Extreme reduction: Mantle-derived oxide xenoliths from a hydrogen-rich environment

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.

Section: Introductionmentioning

confidence: 77%

mentioning

confidence: 99%

Cr2O3 in corundum: Ultrahigh contents under reducing conditions

Griffin

Gain

Saunders

et al. 2021

“…• Griffin et al (2020) believe they identified relicts of the reducing fluid. They assert that voids in their Mt.…”

Section: Ultra-reduced Phases -Textural and Chemical Considerationsmentioning

confidence: 99%

“…Alkali basalts are oxidized around FMQ+1 and their equilibrium volatiles are H 2 O and CO . Griffin et al (2020) realised that problem but circumvented it by stating (rather ad hoc) the fluid infiltration episode was unrelated to the alkali basaltic volcanism. No reasons are given for that assertion.…”

Section: Ultra-reduced Phases -Textural and Chemical Considerationsmentioning

confidence: 99%

Ultra-reduced phases in ophiolites cannot come from Earth’s mantle

Ballhaus

Helmy

Fonseca

et al. 2021

A number of recent papers have purported to find ultra-reduced minerals-as natural examples-within ophiolitic mantle sections, including SiC moissanite, Fe-Si alloys, various metal carbides, nitrides, and borides. All those phases were interpreted to be mantle derived. The phases are recovered from mineral concentrates and are assigned to the deep mantle because microdiamonds and other ultra-high pressure (UHP) minerals are also found. Based on these findings it is claimed that the mantle rocks of ophiolite complexes are rooted in the Transition Zone (TZ) or even in the Lower Mantle, at redox states so reduced that phases like SiC moissanite are stable. 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.

“…In both localities, hibonite is purple and occurs as centimetric euhedral phenocrystals, while grossite occurs as interstitial light violet crystals up to a few millimeters. Furthermore, crystals of both minerals frequently have tubular inclusions of a V-rich phase isostructural with a non-stoichiometric vanadium oxide (approximately V2O) indicating very low oxygen fugacity (Griffin et al 2020). These rocks have formed in presence of high CH4 and H2 fugacity as evidenced by the presence of the first ever reported metal hydride, VH2, in nature (Bindi et al 2019).…”

Section: Introductionmentioning

confidence: 99%

Vanadium-induced coloration in grossite (CaAl4O7) and hibonite (CaAl12O19)

Ardit

Cámara

Hålenius

2021

High concentrations of vanadium cause very unusual coloration in hibonite (purple) and grossite (light violet) crystals in an exotic mineral assemblage from Sierra de Comechingones (Argentina). In the hibonite (CaAl12O19) structure vanadium ions, in various valence states (divalent, trivalent and tetravalent), may be distributed over five crystallographic sites with coordinations corresponding to different polyhedra, namely, three unequal octahedra [M1 (D3d), M4 (C3v) and M5 (Cs)], one M3 tetrahedron (C3v), and one unusual 5-fold coordinated trigonal bipyramid M2 (D3h). Possible locations of vanadium ions in grossite (CaAl4O7) are limited to two crystallographically distinct sites (T1 and T2, both C1) in tetrahedral coordination. The combination of single-crystal X-ray diffraction and absorption spectroscopy techniques aided by chemical analyses has yielded details on the nature of the vanadium-induced color in both hibonite and grossite crystals. In hibonite, both M4 face-sharing octahedral and M2 trigonal bipyramid sites of the R-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.