Textural and Geochemical Evidence for Magnetite Production upon Antigorite Breakdown During Subduction

Abstract Magnetite stability in ultramafic systems undergoing subduction plays a major role controlling redox states of the fluids liberated upon dehydration reactions, as well residual rocks. Despite their relevance for evaluation conditions, systematics and geochemistry oxide minerals have remained poorly constrained subducted Here, we present detailed petrological geochemical study magnetite hydrous rocks from Cerro del Almirez (Spain). Our results indicate that prograde to peak magnetite, ilmenite–hematite solid solution minerals, sulfides coexist both antigorite-serpentinite chlorite-harzburgite at c. 670 °C 1·6 GPa, displaying successive crystallization stages, each characterized by specific mineral compositions. In antigorite-serpentinite, inherited seafloor hydration recrystallized during has moderate Cr (Cr2O3 < 10 wt%) low Al V concentrations. chlorite-harzburgite, polygonal is textural equilibrium with olivine, orthopyroxene, chlorite, pentlandite, minerals. The Cr2O3 contents this are up 19 wt%, higher than any data obtained along V, derived antigorite breakdown, lower Mn This displays conspicuous core rim zoning recognized on elemental maps. Cr–V–Al–Fe3+ mass-balance calculations, assuming conservative behavior total Fe3+ Al, were employed model compositions modes partially dehydrated product starting serpentinite protolith chlorite-harzburgite. disagree measured antigorite-serpentinites chlorite-harzburgites. indicates these two rock types had different initial bulk thus cannot be directly compared. analysis also reveals new formation required across antigorite-breakdown reaction account mass conservation fluid-immobile elements such Cr–V–Al–Fe3+. Complete recrystallization olivine (Mg# 89–91), chlorite ?95), orthopyroxene 90–91), pentlandite buffer released fluid conditions ?1 log unit above quartz–fayalite–magnetite buffer. consistent observation Fe–Ti (hemo-ilmenite ilmeno-hematite) crystallized homogeneous phases exsolved exhumation cooling. We conclude antigorite-dehydration carry only budget therefore may not reason why magmas comparatively oxidized.