Oxide-silicate petrology and geochemistry of subducted hydrous ultramafic rocks beyond antigorite dehydration (Central Alps, Switzerland)

Oxide minerals contained in ultramafic rocks are useful tools to assess the redox conditions of the rock and fluids liberated upon progressive serpentinite dehydration during subduction, as these minerals contain a relevant redox-sensitive element, iron. Previous studies have revealed that magnetite predominates across the antigorite-out reaction. However, the fate of magnetite and other oxides at higher pressure and temperature conditions has remained underexplored. We present a comprehensive petrological and geochemical study of oxide-sulfide-silicate mineral assemblages in metaperidotites beyond antigorite- and chlorite-out reactions (T = 650-850 & DEG;C and P = 1-3 GPa). Several ultramafic lenses, covering different bulk rock compositions and extents of oxidation upon oceanic serpentinization, were investigated from the Central Alps, Switzerland. Results point to two endmember scenarios: (i) Most frequently, metaperidotites have olivine with a Mg# of 89-91 (defined as molar Mg/(Mg + Fe-tot) x 100) and contain low oxide modes (0.06-1.41 vol.%), hematite is absent, and redox conditions are weakly oxidized and buffered by orthopyroxene-olivine-magnetite. (ii) Rare occurrence, high olivine Mg# > 94.5 metaperidotites display coexisting hematite and magnetite, high oxide modes (up to 4 vol.%), and redox conditions are hematite-magnetite (HM) buffered (& UDelta;log(10)fO(2),(QFM) of + 3 to + 4). Spinel displays evolving compositions from magnetite over chromite to Al-Cr-spinel, roughly correlating with increasing temperature. Most of the samples buffered by the olivine-orthopyroxene-magnetite assemblage contain coexisting pentlandite & PLUSMN; pyrrhotite, thus identifying stable sulfides beyond antigorite dehydration for these weakly oxidized samples (& UDelta;log(10)fO(2),(QFM) < 2.5). No sulfides were recognized in the highly oxidized sample. The transition of magnetite to chromite at around 700 & DEG;C goes along with a shift in fO(2) to lower values. At the prevailing oxygen fugacity in the weakly oxidized metaperidotites sulfur in a coexisting fluid is always present in its reduced form. However, oxidized sulfur can be stable in the dehydration fluids released from highly oxidized serpentinites.

Automated summary

Oxide minerals in ultramafic rocks can be used to assess the redox conditions of the rock and fluids during subduction. Previous studies have focused on the dominancy of magnetite, but the fate of magnetite and other oxides at higher pressure and temperature conditions is not well understood. This study investigates mineral assemblages in metaperidotites beyond antigorite-out reactions and identifies two scenarios with different redox conditions. The results provide insights into the behavior of oxides and sulfides in subduction zones.