70th Annual Meteoritical Society Meeting (2007) 5196.pdf

SEYMCHAN: A MAIN GROUP PALLASITE - NOT AN IRON METEORITE D. van Niekerk1, R. C. Greenwood2, I. A. Franchi2, E. R. D. Scott1, and K. Keil1. 1Hawaii Institute of Geophysics and Plane- tology, University of Hawaii, Manoa, Honolulu HI, 96822, USA. E-mail: [email protected]. 2PSSRI, The Open Univer- sity, Milton Keynes, MK7 6AA, UK.

Seymchan was initially classified as a group IIE iron meteor- ite [1], but was subsequently reclassified as Ungrouped [2]. The iron, which was discovered in 1967, consists of two pieces total- ing ~351 kg [3]. A recent (2004) expedition [4] to the same local- ity has led to the discovery of more meteorite masses. Reports suggest that 20% of the recent finds are pallasitic (or partly palla- sitic), while the rest are irons [4]. The recent finds are poorly documented, but photographs of cut slabs attest to the heteroge- neous nature of both metal:olivine (metal veins up to 10’s of cm. vs zones dominated by olivine) and olivine-fragment size (cm- sized to mm-sized). Glorieta Mountain and Brenham also contain large olivine-free metal regions. Neutron activation analysis of the pallasite metal (Tab. 1) reveals that it is identical to the iron meteorite, thus confirming that its occurrence in the same locality is not coincidence and that they are related. The composition of the metal groups Seymchan with the Main Group, but the high Ir indicates that it is anomalous (PMG-am). The metal composition of Brenham and Glorieta Mountain are not closely associated with Seymchan, although they are also classified as PMG-am. Table 1:INAA data for Seymchan [5]. Iron (1984) Iron (1985) Pallasite Cr (µg/g) 24 31 13 Co (mg/g) 5.28 5.26 5.33 Ni (mg/g) 92.8 93.3 95.1 Cu (µg/g) 121 152 140 Ga (µg/g) 24.8 26.3 25.6 As (µg/g) 18.6 18.8 18.2 W (µg/g) 0.52 0.34 0.52 Ir (µg/g) 0.662 0.694 0.676 Au (µg/g) 2.007 2.115 2.132 Electron microprobe analyses and scanning electron microscopy of two thick sections (each ~500 mm2) indicates that the palla- site-textured portion of Seymchan is mostly unweathered, and composed of FeNi metal, olivine, troilite, chromite, schreibersite, and whitlockite. The metal is dominated by low-Ni kamacite, while rims of central plessite regions contain up to 37% Ni. Oli- vine (Fa11.3) is well rounded, and infiltrated by minor troilite along grain boundaries and fractures. Schreibersite is mostly found in contact with olivine, and whitlockite is also a minor phase. Chromite was completely absent in one section, while dominating the second. Oxygen isotope measurements on olivine grains from Seymchan gave the following results (n=2): δ17O = 1.209‰; δ18O = 2.660‰; Δ17O*= -0.186‰. This value falls within the field of Main Group pallasites measured by [6]. References: [1] Scott E. R. D. and Wasson J. T. 1976. Geo- chimica et Cosmochimica Acta 40:103-115. [2] Wasson J. T. and Wang J. 1986. Geochimica et Cosmochimica Acta 50:725-732. [3] Krinov E. L. 1968 Met. Bull. 43. [4] Nazarov M. A. 2007 Personal Communication. [5] Wasson J. T. 2007 Personal Com- munication. [6] Greenwood et. al. 2006. Science 313:1763. Acknowledgements: We thank Dr. J. Wasson for contribut- ing his data (pallasite metal was provided by Dr. M. A. Nazarov).