A new ~3.46 Ga asteroid impact ejecta unit at Marble Bar, Pilbara Craton, Western Australia: A petrological, microprobe and laser ablation ICPMS study
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The Archean rock record contains seventeen asteroid impact ejecta units that represent the terrestrial vestiges of an extended late heavy bombardment (LHB). Correlated impact ejecta units include 3472–3470 Ma impact spherule layers in the Barberton Greenstone Belt, Kaapvaal Craton, South Africa, and the Pilbara Craton, Western Australia, and several ejecta units dated between 3250 and 3220 Ma and between 2630 and 2480 Ma (Lowe and Byerly, 2010; Lowe et al., 2003, 2014). This paper reports the discovery and investigation of a new impact ejecta unit within the Marble Bar Chert Member (MBCM) of the felsic volcanic Duffer Formation, east Pilbara Craton, Western Australia. The age of the MBCM is constrained by a 3459 ± 2 Ma U–Pb zircon date from the uppermost volcanic unit of the Duffer Formation and by a 3449 ± 3 Ma U–Pb zircon date from the overlying felsic volcanic Panorama Formation, stratigraphically above the intervening un-dated Apex Basalt. The ejecta unit, observed in a drill core (ABDP 1) ∼4 km south-southwest of Marble Bar, consists of multiple lenses and bands of almost totally silicified impact spherules 1–2 mm in diameter. All internal primary textures of the spherules have been destroyed. Nonetheless, Fe-rich spherule rims, largely composed of secondary siderite, are well preserved. Chemical analyses of the rims reveal iron-magnesium carbonate displaying high Fe, Mg, Ni, Co and Zn.Whole-rock and in-situ analyses (X-ray fluorescence, Inductively Coupled Plasma Mass Spectrometry (ICPMS), electron-microprobe (EMP) and EMP-calibrated laser ICPMS) reveal that the rims contain high Ni abundances and high Ni/Cr ratios (<50). The spherules are separated by an arenite matrix and spherule lenses also occur within bedded chert. The spherules are particularly visible over some ∼14 m of true stratigraphic thickness in which chert breccia is interpreted to represent a tsunami-generated diamictite affected by hydrothermal fragmentation and veining. Despite the almost total silicification of the MBCM whole-rock analysis by NIS Fire Assay and ICPMS indicates high Ir (2 ppb) and a low Pd/Ir ratio (2.0), consistent with geochemical features of impact ejecta units. Dense concentrations of spherules at the 57–58 m level and the 77 m level of the core, separated by banded chert, raise the possibility of two distinct impact events. Stratigraphic and isotopic age data distinguish between the 3459 and 3449 Ma age of the MBCM ejecta unit and ∼3470.1 ± 1.9 Ma impact ejecta units in the Antarctic Creek Member, Mount Ada Basalt, about 40 km to the west of Marble Bar. In combination with a 3472 ± 2.3 Ma impact unit in the Barberton greenstone belt, these impact ejecta units record large Paleoarchean asteroid impacts, significant for understanding early bombardment rates on Earth and early crustal evolution.
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