Ti3+ in corundum traces crystal growth in a highly reduced magma

Abstract

Aggregates of corundum crystals with skeletal to hopper morphology occur in pyroclastic rocks erupted from Cretaceous basaltic volcanoes on Mt Carmel, N. Israel. The rapid growth of the crystals trapped volumes of the parental Al2O3-supersaturated melt; phenocrysts of tistarite (Ti2O3) in the trapped melts indicate crystallization at oxygen fugacities 6–7 log units below the Iron-Wüstite buffer (fO2 = ΔIW − 6 to − 7), induced by fluxes of mantle-derived CH4-H2 fluids. Cathodoluminescence images reveal growth zoning within the individual crystals of the aggregates, related to the substitution of Ti3+ in the corundum structure. Ti contents are < 0.3 wt% initially, then increase first linearly, then exponentially, toward adjacent melt pockets to reach values > 2 wt%. Numerical modelling indicates that the first skeletal crystals grew in an open system, from a moving magma. The subsequent linear increase in Ti reflects growth in a partially closed system, with decreasing porosity; the exponential increase in Ti close to melt pockets reflects closed-system growth, leading to dramatic increases in incompatible-element concentrations in the residual melts. We suggest that the corundum aggregates grew in melt/fluid conduits; diffusion modelling implies timescales of days to years before crystallization was terminated by explosive eruption. These processes probably operate in explosive volcanic systems in several tectonic settings.

Evidence of the anthropogenic origin of the ‘Carmel sapphire’ with enigmatic super-reduced minerals

“…The composition and morphology of inclusions in the corundum from the Mt Carmel "white breccia" (Fig. S5-S7) correspond well to the SR minerals in the CS described by Griffin et al (2016Griffin et al ( , 2018Griffin et al ( -2021. The photos of "white breccia", mentioned by Griffin et al (2019b) available on the Internet (Fig.…”

“…Besides blue and light-brown fragments of CS grains and aggregates containing a typical SR mineral association, which was described several times by Griffin and co-authors (Figs. S5-S7; Griffin et al, 2016Griffin et al, , 2018Griffin et al, -2021, in the white cement we recognize corundum-free fragments 50-200 m in size. That is firstly aggregates of krotite, CaAl2O4, with grossite, CaAl4O7, and diaoyudaoite, NaAl11O17, inclusions, and also angular grains presented by mullite in glass or cristobalite (Fig.…”

“…This cement is a result of eruption of the ashes on a shallow marine carbonate platform, and the subsequent covering of the ash beds by thick carbonate sequences." Griffin and co-authors (Griffin et al, 2016(Griffin et al, , 2018(Griffin et al, -2021 were not specific about the cement composition or corundum-free fragments of rock in the "white breccia". On only one occasion the amorphous carbon is recorded in the cement of the "white breccia" (Xiong et al, 2017).…”

“…Mineral separates (concentrates) mined by the Shefa Yamim Company (https://www.shefagems.com ) as a result of exploration for gemstones in alluvial deposits are a source of this corundum. Osbornite in the corundum aggregates is associated with unusual SR minerals represented by nitrides, carbides, silicides, FeSi(Ti,Cr,V)-alloys, and Ti 3+ -bearing oxides (Griffin et al, 2016(Griffin et al, , 2018(Griffin et al, -2021Bindi et al, 2020). This corundum with inclusions of SR minerals obtained the trademark "Carmel sapphire" and has been the host of 10 new mineral species discovered in the last five years: carmeltazite (IMA2018-103) ZrAl2Ti4O11 (Griffin et al, 2018); kishonite (IMA2020-023) VH2 (Bindi et al, 2020); oreillyite (IMA 2020-030a) Cr2N (Bindi et al, 2020); griffinite (IMA 2021-110) Al2TiO5 ; magnéliite (IMA2021-111) Ti 3+ 2Ti 4+…”

“…2O7 (Ma et al, 2021); ziroite (IMA2022-013) ZrO2 ; sassite (IMA2022-014) Ti 3+ 2Ti 4+ O5 ; mizraite-(Ce) (IMA2022-027) Ce(Al11Mg)O19 ; toledoite (IMA2022-036) TiFeSi ; and yeite (IMA2022-079) TiSi . Grain aggregates of CS (and also hibonite and moissanite) from Carmel have been the objects of numerous scientific publications (Griffin et al, 2016(Griffin et al, , 2018(Griffin et al, -2022Xiong et al, 2017; https://doi.org/10.1180/mgm.2023.25 Published online by Cambridge University Press Dobrzhinetskaya et al, 2018;Cámara et al, 2019;Bindi et al, 2019Bindi et al, , 2020Huang et al, 2020;Stan et al, 2020;Oliveira et al, 2021;Lu et al, 2022), in which the authors present the highly controversial hypothesis that the corundum rock with SR minerals formed at the crust-mantle boundary as a result of CH4+H2 fluid flowing through the magmatic melt.…”