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

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Marco E. Ciriotti
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Ti3+ in corundum traces crystal growth in a highly reduced magma

Messaggio da Marco E. Ciriotti » sab 03 apr, 2021 18:06

Referenza:
▪ Oliveira, B., Griffin, W.L., Gain, S.E.M., Saunders, M., Shaw, J., Toledo, V., Afonso, J.C., O'Reilly, S. (2021): Ti3+ in corundum traces crystal growth in a highly reduced magma. Scientific Reports, 11, 2439.

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.

The discovery of tistarite (Ti2O3) in the Mt Carmel material was the first report of a terrestrial Ti3+ mineral, and the Mt Carmel examples represent the first recognition of Ti3+ substitution in natural corundum.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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