Textural and chemical variations of olivine and chrome spinel in the East Dover ultramafic bodies, south-central Vermont

Abstract

Regional metamorphism of upper-greenschist to epidote-amphibolite facies produced variations of olivine and chrome spinel textures and compositions within the East Dover ultramafic bodies that can be attributed to differing intensities of recrystallization of the ultramafic rocks. Olivine shows a progressive change from highly strained grains in the least recrystallized rocks to relatively strain-free grains in the most recrystallized rocks. Fo content of olivine bears a direct relationship to texture: with increase in degree of recrystallization, olivine changes composition from Fo 90 to Fo 97 . Chrome spinel grains are ubiquitously rimmed by opaque spinels. Variations in thickness of these rims and shape of the translucent chrome spinel cores occur in a regular manner. Thickest rims occur around the most anhedral cores, and these are found in the most strongly recrystallized rocks. Euhedral chrome spinels with thin opaque spinel rims are found in the least recrystallized rocks. With increasing recrystallization, Mg/(Mg + Fe) and A1/(A1 + Cr) ratios increase in the spinel cores. Zones of intensity of recrystallization indicated by both olivine and spinel textures and compositions are geographically concentric with the map outlines of the ultramafic bodies. Observed textural and compositional variations suggest a reaction involving simultaneous serpentinization and recrystallization of olivine in the presence of an oxidizing fluid. Partial buffering of this fluid (which originated in the country rocks) and subsequent development of a substantial f o 2 gradient between the margins and cores of the larger ultramafic bodies resulted in a more nearly complete reaction at the margins than in the cores of the bodies. This reaction need not have occurred as a single event, and equilibrium was only maintained on the scale of hand specimens ( f o 2 , olivine composition) to fractions of mineral grains (spinel composition).