Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.210c&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 210
Other
Scientific paper
The Watson iron meteorite contains a large silicate mass of H- group composition, igneous texture, and IIE oxygen isotopes, linking anew the IIEs and the H chondrites (1). Its silicate ages are 3.5 Ga [K-Ar] and 3.4 Ga [U-Th-He], similar to the ages of the Kodaikanal IIE meteorite; Watson's exposure age of 8 Ma is similar to that of H chondrites and unlike that of iron meteorites and other IIEs (except Kodaikanal) (2). These observations combined with petrography, thermometry, and metallography permit drawing a number of conclusions about Watson's parent body history. During an impact event H-group chondrite material became isolated within metal, melted at ~1500 degrees C, and cooled forming an igneous texture at ~3.5 Ga. For Watson both the 2-pyroxene and olivine-spinel thermometers record 1100 degrees C. Kodaikanal and Netschaevo have similar 2-pyroxene temperatures, but olivine- spinel values are much lower. This suggests that all three cooled to ~1100 degrees C at similar rates. Below this temperature Watson cooled faster, too fast for intergrain diffusion to permit the olivine-spinel thermometer to record lower temperatures. Antiperthite, K-feldspar exsolved from an albitic host, is present in Watson and indicates that intragrain diffusion continued to somewhat lower temperatures, placing an upper limit on the cooling rate. Watson's metal crystallized as polycrystalline taenite (gamma), trapping minor amounts of the silicate and troilite at gamma/gamma grain boundaries. Calculations based on the Fe-Ni-P equilibrium phase diagram indicate that schreibersite (Ph) first precipitated at ~800 degrees C, with kamacite (alpha) formation starting at ~725 degrees C. Widmanstatten pattern (WP) details suggest cooling at a rate of ~1000 degrees C/Ma to temperatures below 300 degrees C. A late shock event, undoubtedly the 8 Ma impact (2), drastically modified Watson's structure, producing textures that have not been seen before at this scale in iron meteorites. Throughout this large metal mass, particularly along gamma/gamma boundaries, Ph was shock-melted and dissolved several times its volume of surrounding alpha, while leaving nearby areas of WP only slightly, if at all, modified. Dendritic quench structures in the melt pockets indicate cooling rates of 10^4-10^5 degrees C/s. Metal composition and WP development are comparable in both Watson and Kodaikanal, indicating similar cooling rates in the 800 degrees to 400 degrees C range, but Kodaikanal lacks melt pockets, indicating a lower level of shock at final breakup. References: (1) Olsen E., Schwade J., Davis A.M., Clayton R.N., Mayeda T.K., Clarke R.S. Jr., Jarosewich E., and Steele I.M. (1991) Lunar Planet. Sci. (abstract) 22, 999-1000. (2) Schultz L. and Weber H.W. (1992) Lunar Planet. Sci. (abstract) 23, 1229-1230.
Clarke Roy S. Jr.
Olsen Edward J.
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