IMPACTOR FRAGMENTS FROM CRATERS KANMARE AND TABBAN IN THE GULF OF CARPENTRIA

Two crater candidates have been identified in the southeastern region of the Gulf of Carpentaria, Australia [1]. The focus of my research is to confirm these candidates as craters based on the analysis of material found in core samples from the region. Specifically, I will be looking for meteorite fragments. We first analyzed samples from four cores for magnetic susceptibility in order to find likely depths at which impact material might be found, as impact layers typically have higher magnetic susceptibilities relative to surrounding layers. Based on graphs of magnetic susceptibility, we then sieved samples into three or four size fractions: >250 microns, >150 microns, >63 microns and >38 microns.  The 250 micron sieves were only used on samples that appeared to have large portions of very coarse-grained material or large shelly fragments. Potential impactor fragments, as well as impact glasses, melts and spherules were then placed on SEM mounts for analysis. Using the X-ray analyzer on the SEM, we found several objects that appear to be fragments of the impactor. One grain analyzed as Fe-Ni, with three distinct Ni peaks in its spectrum. This piece was bright, visibly metallic and appeared smoothed as though partially melted. Moreover, it analyzed as having two distinct Cr peaks in its spectrum. This may indicate a chondritic origin, as chromium is relatively high in carbonaceous chondrites. The X-ray analysis provided percent atomic weights of 7% for Ni, 12% for Cr, 43% for Fe, 12% for O and 1% for S for this object. This is consistent with the grain being highly metallic. In addition to the Ni-bearing Fe grain, two partially melted pieces analyzed as being calcium phosphate with clear peaks in both Na and Mg. This is indicative of the mineral merrillite, a known extraterrestrial mineral. Additionally, we have found potential melted carbonaceous chondrite fragments. Further evidence that these are legitimate impact craters can be found in the widespread occurrence of impact spherules [2]. The merrillite specimens in particular have been found in the same cores as those containing impact spherules. [1] Abbott, et al. 2006, this volume. [2 ] Martos, et al. 2006 this volume.