Composition and Origin of Ni-rich Spinel from Cretaceous-tertiary Boundary

Introduction: Extraterrestrial spinel is a magnetic mineral that is found in melted micrometeorites, in the fusion crust of meteorites and meteoroid ablation droplets, and in various impact-derived spherules more or less preserved in marine sediments. This mineral can be distinguished from terrestrial spinel on the basis of high Ni content and a high ferric/ferrous ratio resulting from crystallization in an O 2-rich environment, i.e. the Earth's atmosphere [1-4]. Ni-rich spinel is a specific marker of extraterrestrial material which can be used as a tracer of cosmic events in the sedimentary record. Previous Work: The existence of Ni-rich spinel crystals in K/T boundary clays was revealed by [5, 6]. Regional and local (i.e., from site to site) variations in spinel compositions have been also reported (e. It has been shown that the composition of spinel depends primarily on ambient oxygen fugacity and on the initial composition of the crystallizing material [3,4], but their exact formation mechanism is still controversial (e.g., [3, 9, 10]). Two different modes of formation are invoked: condensation in the impact vapor plume [7] and atmospheric ablation of the projectile [1]. Recently, [11, 12] have shown that some impact spinel, so far only for K-T boundary spinel crystals from Bidart (France) and Caravaca (Spain), display compositional zoning from core to rim, with a core depleted in Fe, Ni, and Ti, and enriched in Cr, Al, and Mg. Similar zonations have been reported in spinel crystals present in impact horizons of Upper Eocene sections [13] and from meteorite fusion crusts [3, 12, 14], but these have never been studied in detail. Compositional zoning was recognized using a scanning electron microscope (SEM), [11, 12]. The contact between the Cr-rich core and the Fe-rich rim is very well defined (i.e., sharp), in both, impact spinel and spinel from meteorite fusion crust. Additionally, the depletion in nickel observed in the core of the crystals indicates that the chemical zoning is primary rather than the result of alteration. Ferrière and Robin [11, 12] have suggested that either the variation of the Fe