About Ga Asteroid Bombardment

The most intense meteoritic bombardment identified on the Earth to date is recorded by three 3.26-3.225-Ga multiple impact microkrystite spherule units at the base of the ~3.2-Ga Fig Tree Group, Barberton, eastern Transvaal (Fig. 8.4.A-C). The impact origin of these units is established by their domination by microkrystite spherules, microtektites, Ni-chromites, and 53Cr-52Cr isotope systematics diagnostic of chondrites (Lowe and Byerly, 1986; Lowe et al., 1989; 2003; Shukolyukov et al., 2000; Kyte et al., 2003). As pointed out by Lowe et al. (1989), the location of the impact cluster just above the contact between the mafic-ultramafic volcanic Onverwacht Group and the overlying felsic volcanic and turbidites of the Fig Tree Group may be significant, hinting at major crustal transformation triggered by the impacts. A similar change is recognized in the Pilbara as the transition from the Sulphur Springs Group (Kangaroo Cave Formation) to the Gorge Creek Group (Pincunah Hill Formation). No impact spherules were encountered in the latter unit, whose basal section contains a local olistostrome breccia (Fig. 8.4.C) derived from reactivation bounding faults, possibly as a result of impact-triggered crustal disturbance.

Mass balance calculations based on the Iridium anomalies and spherule size of the impact fallout units (Melosh and Vickery, 1991) indicate asteroids several tens of kilometer in diameter, consistent with impact craters on the scale of 400-600 km (Byerly and Lowe. 1994; Shukloyukov et al., 2000; Kyte et al., 2003; Lowe et al., 2003; Glikson and Allen, 2004). The mafic chlorite-dominated composition of the spherules and the absence of shocked quartz grains militates for derivation of the ejecta from simatic/oceanic loci (Simonson et al., 1998). Siderophile element (Ni, Co), ferrous elements (Cr, V), and Platinum Group Element (PGE) patterns of least-altered microkrys-tite (impact-condensate) spherules and microtektites are consistent with a mafic/ultramafic composition of impact target crust (Glikson and Allen, 2004). A domination of mafic/oceanic crust during the Archaean, estimated as >80% of the Earth crust from models based on rare earth elements (REE) (Taylor and McLennan, 1981) and Sm-Nd isotopes (McCulloch and Bennett, 1994), and encompassing hundreds of kilometer-scale impact basins, is inconsistent with uniformitarian geodynamic models based exclusively on plate tectonic processes.

The spherule data suggest multiple impacts about 3.26-3.225 Ga (Byerly and Lowe, 1994; Shukloyukov et al., 2000). Deposition of S1, S2, and S3

Fig Tree Group Barberton

Fig. 8.4A—C. About 3.24-Ga microkrystite spherules of the S3 impact fallout unit, Mapepe Formation, lower Fig Tree Group, Barberton Mountain Land, eastern Transvaal. (A) right hand - carbonate-rimmed chlorite spherule; center spherule -carbonate dominated; groundmass consists of spherule fragments and iron oxides; plane polarized light (ppl); (B) chlorite-dominated spherules (ppl); (C) reflected light microphotograph of a chlorite-dominated spherules containing a central vesicle and Nickel-rich chromites.

Fig. 8.4A—C. About 3.24-Ga microkrystite spherules of the S3 impact fallout unit, Mapepe Formation, lower Fig Tree Group, Barberton Mountain Land, eastern Transvaal. (A) right hand - carbonate-rimmed chlorite spherule; center spherule -carbonate dominated; groundmass consists of spherule fragments and iron oxides; plane polarized light (ppl); (B) chlorite-dominated spherules (ppl); (C) reflected light microphotograph of a chlorite-dominated spherules containing a central vesicle and Nickel-rich chromites.

spherule units was disturbed by impact-generated tsunami currents, including multiple tsunami wave fronts. These tsunamis may have promoted mixing within a globally stratified ocean, enriching surface waters in nutrients for biological communities (Lowe et al., 2003). This impact period may correlate with a major 40Ar-39Ar 3.18-Ga age distribution peak measured by Culler et al. (2000) in a small sample of lunar impact spherules (Fig. 8.2.C).

Imajenes Recientes Luna

Fig. 8.4D. Olistostrome consisting of boulders (OL) of chert and volcanic rocks several tens of meters across within fragmental conglomerate of the Pincunah Formation, overlying a ~3.24-Ga chert marker that marks the boundary between an underlying volcanic sequence (Sulphur Springs Group) and an overlying turbidite-banded ironstone sequence (Gorge Creek Group) - correlated with the ~3.24-Ga impact cluster of the lower Fig Tree Group, eastern Transvaal. The olistostrome may represent tectonic movements related to the impact cluster.

Fig. 8.4D. Olistostrome consisting of boulders (OL) of chert and volcanic rocks several tens of meters across within fragmental conglomerate of the Pincunah Formation, overlying a ~3.24-Ga chert marker that marks the boundary between an underlying volcanic sequence (Sulphur Springs Group) and an overlying turbidite-banded ironstone sequence (Gorge Creek Group) - correlated with the ~3.24-Ga impact cluster of the lower Fig Tree Group, eastern Transvaal. The olistostrome may represent tectonic movements related to the impact cluster.

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