THE DISTRIBUTION OF MICROSCOPIC ASTEROIDAL AND COMETARY DEBRIS IN MELTS FORMED DURING AIRBURSTS AND SOFT SEDIMENT IMPACTS

The melting of rock and regolith by airbursts and impacts into soft sedimentary targets often produce vesiculated and scoriaceous rocks that resemble materials formed by a variety of terrestrial processes from volcanism to pyrometamorphism. Due to the generally low target impedance and the resulting low peak shock pressures and ultra-high waste heat (conducive to both melting and annealing), the identification of unambiguous shocked minerals is a time-intensive and sometimes futile exercise. When shocked phases are observed, typically they are in entrained ejecta clasts derived from depth during crater excavation, not expected in airburst melts.

Our identification of abundant sand and silt-sized meteoritic microclasts in melt breccias from Bahía Blanca, Argentina (Harris and Schultz, LPSC-50, 2019) and Pica, Chile (Harris et. al, GSA, 2018) provide a method to demonstrate impact provenance, identify impactors, and investigate parent body petrogenesis. In the Argentine example, most meteoritic debris is partially digested and distributed around the outside of melt pods. This occurrence indicates that molten ejecta collected the clasts as they flowed outward along the walls of a crater or while launching through the impact plume. In the Chilean case, a greater portion of meteoric grains are pristine, concentrated along vesicles walls, and increase in abundance toward the top of breccias. They most likely represent dust from the trailing bolide debris stream that was driven down into the quenching melts (Schultz et al., LPSC-50, 2019).

These observations suggest that the presence and distribution of such clasts in enigmatic melts might be used to confirm impact origins and to distinguish crater-forming impacts from low-altitude airbursts. We are analyzing a broad suite of melt breccias, including those from Dakhleh, Edeowie, and Darwin. The meteoritic contamination of the Dakhleh glasses (in particular) is remarkably similar to the Pica glasses, thereby supporting the conclusion by Osinski et al. (MAPS, 2008) that they resulted from a low-altitude airburst, perhaps of a fragmental body compositionally related to the Pica bolide. Preliminary results from Edeowie and Darwin glass also contain preserved meteoritic microclasts and may yield important insights into the impactor flux and impactor origins in the Pleistocene.