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A comparative study of glaciovolcanic palagonitization of tholeiitic and alkaline sideromelane in Helgafell, Iceland and Wells Gray-Clearwater Volcanic Field, BC, Canada Massey, Erica A.


Pleistocene glaciovolcanic eruptions occurred frequently beneath continental-scale ice sheets producing vitric, fragmental volcanic deposits in Helgafell, Iceland (tholeiitic basalt) and Wells Gray, BC, Canada (alkali olivine basalt). They are highly susceptible to hydrothermal alteration that transforms sideromelane (basaltic volcanic glass) into palagonite (early amorphous material) and secondary minerals (i.e. zeolites, clays and sulfides). Compositional controls, mass transfer and geochemical-textural relationships are investigated by optical microscopy and analyses of major (12) and trace (32) elements in glass-palagonite pairs by Electron Microprobe Analysis (EMPA), Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), and geochemical modelling. Helgafell’s thinner (6-10 m vs. 10-20 m) palagonite rims demarcate highly vesicular (30.2% vs. 4.9%) sideromelane that has more secondary minerals (1.4% vs. 0.5%) than Wells Gray’s microlite-rich (12.3% vs. 0.1%) sideromelane. The thicknesses of palagonite rims are similar whether the sideromelane is unaltered or completely altered. Multi-dimensional scaling confirms that sideromelane composition, reflecting igneous processes, strongly controls the chemistry of palagonite. A comparison of element behavior in the palagonite from both localities reveals a tendency for water-soluble cations (Cl, K, Na, Rb, Cu, Mn, P) to correlate with water concentrations. Plots of element ratios (Nb/Y vs La/Nd; Sc/Ta vs Zr/Th) calculated from “immobile” elements show that palagonite from Wells Gray and Helgafell are distinct, and have ratios that are similar to sideromelane that produced the palagonite. Thus some palagonite compositions reflect primary magma compositions based on immobile elements. Gresens mass transfer calculations confirm minimal movement of these elements during palagonitization. However, the same calculations reveal a pattern of Cu, Cl, Ni, Rb and U addition and Na, Ca, Mg, P, V and Mn removal that is similar at both localities. Microprobe traverses identified eight prominent trends across the glass-palagonite interface and palagonite rim, which do not appear to be controlled by sideromelane composition. Several element concentrations decrease in palagonite, including Si (by ~3-10%), Al, Ca and Na, while Ti, Fe and Mg concentrations increase. Locally, the palagonite has an inner Ti-rich zone. The gradual increase in Mg across the palagonite rim may be indicative of changes in solubility and pH.

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