Cr2O3 in corundum: Ultrahigh contents under reducing conditions

Abstract Xenocrysts and xenoliths in Upper Cretaceous pyroclastics on Mount Carmel (northern Israel) represent a series of similar magma-fluid systems at different stages their evolution, recording continuous decrease oxygen fugacity (fO2) as crystallization proceeded. Corundum coexisting with Fe-Mg-Cr-Al spinels, other Fe-Mg-Al-Na oxides, Fe-Ni alloys apparent cumulates crystallized fO2 values near the iron-wüstite (IW) buffer (fO2 = IW±1) is zoned from high-Cr cores to lower-Cr rims, consistent fractional trends. The reconstructed parental melts are Al-Cr-Fe-Mg oxides ca. 2 wt% SiO2. possible that contain Cr-Fe (Fe 45 wt%) has low-Cr still rims. Cr0 IW-5) some cores, but rims (to >30% Cr2O3). These changes zoning patterns reflect strong melting point Cr2O3, relative Al2O3, decreasing fO2. electron energy loss spectroscopy (EELS) analyses show all Cr corundum coexists present Cr3+. This suggests late evolution these reduced melts, Cr2+ disproportionated via reaction 3Cr2+(melt) → 2Cr3+(Crn) + Cr0. most Cr-rich together β-alumina phases including NaAl11O17 (diaoyudaoite) KAl11O17 (kahlenbergite) β″-alumina phases; residual range (K,Mg)2(Al,Cr)10O17 kahlenbergite structure. assemblages appear have been Al-Cr-K-Na-Mg which may be related oxide mentioned above, through or liquid immiscibility. samples less IW than trapped silicate more abundant aggregates IW-6 IW-10). They could considered an earlier stage “ideal” Mt. magmatic system, mafic syenitic magmas were fluxed by mantle-derived CH4+H2 fluids. newly recognized step helps understand element partitioning under highly reducing conditions.