Spinel Formation in an Impact Plume: a Thermodynamic

Introduction. Nickel-rich spinel is recognized as a cosmic event marker [1-3,5]. Spinel is found in K/T boundary sections all over the world [1-3,14], in lower-middle Jurassic hardground [13], in late Pliocene sediments [4] and in upper Eocene sediments [12]. Its composition is characterized by a high nickel oxide concentration (NiO>1 wt%) and a high iron oxidation state (Fe/Fetot>70 mole%) involving formation in an oxygen-rich environment [9]. Robin et al. [5] have shown that oxidation of meteoroids and impact generated products in the atmosphere leads to the formation of Ni-rich spinel. Direct condensation from a gas phase [3] or crystallization from a high temperature melt [1,2,7] generated by a meteorite impact have also been suggested as potential sources for Ni-rich spinel. However, there is to date no compelling evidence of spinel condensation/crystallization from an impact plume. Meteorite impacts produce high temperature and pressure plumes that rapidly expand. During this stage, temperature and pressure decrease allowing condensation reactions to occur. As these conditions are not reproducible experimentally, the only way to determine the condensation sequence is thermodynamic modeling. We report here the first numerical simulation performed on the physical and chemical reactions that occur in an impact plume, depending on temperature and pressure conditions. The purpose of this study is to determine whether the plume can provide the kind of environment (temperature, pressure, oxygen fugacity) needed for the formation of Ni-rich spinel.