Cr2O3 in Corundum: Ultra-high contents under reducing conditions

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