EXTRATERRESTRIAL NI-RICH SPINEL IN UPPER EOCENE SEDIMENTS FROM MASSIGNANO, ITALY

Evidence of a major cosmic event at the end of the Eocene is given by the finding of at least one, possibly two or more, impact horizons con taining microtektites, microkrystites, shocked quartz, and unusually h igh iridium concentrations. We report here the discovery, in the globa l stratotype of the Eocene-Oligocene boundary at Massignano in Italy, of Ni-rich spinel, a mineral that forms by the interaction of meteorit ic bodies with the Earth's atmosphere. A maximum concentration of appr oximate to 800 crystals mg(-1), corresponding to a flux of 2 x 10(7) c rystals cm(-2), is observed in a well-dated layer (35.7 +/- 0.4 Ma) th at also contains shocked quartz and iridium. The crystals are found cl ustered in flattened spheroids, the probable relic of their host bodie s. The chemical and physical characteristics of the spinel crystals in dicate that the spheroids formed by interaction of dust particles in t he upper part of the atmosphere rather than by ablation of large objec ts, as proposed for the spinel-bearing spheroids found at the Cretaceo us-Tertiary boundary. The most likely explanation is that the dust par ticles were generated by a large cometary impact. Whether this impact is the one that produced microtektites and/or microkrystites, or anoth er one that may have occurred shortly after or before, is still questi onable. Additional searching for Ni-rich spinel at other upper Eocene sections may help to answer this question.