Metamorphic forsterite (Fo=98) and diopside (Wo:En:Fs=48.5:49.5:2.0) have been, formed from serpentinite within intensely sheared zones in the large ultramafic complex at the Tuolumne River near Sonora, Calif. Bladelike grains of forsterite are elongate, parallel to c, and have prominent idiomorphic faces developed in (010). Metamorphic diopside occurs as small grains, free of inclusions, in rocks containing forsterite. The formation of forsterite and forsterite +diopside in serpentinite probably took place by the following reactions:
antigorite+magnesite→2 forsterite+ fluid 3 antigorite+calcite→4 forsterite+diopside+fluid
Iron derived from the primary olivine and chromite has been oxidized almost entirely to magnetite and may be treated as an indifferent or accessory component. Stratigraphic reconstruction indicates that the total load pressures probably did not exceed 3 kilobars. At these pressures, the reaction should take place between 400° and 500°C with a fluid composition ranging from nearly pure H2O to less than 5 mole percent CO2. The restriction of forsterite and forsterite-f-diopside to shear zones may be attributed to the presence of carbonate in the rock prior to metamorphism and to the dilution of a CO2-bearing fluid phase by water coming from outside the area of the reaction.
