To be or not to be oxidized: A case study of olivine behavior in the fusion crust of ureilite A 09368 and H chondrites A 09004 and A 09502.

Lidia Pittarello,Vinciane Debaille,Julia Roszjar,Christian Koeberl,Philippe Claeys,Akira Yamaguchi

doi:10.1111/maps.13284

Abstract

Meteorite fusion crusts are quenched melt layers formed during meteoroid atmospheric entry, mostly preserved as coating on the meteorite surface. Antarctic ureilite Asuka (A) 09368 and H chondrites A 09004 and A 09502 exhibit well preserved thick fusion crusts, characterized by extensive olivine crystallization. As olivine is one of the major components of most meteorites and its petrologic behavior is well constrained, it can be roughly considered as representative for the bulk meteorite. Thus, in this work, the evolution of olivine in fusion crusts of the above‐listed selected samples is investigated. The different shape and chemistry of olivine crystallized in the fusion crust, both as overgrown rim on relic olivine clasts and as new crystals, suggest a general temperature and cooling rate gradient. The occurrence of reverse and oscillatory zoning in individual olivine grains within the fusion crust suggests complex redox reactions. Overall, the investigated fusion crusts exhibit a general oxidation of the relatively reduced initial material. However, evidence of local reduction is preserved. Reduction is likely triggered by the presence of carbon in the ureilite or by overheating during the atmospheric entry. Constraining these processes provides a potential analog for interpreting features observed in cosmic spherules and micrometeorites and for calibrating experiments and numerical models on the formation of fusion crusts.

Highlights

Meteorite fusion crust is a blackish layer that generally encloses meteorites and that consists of quenched melt formed by heating due to friction with air molecules during the atmospheric entry of meteoroids, which occurs at high velocities
We focus on olivine as a representative meteoritic material, as olivine occurs as a common rock-forming mineral in meteorites and the major component in the fusion crusts of the selected samples
The polished thin section was cut from this side, where the fusion crust has an average thickness of 1 mm

Summary

Introduction

Meteorite fusion crust is a blackish layer that generally encloses meteorites and that consists of quenched melt formed by heating due to friction with air molecules during the atmospheric entry of meteoroids, which occurs at high velocities (typically >10 km sÀ1; e.g., Love and Brownlee 1994). The thickness of the fusion crust is controlled by the composition of the meteoroid (i.e., stony versus iron); by the spinning of the meteoroid during the atmospheric entry; and by other factors, such as the entry angle and velocity. Fusion crust forms on all kinds of meteorites, but generally it is thicker on iron meteorites than on stony chondrites, possibly due to the high heat conductivity of metal (El Goresy and Fechtig 1967).

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