* Astronomy

The Watson iron meteorite (93kg) was found in 1972 in the Nullarbor Plain, near Watson, South Australia (30° 30` S, 131° 33` E). It was first cut in 1990. The second slice (19 x 24 cm) revealed a large mass of silicate rock (achondritic stone) embedded in the metal, an estimated 30 cm³ and 110 grams in the slab. The total size of the mass is substantially larger. This is part of the largest single silicate mass known within an iron meteorite.


Expand (164kb, 560 x 639)


Title: Watson: A new link in the IIE iron chain
Authors: Olsen, E.; Davis, A.; Clarke, R. S.; Schultz, L.; Weber, H. W.; Clayton, R.; Mayeda, T.; Jarosewich, E.; Sylvester, P.; Grossman, L.; Wang, M.-S.; Lipschutz, M. E.; Steele, I. M.; Schwade, J.

Watson, which was found in 1972 in South Australia, contains the largest single silicate rock mass seen in any known iron meteorite. A comprehensive study has been completed on this unusual meteorite: petrography, metallography, analyses of the silicate inclusion (whole rock chemical analysis, INAA, RNAA, noble gases, and oxygen isotope analysis) and mineral compositions (by electron microprobe and ion microprobe). The whole rock has a composition of an H-chondrite minus the normal H-group metal and troilite content. The oxygen isotope composition is that of the silicates in the IIE iron meteorites and lies along an oxygen isotope fractionation line with the H-group chondrites. Trace elements in the metal confirm Watson is a new IIE iron.
Whole rock Watson silicate shows an enrichment in K and P (each approximately 2X H-chondrites). The silicate inclusion has a highly equilibrated igneous (peridotite-like) texture with olivine largely poikilitic within low-Ca pyroxene: olivine (Fa20), opx (Fs17Wo3), capx (Fs9Wo14)(with very fine exsolution lamellae), antiperthite feldspar (An1-3Or5) with less than 1 micron exsolution lamellae (An1-3Or greater than 40), shocked feldspar with altered stoichiometry, minor whitlockite (also a poorly characterized interstitial phosphate-rich phase) and chromite, and only traces of metal and troilite. The individual silicate minerals have normal chondritic REE patterns, but whitlockite has a remarkable REE pattern. It is very enriched in light REE (La is 720X C1, and Lu is 90X C1, as opposed to usual chondritic values of approximately 300X and 100-150X, respectively) with a negative Eu anomaly. The enrichment of whole rock K is expressed both in an unusually high mean modal Or content of the feldspar, Or13, and in the presence of antiperthite.

Read more