Iridium Anomalies and Shocked Quartz in a Late Archean Spherule Layer from the Pilbara Craton: New Evidence for a Major Asteroid Impact at 2.63 Ga
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Iridium anomalies and shocked quartz in a Late Archean spherule layer from the Pilbara craton: New evidence for a major asteroid impact at 2.63 Ga Birger Rasmussen* School of Earth and Geographical Sciences, University of Western Australia, Crawley, WA 6009, Australia Christian Koeberl* Department of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria ABSTRACT SAMPLES AND METHODS A thin (1±5 mm) spherule layer in ca. 2.63 Ga shale from the Jeerinah Formation Samples were collected in 1999 and 2000 (Pilbara craton, northwestern Australia) has been identi®ed at two new localities. The from diamond core recovered from two drill layers have Ir concentrations as high as 15.5 ppb, signi®cantly higher than the surround- holes ;50 km apart, WRL-1 and DDH 186, ing carbonaceous shale (,1.5 ppb). The sand-sized spherules display quench and devit- located in the Pilbara craton of northwestern ri®cation textures and are interpreted as former silicate melt droplets that were replaced Australia (Fig. 1). The material is from the top by K-feldspar, carbonate, and sul®de during diagenesis. In one spherule-layer sample, an of the Roy Hill Shale Member of the Jeerinah angular quartz grain (;100 mm in size) with planar deformation features was found, Formation (Fortescue Group), in a region that which represents the oldest known shocked grain in distal ejecta by .2000 m.y. The has undergone negligible strain and prehnite- survival of shocked quartz in ca. 2.63 Ga rocks, which have undergone multiple meta- pumpellyite±facies metamorphism (Smith et morphic events, suggests that their absence in other impact ejecta layers may not only be al., 1982) and a low-temperature thermotec- a question of preservation. The presence of shocked quartz and anomalously high Ir tonic event dated by U-Pb analysis of mona- contents in a layer containing melt spherules provides compelling evidence for an extra- zite as 2192 6 5 Ma (Rasmussen et al., 2001). terrestrial impact with a target area that was at least partly silicic, favoring a continental The shale was deposited in a marine shelf or impact site. Estimates based on geochemical data suggest that the spherule layer comprises an upper-slope environment (Blake and Bar- as much as 2±3 wt% of a chondritic meteorite component. If proposed correlations with ley, 1992; Simonson et al., 2000a) and con- the Carawine (eastern Pilbara craton) and Monteville (South Africa) layers are correct, then the combined ejecta blanket represents fallout from a single major impact with an formably passes into the overlying Marra areal distribution of .32,000 km2, among the largest yet documented in the Precambrian Mamba Iron Formation. The age of the Jeer- rock record. inah Formation is between 2684 6 6Ma (Arndt et al., 1991) and 2629 6 5 Ma from Keywords: Archean impact layers, shocked quartz, Ir anomaly, impact ejecta. U-Pb zircon analyses of tuffaceous rocks (Nelson et al., 1999). A tuff in the overlying INTRODUCTION was found in the Oakover River area (Simon- Marra Mamba Iron Formation has an age of Impact events have played a crucial role in son, 1992) (Figs. 1A, 1B). The beds contain 2597 6 5 Ma (Trendall et al., 1998). the geologic and biologic history of Earth. sand-sized spherules interpreted as former Polished thin sections of shale from the up- Many of the larger impacts have left traces in droplets of silicate melt produced during im- per Jeerinah Formation, 15 from WRL-1 and the form of craters or as geochemical and min- pact (Simonson, 1992). All have Ir anomalies 36 from DDH 186, and 9 polished thin sec- eralogical signatures in the stratigraphic (see review by Simonson and Glass, 2004), tions from the spherule beds (7 from WRL-1 record (see Koeberl, 2001, for a review). Al- and two have chromium isotope signatures in- and 2 from DDH 186) were examined by op- though impact structures have not been found dicating an extraterrestrial component (Shu- tical microscope. Key sections were subse- in rocks older than ca. 2 Ga (Grieve et al., kolyukov et al., 2002). Possible equivalents quently studied by scanning electron micro- 1995), possible impact debris layers have been have been found in the Monteville and Reivilo scope (SEM) using backscattered-electron and documented in South African and Australian Formations in South Africa (Simonson et al., SEM-cathodoluminescence techniques. Min- Archean successions (see review by Simonson 2000b; Simonson and Sumner, 2004). The erals were identi®ed by optical microscopy, and Glass, 2004). Shocked quartz, commonly oldest of the three spherule beds from the SEM±energy-dispersive X-ray spectrometry regarded as unambiguous evidence for a hy- Hamersley province occurs in the uppermost (SEM-EDX), and X-ray diffraction analysis. pervelocity impact event (e.g., Grieve et al., Roy Hill Shale Member of the Jeerinah For- The host rock in which the spherule beds 1996), has not yet been found in distal ejecta mation and has been documented in drill hole occur is a carbonaceous shale with thinly in- horizons older than ca. 600 Ma (Gostin et al., FVG-1 (Simonson et al., 2000a) and two out- terbedded siltstone and sandstone. The shale 1986). Only a single Precambrian ejecta layer crop localities, the Hesta railway siding and contains quartz, K-feldspar, muscovite, and (i.e., from the ca. 590 Ma Acraman impact the Tarra Tarra turnoff (Simonson et al., 2002) chlorite, interspersed organic matter (up to 9.0 structure, Australia) is known to contain (Fig. 1A). In the drill hole, the spherule layer wt% total organic carbon; Brocks et al., 1999) spherules, geochemical anomalies, and is #3 mm thick, whereas the layer at the out- shocked grains. crop sites is signi®cantly thicker: ;1.5 m at and trace amounts of heavy minerals. Diage- A series of spherule layers was discovered the Hesta locality and ;10 m at the Tarra Tar- netic and metamorphic minerals include K- in sedimentary successions of Late Archean ra turnoff site (Simonson et al., 2002). We re- feldspar, quartz, chlorite, sericite, calcite, an- age from the Pilbara craton of northwestern port here a spherule layer in the same strati- kerite, sul®des (pyrite, chalcopyrite, sphalerite), Australia (Fig. 1A): three of the layers occur graphic position from two new localities (Fig. phosphates (apatite, xenotime, monazite), tita- in the Hamersley province and a single layer 1A); this layer provides additional mineral- nium oxide, and thorite. Diagenetic pyrite is ogical and geochemical evidence, including common as nodules and thin, bedding-parallel *E-mails: [email protected]; christian. the oldest known shocked quartz, strongly bands, typically surrounded by ®brous quartz [email protected]. supporting an impact origin for the horizon. and sericite pressure shadows. q 2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; December 2004; v. 32; no. 12; p. 1029±1032; doi: 10.1130/G20825.1; 2 ®gures; 1 table. 1029 The spherules, which comprise ,1%±50% of the layer (Figs. 2A, 2B), are now composed of K-feldspar, quartz, calcite, chlorite, pyrite, chalcopyrite, and sphalerite that probably re- placed original constituents after deposition. The spherules are between 0.1 and 0.9 mm (typically ;0.5 mm) in diameter, spherical to elliptical in shape, and range from intact grains (Fig. 2C) to broken fragments. Some spherules and fragments display outlines of fan-shaped, bladed, and lath-shaped crystals that radiate inward from the margins, sugges- tive of nucleation around the outer margin of former melt droplets, possibly along a quenched rim. A few spherules contain round- ed to irregular cores typically composed of quartz, carbonate, chlorite, or sul®de, probably representing vesicles or areas formerly ®lled by glass that were replaced by diagenetic ce- ment. The spherules from the Jeerinah For- mation closely resemble spherules from other impact layers (John and Glass, 1974; Bohor and Glass, 1995), and display quench and de- vitri®cation textures (cf. Lofgren, 1971, 1977) interpreted to be former droplets of silicate melt generated during a large impact (cf. Si- monson and Glass, 2004). SHOCKED QUARTZ Because of the limited amount of drill core, only nine polished thin sections were made across the two spherule beds. In a polished thin section from WRL-1, an angular grain of quartz (;100 mm) with two intersecting sets of planar deformation features (PDFs) was found adjacent to a small (;0.2 mm in di- ameter) spherule (Fig. 2D). The nature of the Figure 1. A: Map showing geology of Pilbara region and locations of drill holes (open grain was con®rmed by SEM-EDX analysis circles) and outcrop localities (x) in Hamersley province and Oakover River area con- and universal-stage studies. Universal stage taining Jeerinah and Carawine spherule layers, respectively. B: Stratigraphic column of work indicates the possible presence of a third upper Fortescue Group and Hamersley Group from Hamersley province and Oakover set of PDFs. The lamellae in the quartz are River area. Geochronology is from Arndt et al. (1991), Trendall et al. (1998), Woodhead et al. (1998), and Nelson et al. (1999). sharp, parallel, and closely spaced (3±10 mm), and extend across the entire grain; they are identical to those of shocked quartz from im- SPHERULE BEDS The spherule beds contain angular, silt- to pact craters and ejecta horizons (cf. Koeberl, In drill hole WRL-1, a spherule layer be- ®ne-sand±sized grains of nonluminescent K- 2001). Shocked quartz is scarce in the strati- tween 1 and 2 mm thick was identi®ed at a feldspar, with lesser amounts of chlorite and graphic record and is restricted to impact cra- depth of 684.1 m, ;5.2 m below the strati- quartz.