WASHINGTON COUNTY IRON METEORITE RECLASSIFIED AS IIIAB Dwijesh Ray and S

77th Annual Meteoritical Society Meeting (2014) 5159.pdf

WASHINGTON COUNTY IRON METEORITE RECLASSIFIED AS IIIAB Dwijesh Ray and S. Ghosh. PLANEX, Physical Research Labor- atory, Ahmedabad 380009, India. E-mail: [email protected]. Introduction: Washington County (WC) was classified as ‘Ataxite’ [1]. Previous metallographic study including the α2- zone of the fusion crust, bulk composition and noble gas revealed WC as chemically ungrouped ataxite with traces of volume cor- related solar noble gases [2, 3]. In an effort to search for the possible carrier phase of solar noble gases, the present study has led to a reappraisal of texture, phase composition and bulk composition of WC, leading to its reclassification. Results: WC iron shows a finely recrystallised matrix (mean size: 30 μm) of granular kamacite (mean Ni: 7.25 ± 0.07 wt%). Evenly dispersed particles (5-20 μm) of high-Ni taenite (Ni: 32.3 ± 1.12 wt%), Ni-rich schreibersite (Ni: 46.6 ± 0.08 wt%) and sporadically distributed angular to subangular particles of high carbon phase (HCP) of variable composition (C: 74- 15 wt%; Ni: 1.2-6.5 wt%) are common in the kamacite. Signatures of annealing are clearly evident from coarsening of kamacite grains (mean size: 70 μm), spheroidisation of taenite and schreibersite particles, development of cavernous (kamacite-filled pits) low-Ni taenite phase (Ni: 22 wt%) and a high degree of decomposition of cohenite to HCP. Bulk chemical data of WC (Co: 0.56 wt%, Cr: 0.01 wt%, Ga: 18 ppm, Ge: 33 ppm, Ir: 0.01 ppm and Cu: 220 ppm) closely correspond to the available data [2, 4]. Discussion: Strong effect of secondary reheating essentially due to high degree of shock (>750 kb) has been identified for complete destruction of all primary textures and formation of the thoroughly recrystallised granulated kamacite matrix decorated with evenly distributed particles of taenite and schreibersite. Based on the abundance of metals and non- metals, their parage- nesis and of course, phase compositions authors assign WC the structural group ‘Reheated Medium Octahedrite’. Shock altera- tion to the extent of kamacite recrystallisation coupled with low Hv ~ 150 ± 5 [2] confirms its well-annealed status. The secondary annealing is natural and not artificial, is indicated from ho- mogeneous equilibrium texture in the interior core and presence of zonal macrostructure that shows progressively decreasing degree of reheating from outer margin to interior core [5]. By con- trast, artificially heated texture mostly as patchy zones of dise- quilibrium melting like Santa Rosa [2] is absent in WC. WC falls within well defined fractionated group IIIAB, more specifically IIIB despite a strong reheating [6, 7]. This chemical grouping reaffirms that the earlier ataxite status for WC needs a revision because ataxites largely belong to chemical group IVB with bulk Ni content of 16 to 30 wt%. Estimated metallographic cooling rate of WC is 100oC/Ma based on its bulk Ni content [8]. HCP could be a possible carrier phase for solar noble gases? Fur- ther studies are in progress. Acknowledgement: We thank Carl Francis for the WC sample. References: [1] Palache C. and Shannon E.V. 1928. Am. Min. 13:406-409. [2] Buchwald V.F. 1975. Handbook of Iron Met. 1426pp. [3] Murty S.V.S. and Ranjith P.M. 2014. LPSC 45, #1110. [4] Wasson J.T. and Schaudy R. 1971. Icarus 14:59-70. [5] Cech R.E. 1962. GCA 26: 993-998. [6] Scott E.R.D. 1979. Min Mag 43:415-421. [7] Yang J. and Goldstein J.I. 2005. MAPS 40:239-253. [8] Yang J. and Goldstein J.I. 2005. GCA 70:3197- 3215.