Fe-monticellite in serpentinites from the Happo ultramafic complex

Abstract

The new discovery of Fe-monticellite in serpentinized peridotites of the Happo ultramafic complex, central Japan, shows it replacing olivine and coexisting with antigorite. The monticellite occurs in several forms: as discrete grains; partially disconnected rings or circular ribbons; fringing mantles or in veins cutting olivine; or as aggregates of small equant grains. It shows a textural contrast to the intensely sheared matrix of antigorite. The coexistence with antigorite and diopside, and the Fe-rich compositions of monticellite indicates that it formed at 300–350 °C. This extremely low-temperature for monticellite formation is consistent with the observed textures indicative of its static formation at a stage of serpentinization that followed mylonitization during high-temperature (400–600 °C) serpentinization in the Happo complex. The monticellite contains minute inclusions of awaruite, which indicates formation under reducing conditions, as is characteristic of serpentinization of olivine. The monticellite is richer in Fe and Mn than relict olivine, being commensurate with deficiencies of these elements in coexisting antigorite. There is no evidence of Ca addition from an external source. However, where tremolite is replaced by serpentine, diopside grows at the opposite side of relict olivine that is partly replaced by monticellite. Thus, tremolite is the likely source of Ca for monticellite formation. These textural relationships and the thermodynamic calculations for paragenesis indicate that monticellite formed by reactions between olivine and aqueous fluids, carrying Ca released from tremolite, that were driven by a local gradient of SiO2 chemical potential during serpentinization.