METEORITIC MICROCLASTS AND ULTRA-HIGH TEMPERATURE DECOMPOSITION OF REFRACTORY PHASES: CONFIRMATION OF AN IMPACT PETROGENESIS FOR PICA GLASSES, ATACAMA DESERT, CHILE

Blanco and Tomlinson (2013) suggested that glasses deposited across the desert near Pica Oasis were formed by a meteoric air blast above the Atacama at the terminal Pleistocene. We collected samples from the Puquio de Nuñez (PN) and Quebrada de Chipana (QC) sites and prepared more than thirty polished sections for microanalysis using a Hitachi SU-3500 VP-SEM at FSC. Chemical compositions of selected phases were determined using an EDAX Element EDS detector. Our observations confirm that Pica glasses formed by melting sediments at temperatures exceeding 1700°C in the presence of a plume of fine meteoritic debris.

Green schlieren-rich glasses from PN are microlite-poor and contain spherical, smooth-walled vesicles toward their tops. The bottoms are marked by elongated vesicles indicative of flow and increased microlites in response to H₂O diffusion fronts rising from wet sediments on which the melts were emplaced. The smooth-walled vesicles contain euhedral troilite crystals, commonly containing 0.5 to 2 wt% Ni. Troilite may be intergrown with Ni-free pyrrhotite and an Fe-Ni-Cu-S phase (close to Fe-rich pentlandite). The sulfides often occur with buchwaldite (NaCaPO₄), an association reported only from extraterrestrial sources.

Although similar assemblages are observed in some glasses from QC, these melts cooled more slowly and allowed time for Ni to escape. Slower quench rates also are demonstrated by abundant microlites in these brownish glasses. The peak temperatures can be estimated from relict zircons partially and completely decomposed to baddeleyite. In melts of similar composition, the transformation to ZrO₂ requires T>1700°C. A few zircon and monazite grains exhibit planar features diagnostic of shock. These glasses also contain other likely meteoritic clasts, including troilite-veined feldspathic chondrules and possible CAIs. The Pica event may have involved a volatile-rich primitive body.

Our conclusions contrast with the grass fires of Roperch et al. (2017) who state that all sulfides in the glass are Ni-free and that the zircon decomposition temperature was never reached, presumably based on a limited number of analyzes. Our microanalysis demonstrates that Pica glasses were formed in an impact event. The evidence that the event was a major air burst is discussed by Schultz et al. (this volume).