FACTORS CONTROLLING COMPOSITIONS OF COSMIC SPINELS: APPLICATION TO ATMOSPHERIC ENTRY CONDITIONS OF METEORITIC MATERIALS

Abstract

During their deceleration through the Earth's atmosphere, meteoritic materials, i.e., interplanetary dust particles, micrometeorites and meteorites, experience thermal shocks which may alter their pristine mineralogy, texture or chemical characteristics. Among these changes, one of the most ubiquitous is the formation of spinels resulting from partial melting and subsequent crystallization of the meteoritic material. These ''cosmic spinels'' differ from terrestrial spinels by their high Ni and Fe3+ contents and show large variations in composition. In order to better understand the factors controlling their chemistry, pulse-heating experiments simulating atmospheric entry of extraterrestrial objects were carried out using Orgueil samples as proxies of meteoritic material. Covering a large range of experimental conditions (temperature 500°C < T <1500°C, duration: 5 s < t < 120 s, and oxygen fugacity: -0.68 < log fO2 < -8), this work shows (1) that the whole range of composition of cosmic spinels analyzed so far at the micrometer scale in fine-grained and scoriaceous micrometeorites, in cosmic spherules or in the fusion crust of several stony meteorites can be reproduced, and (2) that these compositional changes can be expressed as a function of temperature, time and oxygen fugacity.We also show that, due to their fast crystallization kinetics, cosmic spinels can record through their composition, i.e., Al2O3 contents and FeO/Fe2O3 ratio, the diverse conditions of the atmosphere crossed by the extraterrestrial object during its fall towards the Earth's surface. Chemistry of cosmic spinels is thus a powerful tool for constraining the entry conditions in the Earth's atmosphere of any extraterrestrial object, including altitude of deceleration, entry angle and incident velocity. These in turn, may provide valuable information on the origin of the extraterrestrial material.