Serpentinization of the Belvidere Mountain ultramafic body, Vermont; mass balance and reaction at the metasomatic front

Abstract

Chrysotile-asbestos veins that cut the relatively unaltered dunite core of the ultramafic body at Belvidere Mountain, Vermont, are bordered by symmetrical serpentinite zones composed of antigorite, magnetite and brucite. This serpentinite is separated from the dunite host by a sharp reaction front, and the modal mineralogy and mineral chemistry of the two rock-types adjacent to the front were determined in order to characterize the serpentinization reaction. A mass balance of bulk chemistry indicates that the serpentinite formed by the addition of SiO_2, and H_2O, loss of H_2, and without net loss of magnesium. A reaction that most closely represents the serpentinization front is: 100 olivine + 25 SiO_2 + 129 H_2O = 31 antigorite + 4 magnetite + 4 H_2. The presence of essentially monomineralic zones suggests that SiO_2, H_2O and H_2 activities were externally controlled. Local concentrations of brucite indicate that the activities of those components were locally appropriate for the stability of brucite.