Discovery of extraterrestrial component carrier phases in Archean spherule layers: Implications for estimation of Archean bolide sizes

Abstract

In the Barberton greenstone belt (BGB) of South Africa, four distinct spherule horizons (S1–S4) with ages between ca. 3.5 and 3.2 Ga are among the oldest records of large asteroid impacts on Earth. Spherules in these layers are interpreted as molten impact ejecta, condensation products from impact plumes, or impact ejecta melted during atmospheric reentry. Past research has shown that some spherule layer samples from the BGB carry ultrahigh abundances of siderophile elements, including platinum group elements (PGEs), in some cases as much as four times the chondritic abundances. Inferences for very large projectile sizes responsible for these impact layers have been made on the strength of these data. Drilling by the International Continental Scientific Drilling Program has yielded 4 new spherule layer intersections in a 22-cm-long core segment. Scanning electron microscopy–energy dispersive X-ray spectrometry at high spatial resolution identified local areas of PGE enrichment with Ni-rich chromium spinel (Ni-Cr spinel) clusters. They are associated with PGE-rich metal alloy and sulfarsenide phases 600–1400 nm in size. The metal alloys are interpreted as primary objects that formed during the impact process. PGE sulfarsenides are the result of secondary alteration by S- and As-rich fluids. Thus, a micronugget effect caused by PGE phases and Ni-Cr spinel is responsible for the anomalously high extraterrestrial component in some spherule layer samples. This heterogeneous incorporation of meteoritic components due to primary heterogeneous fallout from the vapor plume must be taken into consideration in any attempt to estimate the global fallout of extraterrestrial components and, thus, to constrain projectile sizes.