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Chondritic Meteorite Fragments Associated with the Permian- Triassic Boundary in Antarctica

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Chondritic Meteorite Fragments Associated with the Permian- Triassic Boundary in Antarctica R EPORTS and subrounded (Fig. 2A) and strongly re- semble the so-called “native-iron nuggets” Chondritic Meteorite Fragments reported from the bedded chert layer of the Permo-Triassic section in Sasayama, Japan Associated with the Permian- (13). Most of the grains from our Meishan sample are compositionally similar to the Fe- Triassic Boundary in Antarctica Ni-Si grains reported previously (6) from the base of bed 25 marking the end-Permian in Asish R. Basu,1* Michail I. Petaev,2,3 Robert J. Poreda,1 the Meishan section. Our electron micro- Stein B. Jacobsen,2 Luann Becker4 probe measurements (14) show that the grains are almost pure Fe (99 to 100%) with Multiple chondritic meteorite fragments have been found in two sedimentary up to 0.20 weight % Ni, 0.60 weight % Cr, rock samples from an end-Permian bed at Graphite Peak in Antarctica. The and 0.39 weight % Si. Ni/Fe, Co/Ni, and P/Fe ratios in metal grains; the Fe/Mg and Mn/Fe ratios in The magnetic fraction from the claystone olivine and pyroxene; and the chemistry of Fe-, Ni-, P-, and S-bearing oxide in breccia at the P-T boundary of Graphite Peak the meteorite fragments are typical of CM-type chondritic meteorites. In one consists of abundant opaque grains and rare sample, the meteoritic fragments are accompanied by more abundant discrete aggregates of silicates and opaque minerals. metal grains, which are also found in an end-Permian bed at Meishan, southern Three types of opaque grains were identified China. We discuss the implications of this finding for a suggested global impact in the Graphite Peak samples: (i) predomi- event at the Permian-Triassic boundary. nant Fe-rich metal nuggets, (ii) less abundant Fe,Ni,P,S-bearing oxides, and (iii) rare Meteorite samples that occur as fragments beds are exposed over about 2 m and include Fe,Ni-sulfides. The pure metal grains that are in Earth rocks are rare. There are only a few the last Gondwana coal bed of end-Permian common in the Meishan boundary layer are examples in the literature of which two are age, above a bed with the last occurrence of also abundant (ϳ70%) among the Graphite deep-sea drill core occurrences. One in- Glossopteris flora. Above the coal bed is a Peak opaque grains (Fig. 2B). These grains cludes fragments of an achondrite, the claystone breccia with anomalously low ␦13C are similar but more diverse in composition Eltanin meteorite (1). A feldspar from this value (ϳ–40 ‰). Shocked quartz (8) and ex- material was so well preserved that it yield- traterrestrial fullerenes with trapped noble gases ed a present 87Sr/86Sr ϭ 0.699013 Ϯ11, have been reported from this bed (9). On the only slightly higher than the initial solar basis of paleobotanic and isotopic criteria, and system value. The second is a 2.5-mm-size shocked quartz grains, the claystone breccia meteorite (2) from the Cretaceous-Tertiary (Fig. 1) has been identified as the Permian- (K-T) boundary layer of a North Pacific Triassic (P-T) boundary in Antarctica (7, 8, 10). deep-sea drill core that resembles a carbo- This interpretation was also confirmed and ex- naceous chondrite and is interpreted as a tended by similar observations on P-T sections remnant of the K-T impactor of 65 million at Wybung Head, NSW, Australia (7, 10), as years ago (3). Ordinary chondrite remnants well as by extraterrestrial fullerene evidence were also reported from carbonate rocks of from China and Japan (11). The aforemen- southern Sweden, where the surviving min- tioned findings provided a strong rationale to eral is believed to be chromite (4, 5). This undertake a thorough petrological-mineralogi- paper describes the discovery of chondritic cal study of the magnetic grains from some of meteorite fragments and metallic grains in these end-Permian sedimentary rocks. Specifi- the end-Permian strata at Graphite Peak, cally, high 3He-bearing magnetic grains and Central Transantarctic Mountains, Antarc- their unique noble gas signatures (9) from the tica (Fig. 1) and confirms a previous report Graphite Peak P-T boundary needed to be fully of metallic grains from the end-Permian evaluated in the context of a potential-impact boundary in Meishan (6), southern China tracer. These rock samples used in the previous (Fig. 1). It has been suggested (6) that the study (9) were available in our laboratories. metallic grains from Meishan may have Therefore, we examined four samples of condensed from an impact-generated vapor Graphite Peak claystone breccia at the P-T Fig. 1. Lithostratigraphy of the P-T boundary, as defined isotopically and paleobotanically (7, 8) cloud, although this suggestion is yet to boundary [three of these are subsamples of no. in Graphite Peak near Beardmore Glacier, Cen- gain acceptance. 2060 from (8), and one is no. 315 from (7)] and tral Transantarctic Mountains.This figure Graphite Peak, Antarctica (Fig. 1), exposes another claystone-shale sample [sample no. shows the relative positions of two samples of a 550-m continuous section of the Buckley and 2064 of (8)], ϳ0.8 m below the boundary layer the meteoritic fragments containing claystone Fremouw formations (7, 8). The end-Permian (Fig. 1). We also examined a sample of the breccia, no.2060 from the base and no.315 reddish-black sediment layer believed to mark from 23 cm above.The claystone breccia rests directly above the coal horizon with the last the end-Permian at Meishan, China (6, 12), 1Department of Earth and Environmental Sciences, occurrence of Glossopteris (8).Fullerenes con- University of Rochester, Rochester, NY 14627, USA. where Fe-Ni-Si grains were reported earlier taining extraterrestrial 3He (9) along with 2Department of Earth and Planetary Sciences, Harvard from the same layer (6). shocked quartz grains and a negative shift in University, Cambridge, MA 02138, USA. 3Harvard- We separated magnetic particles from ␦13C were reported (7, 8) earlier from the ho- Smithsonian Center for Astrophysics, 60 Garden these P-T boundary samples at Graphite rizon containing these meteorite-bearing sam- Street, Cambridge, MA 02138, USA. 4Institute of Peak, Antarctica, and Meishan, China. The ples.Our discovery of the meteorite fragments Crustal Studies, Department of Geology, University of in these rocks, along with previous evidence California, Santa Barbara, CA 93106, USA. magnetic particles from Meishan are opaque referred to above (7–9), indicates that the clay- ␮ *To whom correspondence should be addressed.E- and variable in size (20 to 200 m). The stone breccia is a rapidly redeposited unit, in mail: [email protected] outlines of these grains are irregular, curved part from distant impact fallouts. 1388 21 NOVEMBER 2003 VOL 302 SCIENCE www.sciencemag.org R EPORTS than those in Meishan and range in Fe com- the supporting online material (tables S1 to such meteorite fragments have been identified positions up to 100%, with variable amounts S4 and figs. S1 and S2). All fragments in two of the subsamples of no. 2060 and of Ni (up to 0.16%), Cr (up to 12.48%), and display identical poikilitic or granular tex- several dozens of smaller meteorite fragments Si (up to 2.62%). The common occurrence of tures (Fig. 3, A to D). They contain small or mineral grains in no. 315 (see supporting Fe-Ni-Si grains at the Graphite Peak P-T euhedral-to-subhedral forsterite grains en- online material, figs. S1 and S2). boundary layer and at the P-T event strati- closed in larger grains of clinoenstatite, The meteorite fragments contain abun- graphic layer (15, 16) in Meishan provides which shows clear polysynthetic twinning dant Fe-rich opaque phases, which make further evidence that we have sampled the in one of the fragments (Fig. 3D). The the whole fragments magnetic. The elec- P-T boundary at Graphite Peak. These chemical compositions of forsterite and cli- tron microprobe study revealed three major “event-marker” magnetic grains occur only in noenstatite (Table 1) show low concentra- types of opaque phases: (i) Fe,Ni,P,S-bear- the boundary layer and are absent in samples tions of FeO and relatively high Mn/Fe ing oxides, (ii) Fe,Ni,Co,P,Cr-bearing met- above and below, at Meishan (6). Our study ratios. Such compositions do not occur in al grains, and (iii) Fe,Ni-sulfides. The Fe, of the magnetic fraction of the bed 0.8 m terrestrial olivines and pyroxenes and are Ni,P,S-oxides (Fig. 3, A to C) tend to form below the P-T boundary at Graphite Peak indicative of an extraterrestrial origin. For- rounded blobs or irregular masses often yielded only terrestrial Fe oxides and hydrox- sterite has rather high concentrations of interconnected by thin veins of the same ides, lacking Ni, S, and P. CaO and Cr2O3, characteristic of some material. In back-scattered and secondary In the magnetic fractions of the Graphite chondritic meteorites (17). Two minor sil- electron images, these masses display high- Peak samples no. 2060 and no. 315, we also icate phases, Ca-rich clinopyroxene and Al- ly irregular surfaces with numerous pits found a number of particles enriched in rich mesostasis, usually occur as interstitial and caverns indicative of the polymineralic silicate minerals. The results of our elec- anhedral patches. The composition of mes- nature of these materials. The high concen- tron microprobe study of these particles ostasis (Table 1) varies from Ol-normative trations of Ni, P, and S in their chemical (14) are summarized in Tables 1 and 2 and to An-normative and the low Na2O content is compositions along with the low analytical similar to those of type I chondrules.
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