The stability of hydrous phases beyond antigorite breakdown for a magnetite-bearing natural serpentinite between 6.5 and 11 GPa

Abstract

Phase relations for a natural serpentinite containing 5 wt% of magnetite have been investigated using a multi-anvil apparatus between 6.5 and 11 GPa and 400–850 °C. Post-antigorite hydrous phase assemblages comprise the dense hydrous magnesium silicates (DHMSs) phase A (11.3 wt% H2O) and the aluminous phase E (Al-PhE, 11.9 wt% H2O). In addition, a ferromagnesian hydrous silicate (11.1 wt% H2O) identified as balangeroite (Mg,Fe)42Si16O54(OH)40, typically described in low pressure natural serpentinite, was found coexisting with Al-PhE between 650 and 700 °C at 8 GPa. In the natural antigorite system, phase E stability is extended to lower pressures (8 GPa) than previously reported in simple chemical systems. The reaction Al-phase E = garnet + olivine + H2O is constrained between 750 and 800 °C between 8 and 11 GPa as the terminal boundary between hydrous mineral assemblages and nominally anhydrous assemblages, hence restricting water transfer into the deep mantle to the coldest slabs. The water storage capacity of the assemblage Al-PhE + enstatite (high-clinoenstatite) + olivine, relevant for realistic hydrated slab composition along a relatively cold temperature path is estimated to be ca. 2 wt% H2O. Attempts to mass balance run products emphasizes the role of magnetite in phase equilibria, and suggests the importance of ferric iron in the stabilization of hydrous phases such as balangeroite and aluminous phase E.