Petrology of Late Cenozoic Pampean impact melt breccias

HARRIS SCOTT; PETER SCHULTZ; MARCELO ZÁRATE

San Salvador de Jujuy

Congreso; XVII Congreso Geológico Argentino; 2008

Asociación Geológica Argentina

Over the last two decades Schultz and co-workers (e.g., Schultz et al, 1994, 1998, 2004, 2006, Harris et al., 2007a) have identified 8 impact melt breccia deposits in late Cenozoic strata of the Argentine Pampas. 40Ar/39Ar and fission track dating, combined with sequence and biostratigraphic controls, demonstrate that these deposits represent separate events between 9.24 Ma and 6 ka. Pampean impact breccias provide a natural laboratory for characterizing the petrography of impact ejecta formed during “soft target” sedimentary impacts. Although not abundant and heterogeneously distributed, our analysis has shown that traditional signatures of shock metamorphism (>3 GPa) do occur in these breccias. Examples include planar deformation features (PDFs) (Fig. 1) and coesite in quartz, ballen quartz, “asymmetric isotropization” of twinned feldspar grains (Fig. 2), and sigmoidally bent multiple-mechanical twinning (Fig. 3) in ilmenite (e.g., Harris et al., 2005 a, b, 2007a, Harris and Schultz, 2006). Indicators of ultra-high temperatures (>1700ºC) are more common and include lechatelierite, baddeleyite, and molten rutile. Evidence of extremely high-velocity, complex particle interactions (Fig. 4) also provide support for the confirmation of impact breccias formed from loosely consolidated sedimentary targets. Several Pampean melt breccias contain exotic clasts ranging from micron-size grains to mm-size rock fragments (Fig. 4, 5). Their unique mineralogy and geochemistry demonstrate that they are surviving extraterrestrial remains of the impactors. They have allowed us to conclude that the bolides responsible for the 3.27 Ma and 5.28 Ma impacts most closely resembled mesosiderites and angrites, respectively (Harris and Schultz, 2007a). Another feature of some Pampean melt breccias is their ability to trap target water in distinctive, discrete pockets of hydrous melt (Fig. 6). Particularly abundant in 445 ka breccias, we have determined that these melts contain from ~3 to 24 wt% (OH- + H2O) using transmission FTIR, reflectance FTIR, and SIMS (Harris and Schultz, 2005, Harris et al., 2007a). Although melts with such high volatile concentrations are quite different from tektites, which typically are extremely dry (<0.1% OH-), they are becoming more widely recognized in impact materials formed from water and/or carbonate-rich sediments (e.g., Osinski, 2003). Identification of impact ejecta (Fig. 7) surrounding the Pleistocene La Dulce structure (Harris et al., 2007b) suggests that the impact associated with those 445 ka glasses likely occurred in a saturated, riverine environment. We have found that many of the Pampean impact breccia deposits occur with other types of impact products including glass spherules, suevites, small tektites (Fig. 8), and unmelted ejecta. The composition of tektite-like objects associated with the 5.28 Ma melt breccias have allowed us to connect (Harris and Schultz, 2006) this Argentine impact event with Miocene-Pliocene boundary microtektites discovered by Kelly and Elkins-Tanton, (2004) south of Australia. The continued study of Pampean impacts is important for understanding the recent terrestrial impact record and increasing our knowledge of impacts into wet, loess targets on Earth and Mars. Most Pampean impact melts also managed to capture and preserve floral (Fig. 9), faunal, and microbial remains (Schultz and Harris, 2005, Harris et al., 2007) in a manner which experiments have shown likely cannot occur in melts formed by fires or volcanism (Harris and Schultz, 2007b). These glasses may be important to astrobiological investigations.