CONTROLS ON CRYSTAL MORPHOLOGY, SIZE, NUMBER DENSITY, AND OXYGEN ISOTOPE COMPOSITION DURING FORMATION OF FORSTERITE BY SILICIFICATION OF DOLOMITE DURING CONTACT METAMORPHISM

Four samples that experienced the infiltration-driven reaction, 2 dolomite (Dol) + SiO₂(aq) = forsterite (Fo) + 2 calcite (Cal) + 2 CO₂, exhibit correlations among Fo crystal morphology, size, number density, and oxygen isotope ratio (δ¹⁸O). The δ¹⁸O of coexisting Fo, Cal, and Dol 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 Fo with the largest average crystal size (190 μm) and the lowest number density (1.5·10⁵ crystals/cm³ Fo). Fo and Cal are uniform in δ¹⁸O [δ¹⁸O(Fo) = 6.8±0.2 ‰, ±2 SE] with the measured fractionation [Δ¹⁸O(Cal-Fo) = 4.2±0.2 ‰] consistent with equilibrium at the temperature recorded by mineral equilibria (595 °C). Sample B4L from the Beinn an Dubhaich aureole, Scotland, contains tabular Fo with intermediate average size (47 μm) and number density (9.6·10⁶ cm^-3). Fo and Cal are uniform in δ¹⁸O [δ¹⁸O(Fo) = 18.8±0.2 ‰] but the measured fractionation [Δ¹⁸O(Cal-Fo) = 2.6±0.3 ‰] is smaller than the equilibrium value (3.6 ‰) at the temperature recorded by mineral equilibria (680 °C). Samples B1W and B43A from the Beinn an Dubhaich aureole contain rounded, bead-like Fo with the smallest average size (26-36 μm) and largest number density [(2-6)·10⁷ cm^-3]. Fo has variable δ¹⁸O (9.8-17.2 ‰), differing among grains by up to 7.4 ‰ in individual samples and by up to 3.1 ‰ within an individual grain, precluding isotope equilibrium with Cal and Dol. 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 Fo (KP1L: low A, high Pe; B4L: intermediate A and low Pe; B43A: high A, low Pe; B1W: high A, increasing Pe). Ranges in A and Pe are explained by variation in X_CO2 of infiltrating fluid and in either fluid flux or width of the reaction zone, respectively. If infiltrating fluids are initially quartz-saturated, development of Fo (and periclase) from Dol during contact metamorphism must be preceded by reaction of fluid with Cal and Dol marble to produce diopside, wollastonite, and other skarn minerals, and thereby reduce a_SiO2 to the low values required by phase equilibria.