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A study of rubidium, strontium and strontium isotopes in some mafic and sulphide minerals

University of British Columbia

Galena from Pine Point, N.W.T. contains less than 0.3 ppm Sr and less than 0.005 ppm Rb with a Rb⁸⁷/Sr⁸⁶ ratio less than 0.2. Sphalerite from the same locality contains 0.070 ± 0.002 ppm Sr and 0.025 ± 0.003 ppm Rb with a Rb⁸⁷/Sr⁸⁶ ratio of 1.05 ± 0.3. Despite large analytical uncertainties in measuring such small quantities of Rb and Sr, it appears sphalerite has a suitable Rb/Sr ratio for age determination. Calcite, Presqu'ile-type dolomite and sparry dolomite associated with Pine Point mineralization have distinct Sr⁸⁷/Sr⁸⁶ ratios of 0.7153, 0.7080 and 0.7096 respectively. A two point isochron defined by sphalerite and sparry dolomite indicates an age of 165 ± 60 m.y. and an initial ratio of 0.7095 ± 0.0001. Sphalerite from the Bluebell Mine, B.C. contains 0.012 ± 0.003 ppm Rb. Phrrhotite and galena from the same locality contain smaller quantities of Rb (0.0022 ± 0.0002 ppm and 0.0003 ± 0.0001 ppm respectively). No reliable Sr analyses are available due to the extremely small amount of Sr present. The relatively large Rb concentration of sphalerite supports the conclusion drawn from analyses of Pine Point sphalerite that the mineral is suitable for age determination. Olivine and orthopyroxene within lherzolite nodules from Boss Mountain and Jacques Lake, B.C. contain 1-2 ppm Sr and 0.1 - 0.01 ppm Rb with Rb⁸⁷/Sr⁸⁶ ratios close to 0.1. Associated clinopyroxene contains from 22-42 ppm Sr, less than 0.3 ppm Rb, with Rb⁸⁷/Sr⁸⁶ ratios close to 0.01. Two nodules from Jacques Lake show apparent isochron relationships for the constituent olivine, orthopyroxene and clinopyroxene giving ages of 2.0 ± 0.1 b.y. and 4.7 ± 2.4 b.y. with initial ratios of 0.7024 ± 0.0001 and 0.7020 ± 0.0004 respectively. Rb-Sr data for the Boss Mountain nodule does not define an isochron, although Sr isotopic disequilibrium between phases is present. It is not yet clear whether the observed Sr isotopic disequilibrium is due to in situ decay of Rb⁸⁷ or to selective contamination of olivine and orthopyroxene prior to their emplacement in the crust.

Text

2010-02-18

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http://hdl.handle.net/2429/20410

Geological Sciences

University of British Columbia

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