Cr2O3 in corundum: Ultrahigh contents under reducing conditions

Abstract

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 (fO 2) as crystallization proceeded. Corundum coexisting with Fe-Mg-Cr-Al spinels , other Fe-Mg-Al-Na oxides and Fe-Ni alloys in apparent cumulates crystallized at fO 2 near the iron-wüstite (IW) buffer (fO 2 = 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% SiO 2. Corundum in other possible cumulates that contain Cr-Fe (Fe 45 wt%) alloys has low-Cr cores and still lower-Cr rims. Corundum coexisting with Cr 0 (fO 2 = IW-5) in some possible cumulates has low-Cr cores, but high-Cr rims (to >30% Cr 2 O 3). These changes in zoning patterns reflect the strong decrease in the melting point of Cr 2 O 3 , relative to Al 2 O 3 , with decreasing fO 2. EELS analyses show that all Cr in corundum that coexists with Cr 0 is present as Cr 3+. This suggests that late in the evolution of these reduced melts, Cr 2+ has disproportionated via the reaction 3Cr 2+ (melt) → 2Cr 3+ (Crn) + Cr 0. The most Cr-rich corundum crystallized together with -alumina phases including NaAl 11 O 17 (diaoyudaoite) and KAl 11 O 17 (kahlenbergite) and "-alumina phases; residual melts crystallized a range of (K,Mg) 2 (Al,Cr) 10 O 17 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. This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America. The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

Kishonite, VH2, and Oreillyite, Cr2N, Two New Minerals from the Corundum Xenocrysts of Mt Carmel, Northern Israel

“…The coarse (cm-size crystals) V-bearing hibonite + grossite + spinel aggregates containing kishonite appear to represent a late stage of the evolution of this magma [4]. The grain sizes, magmatic microstructures, and evidence for an extended sequence of magmatic crystallization [5,7] clearly rule out more speculative origins such as lightning strikes [24], meteorite impact, or shallow hydrothermal circulation. Petrographic evidence for the reaction liquid + corundum → anorthite, and the stability of grossite suggest that this process operated at or near to the base of the crust (25-30 km depth; [3]), and at temperatures ranging from 1450-1500 • C to a minimum of >1150 • C.…”

“…Colour, luster, streak, hardness, tenacity, cleavage, fracture, density, and optical properties could not be determined because of the small grain size. The sample containing oreillyite was recovered from the Kishon Mid Reach Zone 1 and consists of a 1.7 mm crystal of corundum with a hollow centre and raised rims, indicating hopper growth, and it has an intense purple-red colour [5].…”

“…Oreillyite was found inside a 1.7 mm crystal of corundum with a hollow centre and raised rims, indicating hopper growth, and with an intense purple-red colour (Figure 2; [5]). The surface of the crystal is decorated with balls of native chromium up to 60 microns across, and very small grains of an Fe-Ni alloy.…”

“…The study of the xenoliths occurring in pyroclastic ejecta of small Cretaceous basaltic volcanoes exposed on Mt Carmel [1][2][3][4][5][6][7][8] has revealed a very peculiar, exotic and ultra-reduced mineral assemblage reflecting the interaction of deep-seated basaltic magmas with mantle-derived CH 4 + H 2 at high fluid/melt ratios. During these ongoing studies, two new minerals, kishonite (VH 2 ) and oreillyite (Cr 2 N) have been identified and are described here.…”

“…The sample containing oreillyite was recovered from the Kishon Mid Reach Zone 1 and consists of a 1.7 mm crystal of corundum with a hollow centre and raised rims, indicating hopper growth, and it has an intense purple-red colour [5].…”