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 ϳ50 km apart, WRL-1 and DDH 186, ing carbonaceous shale (Ͻ1.5 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 (ϳ100 ␮m 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 Ͼ2000 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 Ϯ 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 Ͼ32,000 km2, among the largest yet documented in the Precambrian Mamba Iron Formation. The age of the Jeer- rock record. inah Formation is between 2684 Ϯ 6Ma (Arndt et al., 1991) and 2629 Ϯ 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 Ϯ 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 Յ3 mm thick, whereas the layer at the out- shocked grains. crop sites is signi®cantly thicker: ϳ1.5 m at and trace amounts of heavy minerals. Diage- A series of spherule layers was discovered the Hesta locality and ϳ10 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.

᭧ 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 Ͻ1%±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 ϳ0.5 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 (ϳ100 ␮m) with two intersecting sets of planar deformation features (PDFs) was found adjacent to a small (ϳ0.2 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 ␮m), 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, ϳ5.2 m below the strati- quartz. Chlorite aggregates are more abundant ters (ca. 2 Ga or younger) and a few distal graphic contact between the Jeerinah Forma- in the top of the spherule bed, whereas most ejecta horizons (younger than 600 Ma). tion and the overlying Marra Mamba Iron For- quartz grains are scattered in the lower sec- Shocked minerals are considered the strongest mation (Fig. 2). In drill hole DDH 186, the tion. Quartz grains are complexly zoned (seen single evidence for extraterrestrial impacts spherule bed ranges from 1 to 5 mm in thick- by SEM-CL) and comprise composites of two (e.g., StoÈf¯er and Langenhorst, 1994; Grieve ness and is located at a depth of 147.5 m, or more crystals, which share common crystal et al., 1996), and their absence in early Pre- ϳ5.5 m below the contact (Fig. 1B). The base boundaries. A few quartz grains display com- cambrian spherule beds has, in part, been at- of the spherule bed consists of ®ne sandstone plex intergrowths, possibly with igneous feld- tributed to the effects of postdepositional al- ϳ0.5±1.0 mm thick and passes into organic- spar, that have been replaced by nonlumines- teration (Simonson et al., 1998). However, the rich shale laminae (Figs. 2A, 2B). The spher- cent diagenetic K-feldspar. Most quartz grains discovery of shocked quartz in 2630 Ma ules ®rst appear ϳ0.5±1.0 mm above the basal are surrounded by irregular nonluminescent shales shows that (1) in favorable circum- contact, immediately above the organic-rich overgrowths that probably precipitated after stances, such grains can survive diagenesis shale laminae, and grade from coarse (up to deposition. The spherule bed also contains and low-grade metamorphism and (2) the ab- 0.9 mm), intact spherules to smaller spherules trace amounts of sul®de (pyrite, chalcopyrite, sence of such grains in other distal ejecta ho- and broken fragments toward the top of the sphalerite, and galena), apatite, zircon, titani- rizons cannot be dismissed as a problem of bed. um oxide, and sericite. preservation, but may re¯ect the composition

1030 GEOLOGY, December 2004 tion, and texture, and occur in the same strati- graphic position, suggesting that the beds formed during the same event. Uranium-lead analysis of zircon from an andesitic ignimbrite from the uppermost Jeerinah Formation yield- ed an age of 2629 Ϯ 5 Ma (Nelson et al., 1999), which is considered a close approxi- mation of the depositional age of the spherule layer. The horizon from the newly discovered localities is equivalent to spherule beds from the uppermost Jeerinah Formation in drill hole FVG-1 (Simonson et al., 2000a) and nearby outcrop localities (Simonson et al., 2002), ex- tending the Jeerinah spherule layer by thou- sands of square kilometers. Based on petro- graphic similarities and stratigraphic position, the Jeerinah spherule layer was recently cor- related with the Carawine horizon in the Oak- over River area of the Pilbara craton (Fig. 1) (Simonson et al., 2000a, 2002) and the Mon- teville spherule layer (2.65±2.60 Ga) in the Griqualand West Basin, South Africa (Simon- son et al., 1999; Simonson and Glass, 2004). In the Pilbara craton, the layer covers an area of ϳ15,000 km2, whereas in South Africa it covers ϳ17,000 km2 (Simonson et al., 2000b), for a total minimum areal distribution of 2 Figure 2. A and B: Photomicrographs of spherule beds in Jeerinah Formation; ϳ32,000 km . Further geochronology is re- scale bars represent 1 mm. A: Drill hole DDH 186 (147.5 m depth). B: Drill quired to test these correlations, but if correct, hole WRL-1 (684.1 m depth). C: Coarse spherule (~0.9 mm in diameter) com- then the Jeerinah-Carawine-Monteville layer posed of K-feldspar with core displaying planar faces; drill hole WRL-1 (684.1 represents one of the most extensive ®elds of m depth). D: Angular quartz grain with two sets of planar deformation fea- tures; scale bar represents 20 ␮m; drill hole WRL-1 (684.1 m depth). Precambrian impact ejecta yet documented.

CONCLUSIONS of the target. The presence of shocked quartz DDH 186 at 147.50 m depth and in core Our mineralogical, petrographical, and geo- in the Jeerinah spherule bed indicates that the WRL-1 at 684.10 m depth. Both samples have chemical study of the Jeerinah spherule layer impact site contained quartz, favoring a con- signi®cantly elevated Ir contents, at 15.5 and provides compelling evidence for a major as- tinental target rather than an oceanic site. 12.1 ppb, respectively. One other sample has teroid impact ca. 2.63 Ga. Geochemical esti- an Ir content of 0.5 ppb, which is at the de- mates suggest that the layer comprises as GEOCHEMISTRY tection limit. This sample is slightly above the much as 2%±3% projectile material. For the Eight samples (three from DDH 186 and spherule layer and may represent a tailing in ®rst time in these Archean impact layers, ®ve from WRL-1) were analyzed for selected the abundance. The spherule layers also have shocked quartz with planar deformation fea- major and trace element contents by instru- high abundances of Cr, Co, Ni, and Au, and tures was found, indicating that the impact site mental neutron activation analysis. For the the Ir/Au ratios are close to chondritic ratios. contained quartz, and favoring a continental two spherule-layer samples, the cores were cut Owing to minor sul®de mineralization, some site rather than oceanic crust. Such an inter- and then broken, and chips (ϳ200 mg) were of the other samples have elevated contents of pretation is supported by an increase in the used for analysis. Above and below the spher- Au and a few other elements, but without Ir. maximum spherule size (up to 2 mm) and lay- ule layer, ϳ5 g samples were collected and The high contents of Ir and other siderophile er thickness toward the inferred basin margins elements clearly indicate an extraterrestrial were ground to powders in an agate mill; from in the north and east (Simonson et al., 2002). component in the spherule layers, which can these, 200 mg subsamples were analyzed. One If proposed correlations are valid, then the be estimated to represent 2±3 wt% of a chon- sul®de sample chip from WRL-1, ϳ5 cm be- Jeerinah, Carawine, and Monteville layers dritic meteorite. Compositionally the Jeerinah low the spherule layer, was also analyzed to were produced from the same impact event, spherule layers are very similar to the proba- see whether the sul®des had concentrated any demonstrating the likely global extent of the bly coeval spherule layer in the Monteville siderophile trace elements. Details of the an- Formation, South Africa (Simonson et al., ejecta fallout. alytical method were given by Koeberl 2000b); in samples of this spherule layer, from (1993); the only deviation from the method ACKNOWLEDGMENTS the Pering Mine, Ir contents as high as 6.4 ppb described there is that samples were counted We thank S. Brown, M. Doyle, B. Krapez, S. were found, and other trace element contents Sheppard, and B.M. Simonson for discussion; G. for up to 100 h each instead of 24 h, for better are very similar to the Jeerinah samples ana- Broadbent and Rio Tinto Exploration for access to detection limits. lyzed here. WRL-1; the Geological Survey of Western Austra- The results are given in Table 1. In terms lia for access to DDH 186; and the staff of the Cen- of major element contents, the samples are CORRELATION OF SPHERULE tre for Microscopy and Microanalysis, University of Western Australia, for technical help. We are grate- very poor in Na, high in K, and have variable LAYERS ful to John Spray and an anonymous reviewer for Fe contents, depending on pyrite concentra- The spherules in drill holes DDH 186 and constructive comments. Laboratory work by Koe- tions. The spherule-layer samples are in core WRL-1 are similar in size, shape, composi- berl was supported by the Austrian Science Foun-

GEOLOGY, December 2004 1031 TABLE 1. CHEMICAL DATA FOR SAMPLES FROM THE JEERINAH FORMATION IN DRILL HOLES WRL-1 events by SHRIMP U-Pb analysis of monazite AND DDH 186 in shales: Geology, v. 29, p. 963±966. Shukolyukov, A., Castillo, P., Simonson, B.M., and DDH 186 samples WRL-1 samples Lugmair, G.W., 2002, Chromium in Late Ar- Drill depth: 145.1 m 147.50 m 148 m 678.9 m 684.10 m 684.15 m 688.02 m 693.17 m chean spherule layers from Hamersley Basin, shale spherule shale shale spherule shale shale shale Western Australia: Isotopic evidence for extra- bed bed terrestrial component: Lunar and Planetary Na 413 365 375 100 725 475 135 942 Science, v. 33, abs. 1369 (CD-ROM). K (wt%) 8.72 6.18 8.10 0.035 6.15 1.98 0.017 7.66 Simonson, B.M., 1992, Geological evidence for a Sc 19.1 12.5 21.3 12.2 52.9 8.56 1.25 18.7 strewn ®eld of impact spherules in the early Cr 120 157 140 38.2 208 19.3 8.21 172 Precambrian Hamersley Basin of Western Fe (wt%) 3.15 13.8 6.89 5.86 4.27 30.3 18.5 1.88 Australia: Geological Society of America Bul- Co 10.3 42.5 38.7 18.2 15.9 230 1.72 15.7 letin, v. 104, p. 829±839. Ni 30 190 40 60 145 520 13 30 Zn 720 25100 2210 82 2050 280 14 950 Simonson, B.M., and Glass, B.P., 2004, Spherule As 4.51 1.33 27.1 22.4 28.1 161 0.32 7.15 layersÐRecords of ancient impacts: Annual Se 1.8 5.1 4.6 1.7 2.8 5.7 0.5 3.1 Review of Earth and Planetary Sciences, Br 0.4 0.2 0.2 0.4 0.6 0.2 0.3 2.5 v. 32, p. 329±361. Rb 290 241 266 1.6 136 48.7 2 240 Simonson, B.M., and Sumner, D.Y., 2004, A newly Sr 4 5 5 55 53 10 24 5 recognized Late Archean impact spherule lay- Zr 320 90 145 50 225 40 15 230 er in the Reivilo Formation, Griqualand West Sb 0.63 0.56 4.98 2.11 0.70 28.9 0.15 1.17 Basin, South Africa: Lunar and Planetary Sci- Cs 3.6 2.79 4.11 0.32 4.21 1.76 1.98 15.9 Ba 250 220 220 15 200 25 20 280 ence, v. 35, abs. 1689 (CD-ROM). La 19.0 14.1 28.8 10.3 5.25 0.67 2.35 15.6 Simonson, B.M., Davies, D., Wallace, M., Reeves, Ce 44.5 31.5 52.9 18.4 10.9 0.82 4.35 25.1 S., and Hassler, S.W., 1998, Iridium anomaly Nd 24.2 18.1 25.8 12.2 15.2 1.5 2.76 12.7 but no shocked quartz from Late Archean mi- Sm 6.35 5.62 4.07 3.84 6.70 0.56 0.67 2.47 crokrystite layer: Oceanic impact ejecta?: Ge- Eu 1.8 1.88 1.17 1.82 2.83 0.18 0.94 0.85 ology, v. 26, p. 195±198. Gd 5.25 4.44 3.38 4.05 7.75 0.55 0.8 3.87 Simonson, B.M., Hassler, S.W., and Beukes, N.J., Tb 1.0 0.74 0.63 0.66 1.67 0.095 0.17 0.75 1999, Late Archean impact spherule layer in Tm 0.59 0.31 0.37 0.30 0.77 0.055 0.11 0.46 Yb 4.21 1.92 2.48 1.79 5.10 0.38 0.77 3.09 South Africa that may correlate with a West- Lu 0.66 0.29 0.38 0.27 0.77 0.058 0.13 0.46 ern Australian layer, in Dressler, B.O., and Hf 7.11 2.45 3.86 0.86 4.61 0.89 0.13 4.80 Sharpton, V.L., eds., Large meteorite impacts Ta 0.81 0.35 0.80 0.19 0.99 0.27 0.02 1.05 and planetary evolution II: Geological Society W 2.7 2.5 2.8 0.6 1.8 0.7 0.08 0.05 of America Special Paper 339, p. 249±261. Ir (ppb) 0.5 15.5 Ͻ0.7 Ͻ0.5 12.1 Ͻ1.5 Ͻ0.5 Ͻ1 Simonson, B.M., Davies, D., and Hassler, S.W., Au (ppb) 0.8 5.1 2.4 1.2 4.5 24.5 0.3 0.9 2000a, Discovery of a layer of probable im- Th 7.11 3.21 8.68 2.40 9.32 0.64 0.30 10.1 pact melt spherules in the Late Archean Jeer- U 3.63 1.59 2.32 0.70 2.71 0.23 0.062 4.13 inah Formation, Fortescue Group, Western Note: Data are in ppm, except as noted. Australia: Australian Journal of Earth Scienc- es, v. 47, p. 315±325. Simonson, B.M., Koeberl, C., McDonald, I., and dation. This work was supported by an Australian ture and experiment: II. Signi®cance in geo- Reimold, W.U., 2000b, Geochemical evidence Research Council fellowship and grant to science: Meteoritics and Planetary Science, for an impact origin for a Late Archean spher- Rasmussen. v. 31, p. 6±35. ule layer, Transvaal Supergroup, South Africa: John, C., and Glass, B.P., 1974, Clinopyroxene- Geology, v. 28, p. 1103±1106. 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1032 GEOLOGY, December 2004