.13 C0 2 etc 1.13 0.33 Ba Ce Cr ' Nb 5 11 Nd Nl 275 94 Rb 4 5 Sr 138 152 V Y 24 36 Zr 81 136 La/Sm ef 0.60 Zr/Nb 16.2 12.4 S r 8 7 / S r 8 6 Qtz Ne - - Or 1.07 1.54 Ab 20.77 26.60 An 33.43 23.37 Di 4.64 25.42 Hy 21.9.2 0.92 01 2.45 3.72 Ilm 2.45 3.72 Mag 2.19 2.52 Ap 0.23 0.49 100 (Mg/ Mg+Fe 2 +) 64.16 56.55 Class. K-poor A l k a l i T h o l e i i t e Basalt 48.28 48.43 45.43 1.94 1.78 0.86 13.28 13.28 14.38 13.25 12.83 12.96 0.22 0.22 0.24 8.12 8.73 14,34 11.59 12.17 8.74 2.88 2.30 1.54 0.27 0.21 0.09 0.20 0.18 0.10 0.43 0.41 0.49 0.41 0.35 2.03 ' '25 11 336 13 11 20 100 97 6 5 2 143 135 100 296 35 32 132 125 1.00 0.59 10.2 11.4 1.61 1.25 0.48 24.79 19.75 17.63 22.58 25.39 25.76 25.75 26.13 4.57 l i : 9 4 17.24 3.71 8.37 27.78 3.71 3.41 1.39 2.58 2.50 3.22 0.47 0.42 0.16 57.42 59.97 75.20 K-poor K-poor P i e r i t e T h o l e i i t e T h o l e i i t e Basalt Weathered sample. Not included i n diagrams i n text. Paul Revere Ridge 72-22-7-1 70-25-6-14 Explorer Deep 70-25-17-1 79-6-32-39 79-6-32-41 S i 0 2 48.24 50.09 T i 0 2 3.53 . 1.42 A1 20 3 10.69 16.78 Fe 203* 19.33 8.80 MnO 0.30 0.13 MgO 5.44 7.10 CaO 8.38 13.78 Na 20 2.79 2.58 K 20 0.53 0.10 P 2 0 5 H20+ 0.39 1.10 0.16 C0 2 etc 0.37 Ba 21 Ce 76 Cr 171 267 Nb 19 Nd 36 Ni 147 347 Rb 10 3 Sr 133 153 V 400 Y 65 38 Zr 280 90 La/Sm ef 1.18 Zr/Nb 14.7 S r 8 7 / S r 8 6 .70254 Qtz 1.60 — Ne - -Or 3.19 0.59 Ab 24.35 22.10 An 15.24 33.75 Di 18.59 27.15 Hy 24.59 6.06 01 - 5.67 Ilm 6.84 2.69 Mag 3.78 1.71 Ap 0.92 0.37 100 (Mg/ M g + F e 2 + ) 38.24 63.97 49.32 49.11 48.64 •1.76 1.75 1.78 15.10 13.44 12.99 11.51 12.27 12.23 0.19 0.20 0.22 7.81 8.95 10.05" 11.43 11.29 11.50 2.44 2.23 1.96 0.45 0.43 0.40 0.21 0.20 0.20 0.50 0.42 0.63 0.08 0.46 0.20 106 102 17 14 334 393 15 20 18 20 22 163 136 145 10 9 9 161 155 163 274 283 32 26 28 144 137 124 1.54 2.03 9.6 6.9 6.5 .70252 2.68 2.56 2.39 20.99 19.17 16.89 29.06 25.53 25.65 21.37 21.74 23.83 13.96 19.05 17.24 5.66 4.66 . 4.57 3.37 3.35 3.41 2.25 2.39 2.39 0.49 0.47 0.47 59.89 61.61 64.39 Class. K-poor K-poor T h o l e i i t e T h o l e i i t e Average T h o l e i i t e Average T h o l e i i t e K - r i c h T h o l e i i t e 87, S i 0 2 TiU2 Al 203 Fe20 3 A MnO MgO CaO Na2<3 K 20 P2O5 H 20 + C0 2 etc Explorer Deep 79-6-32-42 48.68 1.79 12.94 12.41 0.21 . 9.85 11.55 2.03 0.40 0.20 0.49 0.24 Ba Ce Cr Nb Nd Ni Rb Sr V Y Zr La/Sm ej Zr/Nb S r 8 7 / S r 8 6 Qtz Ne Or Ab An Di Hy 01 Ilm Mag Ap 100 (Mg/ Mg+Fe 2 +) Class. 19 165 11 163 30 126 6.6 2.38 17.47 25.16 24.20 16.47 7.44 3.43 2.42 0.47 63.59 K-ri c h T h o l e i i t e Southern Explorer Ridge 77-14-36-X 77-14-36-G 77-14-36- 48.72 1.62 13.57 12.71 0.21 8.87 12.12 2.26 0.19 0.14 0.22 0.22 78 5 122 26 100 0.96 14.3 ,70254 1.13 19.40 26.39 25.92 12.30 8.45 3.09 2.47 0.33 60.56 K-poor T h o l e i i t e 48.66 1.63 13.25 12.67 0.22 9.40 12.04 2.19 0.17 0.13 0.22 0.27 84 4 123 28 100 14.4 1.01 18.80 25.89 25.75 13.37 5.20 3.11 2.46 0.30 62.01 K-poor T h o l e i i t e 49.12 1.58 13.57 12.81 0.20 8.63 11.93 2.15 0.21 0.14 0.26 0.21 28 4 285 8 14 75 5 118 281 27 99 0.92 14.9 1.25 18.47 26.85 24.89 17. 4. 3. 2. 32 89 02 49 0.33 59.72 Average T h o l e i i t e 77-14-36-13 48.89 1.58 13.32 13.00 0.21 8.68 11.88 2.14 0.23 0.13 0.32 0.44 51 329 8 14 80 5 120 278 28 105 13.1 1.37 18.40 26.17 24.14 18.35 4.70 3.02 2.53 0.30 59.50 Average T h o l e i i t e 88. Southern Explorer Ridge 77-14-36-32 77-14-36-35 77-14-36-36 73-26-13-3 73-26-13-4 S i 0 2 48 .41 49.32 T i 0 2 1 .61 1.42 AI2O3 13 .06 15.28 Fe2C>3* 13 .17 10.78 MnO 0 .23 0.18 MgO 9 .27 8.44 CaO 11 .97 12.39 Na 20 2 .06 2.45 K 20 0 .21 0.24 P 205 H 20* 0 0 .13 .48 0.16 0.15 C0 2 etc 0 .27 0.06 Ba Ce Cr Nb 8 3 Nd Ni 79 112 Rb 6 6 Sr 123 121 V Y 29 22 Zr 101 86 La/Sm ef Zr/Nb 12 .6 28.7 S r 8 7 / S r 8 6 48.73 49.27 49.02 1.70 1.56 1.52 14.25 14.28 14.34 11.98 12.71 12.12 0.19 0.21 0.20 9.58 7.03 7.50 10.84 11.80 12.16 2.47 2.58 2.58 0.18 0.42 0.35 0.20 0.18 0.16 0.47 0.64 0.56 0.23 0.14 0.33 8 4 .213 10 6 9 15 251 78 107 4 11 7 127 110 110 275 33 28 27 118 104 100 0.86 11.8 17.3 11.1 Qtz Ne - - Or 1.25 1.42 Ab 17.74 20.98 An 25.9.1 29.97 Di 25.63 24.58 Hy 15.00 8.48 01 4.61 9.25 Ilm 3.08 2.70 Mag 2.57 2.09 Ap 0.30 0.37 100 (Mg/ Mg+Fe 2 +) 60.77 63.27 Class. Average Average T h o l e i i t e T h o l e i i t e 1.07 2.51 2.09 21.24 22.24 22.21 27.38 26.32 26.64 19.51 25.27 25.43 14.57 11.02 9.25 5.99 6.41 7.97 3.25 2.99 2.91 2.34 2.48 2.36 0.47 0.42 0.36 63.76 54.90 57.66 K-poor Average Average T h o l e i i t e T h o l e i i t e T h o l e i i t e Southern Explorer Ridge 70-25-15-3 70-25-15-8 70-25-15-29 71-15-70-1 71-15-70-8 S i 0 2 49. .06 49. ,24 48. ,95 48. ,07 48, ,23 Ti02 1, .77 1. .80 1. ,79 1. ,98 2, .11 Al 203 13. .35 13. .54 13. .62 13. .99 13, .32 F e 2 0 3 * 12, .14 12, .39 12. .12 13. ,30 13, .47 MnO 0. .19 0. .20 . 0. ,20 0. ,21 0, .22 MgO 8, .33 8. .30 8. ,13 8. ,38 9, .27 CaO 11, .81 11, .70 11. ,94 10. .44 10, .61 Na 20 2, .58 2, .33 2. .64 2. .55 2 .36 K 20 0, .38 0. .37 0. ,40 0. .50 0, .30 P 2 0 5 0, .19 0, .20 0. ,19 0. .20 0 .22 H20+ 0, .60 0, .50 0. .46 0, .83 0 .37 C0 2 etc 0 .35 0, .20 0. .35 0, .44 0 .39 Ba 29 34 33 Ce 3 25 Cr 398 393 Nb 12 12 13 8 10 Nd 17 22 Ni 55 90 82 243 218 Rb 8 7 9 11 6 Sr 158 180 162 145 129 V 290 308 Y 29 30 29 39 39 Zr 137 136 141 155 154 La/Smef 1 .35 0 .87 Zr/Nb 11 .4 11 .3 10 .8 19 .4 15 .4 S r 8 7 / S r 8 6 .70249 Qtz Ne Or 2 .27 2 .21 2 .38 2 .99 1 .79 Ab 22 .19 20 .05 22 .71 22 .04 20 .31 An 23 .89 25 .54 24 .23 25 .45 24 .97 Di 25 .93 24 .84 26 .12 18 .67 19 .68 Hy 10 .67 15 .62 .8 .21 13 .08 16 .96 01 ,8 .02 4 .93 9 .30 9 .86 8 .21 Ilm 3 .40 3 .45 3 .43 3 .81 4 .04 Mag 2 .37 2 .42 2 .35 2 .60 2 .62 Ap 0 .44 0 .47 0 .44 0 .47 0 .51 !00 (Mg/ Mg+Fe 2 +) 60 .16 59 .58 59 .61 58 .11 60 .23 Class. Average Average Average Average Averag T h o l e i i t e T h o l e i i t e T h o l e i i t e T h o l e i i t e Th o l e i i i PRECISIONS OF STANDARDS AND UNKNOWNS 90. FOR EACH ELEMENT Percent Mean Deviations From Recommended Values For Standards (Abbey, 1980). Howarth and Thompson 95% Confidence Limits From Pre c i s i o n P l o t s . S i 0 2 1.5% 1% T i 0 2 2.3% 7% A1 20 3 3.4% 3% F e 2 0 3 * 2.9% 3% MnO 2.5% 5% MgO 2.8% 5% CaO 2.4% 2% Na 20 9.7% 10% K 20 2.7% 5% P 2 0 5 15.0% 8% One Standard Deviation, Based On F i t s To Working Curves Ba ±2 ppm 60% Ce ±21 ppm 60% Cr ±17 ppm 5% Nb ±1 ppm 15% Nd ±2 ppm 22% Ni ±5 ppm 15% Rb ±2 ppm 15% Sr ±5 ppm 6% V ±15 ppm 4% Y ±5 ppm 15% Zr ±5 ppm 7% F i n a l Data for Standards/Used i n Construction of Working Curves for Major Element Analysis EXPLORER RIDGE MAJOR ELEMENTS IDENT SiO 2 A1 20 3 F e 2 0 3 :MgO A6V1 60 13 15 18 7 05 1 . 77 60 73 15 33 7 12 1 . 79 59 61 17 19 6 81 1 . 52 1 12 -1 86 0 31 0 27 JB1 53 01 14 61 9 04 7 55 52 93 14 59 9 02 7 54 52 60 14 62 9 04 7 76 0 33 -0 03 -0 02 -0 22 BCR1 55 13 13 14 12 77 3 43 55 20 13 15 12 78 3 43 54 53 13 72 13 42 3 48 o 67 -o 57 -0 64 -0 05 MRG1 40 42 8 57 1.8 21 13 50 39 61 8 40 17 84 13 23 39 32 8 50 17 89 13 49 0 29 -0 10 -0 05 -0 26 NIMN '51 41 16 92 9 73 7 35 51 03 16 79 9 66 7 30 52 64 16 50 8 90 7 50 - 1 61 0 29 0 76 -0 20 W1 51 86 15 13 1 1 32 6 45 51 60 15 05 1 1 27 6 42 52 72 15 02 1 1 09 6 63 -1 12 0 03 0 18 -0 21 BHVO 49 74 14 37 1 1 86 7 35 49 .37 14 27 1 1 . 78 7 30 49 .90 13 70 12 . 14 7 20 -0 .53 0 57 -0 . 36 0 10 STANDARDS CaO Na 20 K?0 T i 0 2 MnO P 205 5 . 33 4 1 1 2 91 1 15 0. 10 0. 50 5. 38 4 15 2 94 1 16 0. 10 0. 50 4. 95 4 32 2 92 1 06 0 10 0. 51 0 43 -0 17 0 02 0 10 0 00 -0. 01 9 12 2 32 1 45 1 40 0 16 0 31 9 10 2 31 1 45 1 40 0 16 0 30 9 35 2 79 1 42 1 34 0 15 0 26 -0 25 -0 48 0 03 0 06 0 01 0 04 6 83 3 43 1 70 2 21 0 18 0 39 6 84 3 44 1 70 2 21 0 18 0 39 6 97 3 30 1 70 2 26 0 18 0 36 -0 13 0 14 -0 00 -0 05 -0 00 0 03 14 77 0 37 0 20 3 74 0 17 0 10 14 48 0 36 0 20 3 67 0 17 0 10 14 77 0 71 0 18 3 69 0 17 0 06 -0 29 -0 35 0 02 -0 02 -0 00 0 04 1 1 92 2 24 0 28 0 21 0 19 0 05 1 1 84 2 23 0 28 0 21 0 19 0 05 1 1 50 2 46 0 25 0 20 0 18 0 03 0 34 -0 23 0 03 0 01 0 01 0 02 10 99 2 04 0 69 1 06 0 17 0 20 10 94 2 03 0 69 • 1 05 0 17 0 20 10 98 2 15 0 64 1 07 0 17 0 14 -0 04 -0 12 0 05 -0 02 -0 00 0 06 1 1 28 2 60 0 48 2 67 0 17 0 23 1 1 19 2 58 0 47 2 65 0 16 0 23 1 1 40 2 30 0 53 2 .70 0 17 0 28 -0 21 0 28 -0 06 -0 .05 -0 01 -0 05 H 20 C0 2 T 0 T A L 0 78 0. 02 99 . 03 FINAL VALUE 0 78 0 02 NORM. VALUE 0 78 0 02 99 79 RECCOM. VALUE 0 0 0 0 NORM.-RECC. 1 01 0 18 100 16 FINAL VALUE 1 01 0 18 NORM. VALUE 1 01 0 18 100 52 • RECCOM. VALUE 0 0 0 0 NORM.-RECC. 0 67 0 02 99 88 FINAL VALUE 0 67 0 02 NORM. VALUE 0 67 0 02 100 61 RECCOM. VALUE 0 0 0 O NORM.-RECC. 0 98 1 00 102 04 FINAL VALUE 0 98 1 00 NORM. VALUE 0 98 1 00 100 76 RECCOM. VALUE 0 0 0 0 NORM.-RECC. 0 33 0 10 100 74 FINAL VALUE 0 33 0 10 ( NORM. VALUE 0 33 0 10 100 59 RECCOM. VALUE 0 0 0 0 NORM.-RECC. 0 53 0 06 100 50 FINAL VALUE 0 53 0 06 NORM. VALUE 0 53 0 06 101 20 RECCOM."VALUE 0 0 0 0 NORM.-RECC. 0 0 0 0 100 75 FINAL VALUE 0 0 0 0 NORM. VALUE 0 0 0 0 100 32 RECCOM. VALUE 0 .0 0 .0 NORM.-RECC. APPENDIX 3 P r e c i s i o n plots f o r major and trace element p r e c i s i o n a n a l y s i s , a f t e r the method of Howarth & Thompson (1976) (i) Inset on each plot are the duplicate data pairs used i n the P r e c i s i o n a n a l y s i s . ( i i ) The upper and Lower p r e c i s i o n l i n e s are the 99% and 90% p r e c i s i o n l i n e s , r e s p e c t i v e l y X 393 376 393 384 284 267 436 415 33G 329 500 463 105 1 18 408 376 278 298 140 T 1 _ 137 T 1 I 1 II 10' ]0» 10J 10* (Xl*X2/2) CHROMIUM PRECISION 5% IX1TX2/2) YTTRIUM PRECISION 15% 11.99 11.95 1 13.24 13.14 12.59 12.49 9.47 9.15 7.67 7.61 8.91 8.86 10.35 10.37 12.89 12.67 19.78 18.94 MRGNESIUM PRECISION 5 % APPENDIX 4 P h i l l i p s PW-1410 X-ray fluorescence spectrometer operating conditions for major element and trace element an a l y s i s , and d e s c r i p t i o n of computer program action used i n the reduction of major element XRF data. ELEMENT k tljj. If Jij. & K • K %' Si ikn LINE —— —>- —>- - »~ v- » 11J.30 20 57-5M ey .00 113.17 IIO. U|0 • 134.75 1 5 2 . IS ion. IL| ,v TARGEJ CRYSTAL Or »—' tifloo *— per kV/mA .5o/35 50/16 -, 5 o /as SO/IJO COLLIMATOR- F —,. 1 *— COUNTEU F • r— VACUUM OH GAIN 12.8 COUNTER kV 8.01 < Z LOWER LEVEL 15b WINDOW. T° COUNT TIME 10 tec ELEMENT fit AL % P ML % m, LINE l<« — >- »- — - - — - . ?. 0 / H S M f ) 139.00 89.6I 92.60 H5.o\ ^4.00 55.50 63.00 6V.00 TARGET O- w CRYSTAL Per T L f l P 15 F Zoo KV/mA 50/40 1 ~ ' - — ••— - 1~ COLLIMATOR COUNTER c F F - - — — VACUUM otf GAIN COUNTER UV 128 •6 , —; : — LOV/ER LEVEL 15b lM°v \<30 —————— ISO r WINDOW 7 ° ° : J f l ° _ loo 5«c 700 COUNT TIME ;o s e c ————— — K> ice loo S e t 10 a t e 2o Sec . XRF P R E S S E D P E L L E T ANALYSES COMMENTS U z e ^ot vuh. tun. . XRF Major Element A n a l y s i s Machine C o n d i t i o n s 97. MAJOR ELEMENT ANALYSIS Data Reduction Program Action (i ) Intensity r a t i o s for the standards are regressed against t h e i r known chem- i c a l analyses and the r e s u l t i n g quadratic equation i s applied to the inten- s i t y r a t i o s f o r a l l standards and unknowns to obtain the f i r s t approximate r e s u l t s . ( i i ) Total mass absorption c o e f f i c i e n t s f o r the standards are calculated from the known chemical analyses and mass absorption c o e f f i c i e n t s , and are used generate corrected standard i n t e n s i t y r a t i o s , which are then f i t t e d against the known chemical analyses to derive a new set of quadratic regression l i n e c o e f f i c i e n t s . ( i i i ) F i r s t approximate r e s u l t s from ( i ) are used i n conjunction with known mass absorption c o e f f i c i e n t s to generate total-approximate mass absorption c o e f f i c i e n t s f o r standards and unknowns. A serie s of r a t i o s , crudely cor- r e c t e d for mass absorption, i s then derived f o r standards and unknowns. The regression c o e f f i c i e n t s obtained i n ( i i ) are applied to these crudely corrected r a t i o s to obtain a new a n a l y t i c a l r e s u l t , which i s then recycled i t e r a t i v e l y to generate new mass absorption corrected i n t e n s i t y r a t i o s and a further r e f i n e d analysis, u n t i l successive r e s u l t s ( t o t a l s ) for each sam- ple converge to a differ e n c e of l e s s than 0.001 weight % oxide. (iv) The f i n a l , mass absorption corrected analyses f o r the standards derived i n ( i i i ) are regressed against t h e i r known chemical analyses. This f i t should give s t r a i g h t l i n e s of unit gradient f o r each element, but minor deviations often occur. Therefore: (v) F i n a l a n a l y t i c a l r e s u l t s are generated by applying the quadratic function of regression from (iv) to the i t e r a t e d analyses from ( i i i ) . In t h i s way, wet chemical discrepancies are smoothed out and each standard i s e f f e c t i v e l y "standardized" against the remainder of the standard block. For a more d e t a i l e d program d e s c r i p t i o n , and a l i s t i n g of the program, see van der Heyden (1982). XRF TRACE ELEMENT ANALYSIS MACHINE CONDITIONS SR/RB ELEMENT Rb K a i Sr Kai 25.11 37.93 Molybdenum'^ - CRYSTAL LIF(200) LIF(220) kV/mA 60/40 COLLIMATOR f i n e S c i n t i l l a t i o n o f f 128 LINE 29 TARGET COUNTER VACUUM GAIN COUNTER kV 10.9 LOWER LEVEL280 WINDOW 420 Compton Peak 30.13 LIF(220) COUNT TIME 3 x 10 sees REDUCTION Feather and W i l l i s REMARKS Duplicates done using Berman method. CE/ND bkg Nd La^ 72.13 bkg Nd+1.3 Ce L3 X Ce-.85 71.62 Molybdenum ' LIF(200) 60/40 f i n e Flow Proportional and S c i n t i l l a t i o n on 128 FPC: 8.4 Sc: 9.5 200 500 40 s. 100 s. 100 s. 40 s. Berman BA T i Ba Lai 87.17 bkg 86.09 Ba+4.0 Chromium LIF(200) 50/40 f i n e Flow Proportional on 128 8.7 360 320 10.s. 10.s. 10 s. Berman T i c o r r e c t i o n on Ba. vo 00 NI ELEMENT bkg Ni bkg LINE Ka1 2_6 Ni-.63 48.67 Ni+1. TARGET Molybdenum. CRYSTAL LIF(200) kV/mA 60/40 COLLIMATOR coarse COUNTER Flow Proportional and S c i n t i l l a t i o n VACUUM on GAIN 128 COUNTER kV FPC: 8.6 Sc: 10.8 LOWER LEVEL 300 WINDOW 300 COUNT TIME 20 sees. 40 20 REDUCTION Berman REMARKS Use aluminium f i l t e r . CR/V bkg Cr bkg "bkg V T i K o 1 > 2 Rax KB!,3 Cr-2.20 69.36 Cr+1.5 V-1.86 76.89 77.27 Molybdenum . LIF(200) 60/40 - - - - - coarse - - - - _ _ _ _ f i n e - - - - - Flow Proportional and S c i n t i l l a t i o n on 128 FPC: 8.6 Sc: 10.8 280 440 40 sees. 100 40 40 100 40 Berman T i interference on V corrected using Ti-spiked p e l l e t . Cr corrected for V interference. VO VO NB/ZR/Y/SR/RB ELEMENT bkg Nb Zr bkg Y bkg Sr bkg ' Rb bkg LINE K 0 4 K 0 4 K o t j Kai Kai 26 Nb-.35 21.36 22.51 Y-.25 23.76 Sr-.70 25.11 Rb-.74 26.58 Rb+.50 TARGET Tungsten CRYSTAL LIF(200) kV/mA 50/40 COLLIMATOR f i n e COUNTER Flow Proportional and S c i n t i l l a t i o n VACUUM on GAIN 128 COUNTER kV FPC: 8.6 Sc: 10.95 LOWER LEVEL 300 WINDOW 450 COUNT TIME 20 sees. 40 40 20 40 20 40 20 40 20 REDUCTION Berman REMARKS Rb interference on Y, Sr interference on Zr corrected using Sr-Rb-spiked p e l l e t .

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