Formation of Forsterite by Silicification of Dolomite during Contact Metamorphism

摘要

Four samples that experienced the infiltration-driven reaction 2 dolomite + SiO₂(aq) = forsterite + 2 calcite + 2 CO₂ exhibit correlations among forsterite crystal morphology, size, number density (number of Fo crystals per cm3 Fo), and oxygen isotope ratio (δ18O). The δ18O of coexisting forsterite, calcite, and dolomite were determined by in situ ion microprobe analysis with a spatial resolution of ∼15 µm. Sample KP1L from the Twin Lakes pendant, Sierra Nevada, California, contains blocky forsterite with the largest average crystal size and the lowest crystal number density. Forsterite and calcite are uniform in δ18O with the measured fractionation consistent with equilibrium at the temperature inferred for the reaction (595°C). Sample B4L from the Beinn an Dubhaich aureole, Scotland, contains tabular forsterite with intermediate average size and number density. Forsterite and calcite are uniform in δ18O but the measured fractionation is smaller than the equilibrium value at the temperature inferred for the reaction (680°C). Samples B1W and B43A from the Beinn an Dubhaich aureole contain rounded forsterite with the smallest average size and largest number density. Forsterite has variable δ18O differing among grains by up to 7·4‰ in single samples and by up to 3·1‰ within a single grain, precluding isotope equilibrium with calcite and dolomite. Crystal morphology, size, number density, δ18O(Fo), and Δ18O(Cal–Fo) can be understood in terms of the interplay between reaction affinity (A) and Peclet Number (Pe) during formation of forsterite (KP1L: low A, high Pe; B4L: intermediate A and low Pe; B43A: high A, low Pe; B1W: high A, increasing Pe). Differences in A were controlled by variations in in the infiltrating fluid. Differences in Pe were controlled by variations in fluid flux and/or width of the reaction zone. If the infiltrating fluid is initially quartz-saturated, phase equilibria require that development of forsterite and periclase from dolomite during contact metamorphism must be preceded by reactions that produce diopside, wollastonite, and/or other skarn minerals upstream.