Chondritic Meteorite Fragments Associated with the Permian-Triassic Boundary in Antarctica
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Magnetic grains were separated from ∼2 g material of the dark fine-grained reddish layer at the base of bed 25 of the Meishan section. These grains of variable size (∼5 to ∼300 μm) were mounted on epoxy in glass slides and polished with Al 2 O 3 powder (3 1 and 0.3 μm size) on a soft woven mat. This polishing process removed many of the finer grains (<20 μm). The remaining grains were examined optically and many were analyzed with the electron microprobe. The yield of the magnetic grains was ∼0.02%. For the Graphite Peak samples we followed the same procedure as above. The initial sample size of no.2060 was ∼50 g it was clay-rich material containing substantial amounts of sand-sized minerals and polycrystalline aggregates which were presorted to a 0.2-g fraction. This sample was not sieved but disaggregated in agate mortars. Magnetic grains from this material were separated with a yield of ∼0.05% of mostly opaque and silicate fragments (for details see supporting online material). Sample no.315 is similar to sample no.2060 both in mineral content and texture. This sample was crushed in a stainless steel mortar. About 0.2 g of the crushed rock sample no.315 was subjected to the same magnetic separation and yielded ∼0.03% meteorite fragments. These grains were mounted polished and analyzed. Quantitative analysis of the mineral and metal grains was conducted with a JEOL 733 Superprobe at the Harvard-Smithsonian Center for Astrophysics using wavelength dispersive spectrometers natural and synthetic metal and silicate standards. Accelerating voltage beam current beam size and counting time were 15 kV 20 nA ∼1 μm and mostly 40 s respectively.
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We can eliminate the possibility that the meteorite fragments and metal grains were found in the samples because of laboratory contamination. We have never crushed or separated any chondritic meteorite in the clean laboratory at the University of Rochester where we separated the magnetic meteorite fragments from the Antarctic samples. We received on two separate occasions unprocessed pieces of the similar looking material of sample no.2060 of the P-T boundary layer claystone breccia from G. Retallack's laboratory. We extracted the magnetic fragments in two separate experiments at Rochester—one for each subsample received. Thus we repeated our separation twice and in each case we mounted the magnetic fragments on separate glass slides that were later analyzed. From our electron microprobe analyses the same type of meteorite fragments and the metal nuggets were identified in both subsamples. An important observation is the presence of metal grains in association with the meteorite fragments in the Antarctic samples. These metal grains are neither meteoritic nor are they found in any terrestrial rocks except that similar metal grains are also found in the P-T boundary at Meishan in China. Most of these metal grains also have compositions that are clearly different from metals used in laboratory tools that may have come in contact with the samples. Also other similar samples that were not from the P-T boundary but were separated in the same laboratory at the same time did not show these metal particles or meteoritic fragments. Thus we conclude that it is improbable that the Antarctic P-T sample was contaminated with these metal grains and meteorite fragments. Later we obtained yet another rock fragment of no.2060 (from G. Retallack) with a different (whitish) color from the original pieces (see supporting online material) as well as a different sample no. 315 stratigraphically 23 cm above no.2060 and received directly from E. Krull. The latter sample also contains meteorite fragments.
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We are grateful to G. J. Retallack and E. Krull for providing the Graphite Peak samples from Antarctica and to S. D'Hondt for the Meishan base 25 ferruginous sample from China that was collected in collaboration with D. H. Erwin and Y. G. Jin. Research supported by the Division of Earth Sciences (EAR) NSF (A.R.B.); the Division of Ocean Sciences (OCE) NSF (L.B. and R.J.P.); NASA (Exobiology L.B.); and NASA (Origin of Solar System and Cosmochemistry S.B.J.) grants.