Four samples that experienced the infiltration-driven reaction 2 dolomite þ SiO 2 (aq) ¼ forsterite þ 2 calcite þ 2 CO 2 exhibit correlations among forsterite crystal morphology, size, number density (number of Fo crystals per cm 3 Fo), and oxygen isotope ratio (d 18 O).The d 18 O of coexisting forsterite, calcite, and dolomite were determined by in situ ion microprobe analysis with a spatial resolution of $15 mm. Sample KP1L from the Twin Lakes pendant, Sierra Nevada, California, contains

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... y forsterite with the largest average crystal size and the lowest crystal number density. Forsterite and calcite are uniform in d 18 O with the measured fractionation consistent with equilibrium at the temperature inferred for the reaction (5958C). 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 d 18 O but the measured fractionation is smaller than the equilibrium value at the temperature inferred for the reaction (6808C). 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 d 18 O differing among grains by up to 7·4ø in single samples and by up to 3·1ø within a single grain, precluding isotope equilibrium with calcite and dolomite. Crystal morphology, size, number density, d 18 O(Fo), and Á 18 O(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 X CO2 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.