Earth and Planetary Science Letters 376 (2013) 87–98 Contents lists available at SciVerse ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Ni-rich spinels and platinum group element nuggets condensed from a Late Archaean impact vapour cloud Steven Goderis a,b,∗ , Bruce M. Simonson c , Iain McDonald d , Scott W. Hassler c,e , Andrei Izmer b , Joke Belza a , Herman Terryn f , Frank Vanhaecke b , Philippe Claeys a a Earth System Science, Dept. of Geology, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium b Dept. of Analytical Chemistry, Ghent University, Krijgslaan 281–S12, B-9000 Ghent, Belgium c Geology Dept., Oberlin College, Oberlin, OH 44074, USA d School of Earth & Ocean Sciences, Cardiff University, Cardiff CF10 3YE, UK e The Wilderness Society, San Francisco, CA 94104, USA f Research Group Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium article info abstract Article history: Received 2 February 2013 Received in revised form 30 May 2013 Accepted 12 June 2013 Available online 8 July 2013 Editor: B. Marty Keywords: impact Late Archaean Paraburdoo spherule layers Ni-rich spinel platinum group elements Deciphering Earth’s impact history before ∼2 Ga relies heavily on the lunar record and terrestrial spherule layers, which are distal ejecta from large impacts. This study focuses on the Paraburdoo and Reivilo spherule layers in Western Australia and South Africa respectively, that were probably formed by one impact around 2.57 Ga. Both layers contain an aggregate thickness of ∼ 2 cm of spherules, known as microkrystites. These spherules are up to ∼ 0.6 mm in diameter and crystallized during flight, but were diagenetically replaced by K-feldspar and phlogopite with remarkable textural retention. Unlike any other Archaean layer, except for the 3.2 Ga S3 layer in the Barberton greenstone belt, the Paraburdoo and Reivilo spherules contain Ni-rich spinel crystals and high concentrations of meteoritic material (up to 357 ng g −1 Ir for bulk samples of several gram). These exceptional characteristics shed new light on the distribution of the meteoritic component carrier phases (metallic alloys dispersed in the pristine glass) and the processes involved in impact spherule formation and secondary alteration.  2013 Elsevier B.V. All rights reserved. 1. Introduction Extrapolation of the lunar cratering record implies that Earth was heavily bombarded during the Hadean and Archaean eras (Neukum and Ivanov, 1994; Bottke et al., 2012; Glass and Si- monson, 2013; Johnson and Melosh, 2012). While claims for large Archaean impact craters remain controversial (Garde et al., 2012; Jourdan et al., 2012; Reimold et al., 2013; Garde et al., 2013), the most widely accepted evidence for this bombardment arises from spherule-rich layers in Archaean and Early Proterozoic sedimentary and volcanic strata on the Pilbara (Western Australia) and Kaapvaal (South Africa) cratons (Bottke et al., 2012; Glass and Simonson, 2013; Johnson and Melosh, 2012). At least four discrete impact events occurred close to the Archaean–Proterozoic Boundary (APB) over about 140 Ma, from approximately 2.63 to 2.49 Ga (Fig. 1; Hassler et al., 2011; Simonson et al., 2009a, 2009b); spherules from one of these events are the focus of this study. * Corresponding author at: Earth System Science, DGLG Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium. Tel.: +32 26291480, fax: +32 26293391. E-mail address: Steven.Goderis@vub.ac.be (S. Goderis). Ejecta found more than ∼ 10 crater diameters away from a large crater consists primarily of glassy spherules – droplets so- lidified from melt or condensed from vapour (Glass and Simon- son, 2012, 2013). Most impact spherules are smaller than 1 mm in diameter and are either microtektites, consisting of pure glass, or microkrystites, containing primary crystallites in addition to glass. Microkrystites are generally more mafic in composition, can be highly contaminated by the impactor as indicated by elevated siderophile element contents (French and Koeberl, 2010), and may contain unusually Ni-rich spinels. As such, microkrystites probably represent droplets condensed from vaporized target rocks and im- pactor (Koeberl et al., 2012). The Paraburdoo and Reivilo spherule layers have compara- ble stratigraphic positions in the sedimentary sequences of the Hamersley (Pilbara craton) and Griqualand West (Kaapvaal cra- ton) basins, respectively (Hassler et al., 2011), and show similar interpolated U–Pb ages of around 2.57 Ga (Trendall et al., 1998; Sumner and Beukes, 2006; Fig. 1). These layers also share nu- merous characteristics not seen in any of the other APB spherule beds (Hassler et al., 2011). Both consist of highly crystallized mi- crokrystites with abundant pseudomorphs of plagioclase and skele- tal ferromagnesian (olivine and possibly clinopyroxene) crystals re- placed mainly by K-feldspar and a phlogopite-like phyllosilicate, 0012-821X/$ – see front matter  2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2013.06.027