Magnetic Fe,Si,Al-rich Spherules From the P-T Boundary Layer at Graphite Peak, Antarctica

Recently we have reported on the discovery of meteoritic fragments of the CM chondrite affinity in two samples (#2060 and #A315, see Basu et al, submitted to Science and Basu et al., this meeting) from the P-T boundary at Graphite Peak, Antarctica. Here we report on meteorite fragments from a third sample (#A314) from the P-T boundary in this location associated with numerous small (0.018 - 0.053 mm across) vesicular spherules enriched in SiO2 (32 - 60 wt %), Al2O3 (7 - 22 %), and FeO (12 - 62 %). Most spherules typically consist of a silicate glass which often contains either blocky crystals of a SiO2 polymorph in the SiO2-enriched spherules or skeletal magnetite crystals in the FeO-enriched spherules. Few spherules consist mainly of magnetite with rare interstitial patches of a silicate glass. Although the chemical composition of the glass varies from spherule to spherule, the high concentrations of TiO2 (0.48 - 2.48 %), Al2O3, and K2O (0.58 - 3.10 %) along with the lack of Cr2O3, MnO, and NiO unequivocally point to the terrestrial crustal origin of the spherules. Moreover, high FeO and low CaO (0.75 - 4.93 %) concentrations suggest either a sedimentary or metamorphic rather than an igneous precursor. The wide compositional variations along with the quenched textures of the spherules are consistent with their impact origin. Thus, the association of the spherules with meteorite fragments at the P-T boundary lends further support to the catastrophic collision of Earth with a large celestial body in the end-Permian which might have been responsible for the global extinction event at the P-T boundary.