Oxygen isotope fractionation in decarbonation metamorphism: the Mottled Zone event |
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| Authors: | Alan Matthews Yehoshua Kolodny |
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| Institution: | Department of Geology, The Hebrew University of Jerusalem, Jerusalem Israel |
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| Abstract: | Calculated univariant equilibria and oxygen isotope compositions of silicates and carbonates support the proposal that the “Mottled Zone Event” is a low-pressure (1–25 atm), high-temperature (200° < T < 1300°C) metamorphism of calcareous siliceous sediments in which the thermal energy is provided by combustion of organic matter. δ18O of silicates decreases systematically with increasing metamorphic grade from averages of 18.1‰ in protolith shales, to 16.6‰ in grossular-diopside-zeolite rocks, 15.6‰ in wollastonite and anorthite-diopside-gehlenite-grossular fels, 14.1‰ in spurrite-brownmillerite marbles and 11.7‰ in the highest-grade larnite-gehlenite-brownmillerite assemblages. Decarbonation is the principal mechanism influencing the oxygen isotope compositions. The progressive decrease of δ18O in silicates can be modelled as a Rayleigh distillation of CO2 approximately 16‰ enriched in 18O relative to whole rock assemblages i.e., of initial isotopic composition 8.5‰ heavier than the parent carbonates. The mineral assemblage of one sample with an unusual granoblastic texture is in apparent isotopic equilibrium at a temperature of 540°C. |
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