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

Archean spherule layers represent the only currently known remnants of the early impact record on Earth. Based on the lunar cratering record, the small number of spherule layers identified so far contrasts to the high impact flux that can be expected for the Earth at that time. The recent discovery of several Paleoarchean spherule layers in the BARB5 and CT3 drill cores from the Barberton area, South Africa, drastically increases the number of known Archean impact spherule layers and may provide a unique opportunity to improve our knowledge of the impact record on the early Earth. This study is focused on the spherule layers in the CT3 drill core from the northeastern Barberton Greenstone Belt. We present highly siderophile element (HSE: Re, Os, Ir, Pt, Ru, and Pd) concentrations and Re-Os isotope signatures for spherule layer samples and their host rocks in order to unravel the potential presence of extraterrestrial fingerprints within them. Most spherule layer samples exhibit extreme enrichments in HSE concentrations of up to superchondritic abundances in conjunction with, in some cases, subchondritic present-day Os-187/Os-188 isotope ratios. This indicates a significant meteoritic contribution to the spherule layers. In contrast to some of the data reported earlier for other Archean spherule layers from the Barberton area, the CT3 core is significantly overprinted by secondary events. However, HSE and Re-Os isotope signatures presented in this study indicate chondritic admixtures of up to (and even above) 100% chondrite component in some of the analyzed spherule layers. There is no significant correlation between HSE abundances and respective spherule contents. Although strongly supporting the impact origin of these layers and the presence of significant meteoritic admixtures, peak HSE concentrations are difficult to explain without postdepositional enrichment processes

Major and trace element compositions of melt particles and associated phases from the Yaxcopoil-1 drill core, Chicxulub impact structure, Mexico

Melt particles found at various depths in impactites from the Yaxcopoil-1 borehole into the Chicxulub impact structure (Yucatán) have been analyzed for their major and trace element abundances. A total of 176 electron microprobe and 45 LA-ICP-MS analyses from eight different melt particles were investigated. The main purpose of this work was to constrain the compositions of precursor materials and secondary alteration characteristics of these melt particles. Individual melt particles are highly heterogeneous, which makes compositional categorization extremely difficult. Melt particles from the uppermost part of the impactite sequence are Ca- and Na-depleted and show negative Ce anomalies, which is likely a result of seawater interaction. Various compositional groupings of melt particles are determined with ternary and binary element ratio plots involving major and trace elements. This helps distinguish the degree of alteration versus primary heterogeneity of melt phases. Comparison of the trace element ratios Sc/Zr, Y/Zr, Ba/Zr, Ba/Rb, and Sr/Rb with compositions of known target rocks provides some constraints on protolith compositions; however, the melt compositions analyzed exceed the known compositional diversity of possible target rocks. Normalized REE patterns are unique for each melt particle, likely reflecting precursor mineral or rock compositions. The various discrimination techniques indicate that the highly variable compositions are the products of melting of individual minerals or of mixtures of several minerals. Small, angular shards that are particularly abundant in units 2 and 3 represent rapidly quenched melts, whereas larger particles (>0.5 mm) that contain microlites and have fluidal, schlieric textures cooled over a protracted period. Angular, shard-like particles with microlites in unit 5 likely crystallized below the glass transition temperature or underwent fragmentation during or after deposition.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202

Geochemistry of 2.63-2.49Ga impact spherule layers and implications for stratigraphic correlations and impact processes

Thin layers rich in spherules formed during impacts by large extraterrestrial objects have the potential to shed new light on impact processes and aid in the long-distance stratigraphic correlation of Precambrian successions. Seven formations in Western Australia and South Africa clustered around the Archean-Proterozoic boundary each contain a single spherule layer, all of which were deposited between ca. 2.49 and 2.63Ga. Analyses of 25 samples from 6 of the 7 spherule layers and 23 samples from closely associated strata free of spherules revealed an extraterrestrial component (ETC) in all six layers, based on PGE ratios and/or Cr isotopic composition. The amount of ETC varies from marginally detectable to clear and obvious; it generally amounts to ca. a few percent. Both PGE ratios and Cr isotopic anomalies indicate most if not all of the impactors were ordinary chondritic in composition. In contrast, all spherule layers older than ca. 3.0Ga that have been analyzed were produced by carbonaceous chondritic impactors. Normalized rare earth element patterns suggest most target rocks were basaltic in composition and variability in PGE ratios favors ballistic emplacement of melt droplets rather than spherule formation via vapor condensation. The geochemical data also provide a means to test proposed intra- and intercontinental stratigraphic correlations. Cr isotopic compositions are consistent with the formation of the oldest layers on both continents by a single impact event about 2.63Ga. In contrast, clear geochemical differences between the middle layers, both deposited ca. 2.54Ga, suggest they were not produced by the same impact event. The youngest, banded iron formation-hosted layers on both continents have also been correlated, but no geochemical data are available on the African layer to test this at present

Is Bedout an Impact Crater? Take 2

In their Research Article “Bedout: a possible end-Permian impact crater offshore of northwestern Australia,” L. Becker et al. report having identified a buried impact structure, which they link to the Permian-Triassic mass extinction (4 June, p. 1469; published online 13 May; 10.1126/science.1093925). Becker et al. have scarcely extended the suggestion made by Australian petroleum workers (in industry trade journals). Our scrutiny of the alleged evidence indicates that there is no substantiation that this alleged structure is an impact crater. The gravity map (fig. 11) actually highlights the differences between Bedout and confirmed impact structures. There is actually no crater defined by the geophysical data, only a noncircular high in the seismic data, claimed to be a “central uplift.” In comparison, the central uplift feature of a large impact structure, such as the 250- to 300-km-diameter Vredefort Structure, would reveal a significant central positive gravity anomaly due to the uplift of relatively denser mid- to lower crustal material. The highly altered rocks described by Becker et al. as impact products strongly resemble volcanic breccias and lack impact diagnostic textures. No true shock features are described from any of the samples. No mineralogical or geochemical evidence is provided that the purported “diaplectic glass” or “maskelynite” are indeed glasses, and mineral chemical information is missing. The “shock features” claimed to be presented in quartz grains from “ejecta horizons” (which remain of uncertain stratigraphic relation either to the alleged Bedout feature or to the end-Permian extinction) do not show any of the characteristics of unambiguous shocked minerals

Systematic survey of K, Th, and U signatures in airborne radiometric data from Australian meteorite impact structures : possible causes of circular features and implications

International audienceAirborne radiometric (gamma-ray) data provide estimates of the concentrations of potassium (K), thorium (Th), and uranium (U) in soil, regolith, and bedrock. Radiometric data constitute an important source of geochemical information, commonly used in mineral exploration and for geological mapping of Earth and other planets. Airborne radiometric data have rarely been applied to the exploration and analyses of impact structures, in contrast with other conventional geophysical tools (e.g., gravimetry, magnetism, and seismic reflection/refraction). This work represents the first systematic survey of the K, Th, and U radiometric signatures of Australian impact structures, based on the continent-wide airborne radiometric coverage of Australia. We first formulated several hypotheses regarding the possible causes of formation of circular radiometric patterns associated with impact structures. Then, the radiometric signatures of 17 exposed impact structures in Australia were documented. Our observations confirmed the supposition that impact structures are commonly associated with circular radiometric patterns. We then selected the five structures with the most prominent circular radiometric patterns (Gosses Bluff, Lawn Hill, Acraman, Spider, and Shoemaker), and we discuss the possible origin of these anomalies. Based on these five case studies, we argue that such patterns result from either crustal deformation induced by the impact event and/or from postimpact superficial processes controlled by the crater topography. This work also suggests that airborne radiometric data may be useful, in combination with other geophysical tools, in the search for new possible impact structures

The origin of the potassium-rich annular zones at the Bosumtwi impact structure, Ghana, investigated by field study, radiometric analysis, and first cosmogenic nuclide data

International audienceThe 10.5-km-diameter, 1 Ma Bosumtwi impact structure in Ghana is one of the youngest, large impact structures known on Earth. The preservation of the morphology of its ejecta deposits, with an annular moat and outer ridge resembling those of rampart impact craters on Mars, makes Bosumtwi a remarkable impact structure on the African continent. An airborne radiometric survey of the southwestern part of Ghana reveals enigmatic circular feature enriched in potassium, coinciding with the crater rim and an outer ejecta ridge at Bosumtwi. The goal of this study is to investigate possible origins of these features, by impact processes (shock metamorphic effects, impact-induced hydrothermal systems) or postimpact surficial processes (erosion, weathering). The origin of these features is discussed here based on field observations, ground-based radiometric measurements, and first cosmogenic nuclide analyses (10Be). The data indicate that the rim and outer ridge were eroded more rapidly than the rest of the impact structure. Accordingly, the downward advance of the weathering fronts in the annular moat, after ejecta emplacement, are responsible for leaching of K from the lateritic residual observed at the surface. The Bosumtwi impact structure is, therefore, a valuable natural laboratory to investigate the factors controlling erosion and weathering processes in the Ashanti belt since impact 1 Ma ago. Simulations of vertical profiles of 10Be concentration further constrain local variations of the erosion rate. In light of this study, circular K anomalies in radiometric surveys might be indicative of potential impact structures in tropical regions

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

Archean spherule layers represent the only currently known remnants of the early impact record on Earth. Based on the lunar cratering record, the small number of spherule layers identified so far contrasts to the high impact flux that can be expected for the Earth at that time. The recent discovery of several Paleoarchean spherule layers in the BARB5 and CT3 drill cores from the Barberton area, South Africa, drastically increases the number of known Archean impact spherule layers and may provide a unique opportunity to improve our knowledge of the impact record on the early Earth. This study is focused on the spherule layers in the CT3 drill core from the northeastern Barberton Greenstone Belt. We present highly siderophile element (HSE: Re, Os, Ir, Pt, Ru, and Pd) concentrations and Re‐Os isotope signatures for spherule layer samples and their host rocks in order to unravel the potential presence of extraterrestrial fingerprints within them. Most spherule layer samples exhibit extreme enrichments in HSE concentrations of up to superchondritic abundances in conjunction with, in some cases, subchondritic present‐day 187Os/188Os isotope ratios. This indicates a significant meteoritic contribution to the spherule layers. In contrast to some of the data reported earlier for other Archean spherule layers from the Barberton area, the CT3 core is significantly overprinted by secondary events. However, HSE and Re‐Os isotope signatures presented in this study indicate chondritic admixtures of up to (and even above) 100% chondrite component in some of the analyzed spherule layers. There is no significant correlation between HSE abundances and respective spherule contents. Although strongly supporting the impact origin of these layers and the presence of significant meteoritic admixtures, peak HSE concentrations are difficult to explain without postdepositional enrichment processes.© 2019 The Author