Transmission electron microscopy of impact‐generated platinum group element alloys from Barberton spherule layers: New clues to their formation

Abstract

The oldest known large bolide impacts onto Earth are represented by approximately 3.47–3.2 Ga old Archean spherule layers of the Barberton Greenstone Belt (BGB) in South Africa and the Pilbara craton in West Australia. These layers were recognized as impact deposits by their excessively high platinum group element (PGE) contents that are indicative of an extraterrestrial component. This was followed by measurements of extraterrestrial Cr isotopic ratios, in some cases. Recently, the extraterrestrial PGE signature in Archean spherule layers from the BGB was localized and positively associated with the presence of submicrometer PGE alloy micronuggets associated with Ni,Cr‐rich spinel. The actual formation of these platinum group mineral (PGM) phases has, however, not yet been resolved. Primary meteoritic particles from the impacting body, the products of impact melting, or condensation from impact vapor plumes have all been proposed as possible genetic process. Resolving this requires detailed microanalytical investigation of the internal microchemical and microstructural compositions, textural characteristics, and crystallographic relationships between the different phases. Here, we report the results of a first transmission electron microscopy (TEM) study of six such PGE microparticles enclosed in Ni‐Cr spinel or occurring in groundmass of Barberton spherule layers from the BARB5 ICDP drill core and from the CT3 exploration core. Results include a variety of chemical and structural PGM compositions that are difficult to explain by a single process, leading to the conclusion that several processes may have been involved in the formation of PGMs in Archean spherule layers from the BGB. There is evidence supporting formation of these PGMs by exsolution from the spinel host phase, precipitation from a melt phase, and condensation from a gas phase (of the impact vapor plume).

Meteoritic highly siderophile element and Re‐Os isotope signatures of Archean spherule layers from the CT3 drill core, Barberton Greenstone Belt, South Africa

“…In this scenario, chromium spinel could have been associated in the spherules but could also have been incorporated into shales by postimpact wave action in the shallow sea, also noted by Mohr‐Westheide et al. (). However, the modal abundance of such spinels in our samples might not be high enough to fully explain the amount of meteoritic component present.…”

“…Notably, a recent study by Mohr‐Westheide et al. () confirmed an association of HSE phases and Ni‐Cr spinels in CT3 layers, which were found in spherule layers and country rocks (B‐SL5b, B‐SL8, B‐SH13, B‐SH19s, B‐SL10, and C‐SL17). They also presented HSE alloys as the reason of a nugget effect which causes these extreme HSE enrichments, together with secondary alterations such as HSE‐bearing sulfarsenides observed in spinel fragments.…”

“…(), point toward significant secondary overprint in the CT3 compared to the BARB5 spherule layer sections; All CT3 samples define rough linear mixing trends connecting impact‐unrelated Fig Tree Group sediments with chondrites in HSE interelement diagrams, strongly supporting the existence of varying amounts of an extraterrestrial component in the samples; The presence of near‐chondritic or even superchondritic HSE concentrations in spherule samples and country rocks (in close vicinity to the next spherule layer) suggests the presence of fragmented or otherwise crushed spherules in the country rocks, or postdepositional mechanical enrichment of chromium spinel (HSE carrier phases; Mohr‐Westheide et al. , ). Alternatively, superchondritic HSE abundances might be best a result of fractionation processes within the impact vapor plume, maybe accompanied by postdepositional mechanical or chemical enrichment of HSE carrier phases in the rocks, making quantification of inherited meteoritic admixtures problematic; Initial 187 Os isotope signatures of most samples (back‐calculated to ˜3.2 Ga) range from chondritic (˜0.105; Shirey and Walker ) to superchondritic (up to 0.165), arguing for a chondritic impactor (or a differentiated asteroid with similar bulk composition, possibly not represented in our present‐day meteorite inventory); Present‐day 187 Os values in some CT3 samples are slightly subchondritic, resulting from isotope evolution with a subchondritic Re/Os ratio, probably resulting from Re‐loss at the time of impact via volatilization, or in the wake of the impact event via hydrothermal activity or mechanical enrichment.…”

“…However, Mohr‐Westheide et al. (, ) reported the occurrence of HSE‐rich metal alloys associated with Ni‐rich chromium spinels within and detached from the spherules (i.e., embedded in the shale groundmass) in the BARB5 and CT3 spherule layer sections. They identified these alloys as the likely carrier phases of the extraterrestrial component.…”

“…Moreover, the country rocks include Ni‐rich chromium spinels (a suggested carrier phase of the meteoritic component in CT3 drill core; see Mohr‐Westheide et al. ), which are either detached from spherules or included in tiny spherule fragments (see also explanations in Ozdemir et al. ).…”