Spherule Beds 3.47–3.24 Billion Years Old in the Barberton Greenstone Belt, South Africa: A Record of Large Meteorite Impacts and Their Influence on Early Crustal and Biological Evolution

Abstract

Four layers, S1-S4, containing sand-sized spherical particles formed as a result of large meteorite impacts, occur in 3.47-3.24 Ga rocks of the Barberton Greenstone Belt, South Africa. Ir levels in S3 and S4 locally equal or exceed chondritic values but in other sections are at or only slightly above background. Most spherules are inferred to have formed by condensation of impact-produced rock vapor clouds, although some may represent ballistically ejected liquid droplets. Extreme Ir abundances and heterogeneity may reflect element fractionation during spherule formation, hydraulic fractionation during deposition, and/or diagenetic and metasomatic processes. Deposition of S1, S2, and S3 was widely influenced by waves and/or currents interpreted to represent impact-generated tsunamis, and S1 and S2 show multiple graded layers indicating the passage of two or more wave trains. These tsunamis may have promoted mixing within a globally stratified ocean, enriching surface waters in nutrients for biological communities. S2 and S3 mark the transition from the 300-million-year-long Onverwacht stage of predominantly basaltic and komatiitic volcanism to the late orogenic stage of greenstone belt evolution, suggesting that regional and possibly global tectonic reorganization resulted from these large impacts. These beds provide the oldest known direct record of terrestrial impacts and an opportunity to explore their influence on early life, crust, ocean, and atmosphere. The apparent presence of impact clusters at 3.26-3.24 Ga and approximately 2.65-2.5 Ga suggests either spikes in impact rates during the Archean or that the entire Archean was characterized by terrestrial impact rates above those currently estimated from the lunar cratering record.

Sedimentology of the ∼3.3 Ga upper Mendon Formation, Barberton Greenstone Belt, South Africa

“…These spherule beds are not a focus of this study, but rare impact spherules are observed in some intraclast breccias of the upper Mendon Formation. These spherules are texturally similar to those described previously (Lowe and Byerly, 1986;Lowe et al, 2003). Spherules have nearly circular cross sections and are typically composed of microcrystalline quartz and phyllosilicates (cf.…”

“…Previous authors have identified several distinctive beds of impact spherules within the (Lowe and Byerly, 1986;Kyte et al, 2003;Lowe et al, 2003). These spherule beds are not a focus of this study, but rare impact spherules are observed in some intraclast breccias of the upper Mendon Formation.…”

Events in the Precambrian history of the Earth: Challenges in discriminating their global significance

“…Analogy with the K/T impact event and concomitant spherule occurrence globally (e.g., Smit, 1999) suggests that the c. 3.47 Ga and c. 2.63 Ga impact events may equally have been global in compass; however, the enclosing facies of the spherule beds indicate deposition mostly beneath wave base and extensive sedimentary, mass-flow and catatrophic (e.g. tsunami) reworking processes (e.g., Simonson, 1992;Glikson, 2001;Hassler and Simonson, 2001;Lowe et al, 2003). The geochemistry of the spherules suggests they might be distal rather than proximal deposits relative to the impact sites themselves .…”

The Geology and Habitability of Terrestrial Planets: Fundamental Requirements for Life

“…If life did originate on Mars first, could the transfer of planetary material from Mars during the period of late heavy bombardment have inoculated Earth? Whether or not this transfer occurred, the Noachian (Phyllosian) age rocks could harbour a record of fossilised 'bacteria' or life forms similar to bacteria, and mineralogical biomarkers similar to those produced by life on Earth (Banfield et al 2001;Bazylinski 1996;Beveridge 1989;Cady et al 2003;Ferris et al 1986;Lowenstam 1981;Toporski et al 2002).…”