SPATIAL DISTRIBUTION OF METEORITIC CARRIER PHASES IN PALEOARCHEAN SPHERULE LAYERS FROM SOUTH AFRICA

Paleoarchean impact spherule layers from the Barberton Greenstone Belt have been known since the late 1980s; Highly siderophile element (HSE) abundances were recently reported for some new locations by our group (e.g., Schulz et al., 2017; Oezdemir et al., 2019, Meteoritics and Planetary Science, doi: 10.1111/maps.13234). These studies revealed meteoritic components within bulk samples consisting of spherule-groundmass assemblages (mixtures of spherules and shales of the Fig Tree Group). Reported near-chondritic HSE concentrations and interelement ratios, however, invoke the question, whether or not the carriers of the extraterrestrial admixtures are confined to the spherules, or if a significant portion is hosted within the spherule-free groundmass. Petrographic inspections and detailed element mapping revealed the occurrence of grains of PGE alloys and Ni-rich Cr spinels, suspected carrier phases of the chondritic component, hosted within and outside the spherules (e.g., Mohr-Westheide et al., 2018, Meteoritics and Planetary Science 53, 1516-1536). We calculate that it would require 0.01 to 1 ppm of such carrier phases within the bulk samples in order to explain the enormous HSE concentrations reported by Schulz et al. (2017, Geochimica et Cosmochimica Acta 211, 322-3407). This might be a reasonable result, supporting the hypothesis that Ni-rich Cr-spinel-related PGE phases are the long sought meteoritic carrier phase.

In an attempt to further address the question of the spatial distribution of the meteoritic carriers within spherule layers from the Barberton area, we report on HSE analyses of pure spherule and groundmass separates of a bulk sample from the BARB5 drill core (Schulz et al., 2017, op.cit.). Our preliminary results reveal that the groundmass separate exhibits consistently higher PGE concentrations compared to the pure spherule sample (e.g., 171 ppb Os vs. 84 ppb and 205 ppb Ir vs. 70 ppb), but that both separates contain enormous HSE enrichments compared to non-impact related sediments from the region. This finding, in conjunction with the earlier published in-situ investigations, may provide further constraints for understanding spherule formation within the impact vapor plume and the subsequent emplacement process.