THEIL MOUNTAINS

Geologists from the US Geological Survey found two fragments lying about 200 m apart on the ice at the NE base of Mt. Wrather, in the Theil Mountains of Antarctica. This was only the third meteorite ever found in Antarctica. The larger fragment weighed 18 kg and the smaller one weighed 10.6 kg. The fusion crust had been completely stripped and the surface smoothed and polished due to sandblasting by windblown rock and ice particles.

Theil Mountains is a typical member of the main-group pallasites. Trace element and O-isotopic studies suggest that pallasite metal crystallized from IIIAB liquids during fractional crystallization of the core and mantle. However, some recent studies rule out this scenario (Yang and Goldstein, 2006). Metallographic cooling rates of pallasites are not what would be expected given an origin at the core–mantle boundary. Instead, based on the size of the island phase in the cloudy zone of the pallasites, the cooling rates are 20× lower than those of IIIAB irons. Moreover, the Re–Os chronometer suggests that pallasites formed 60 m.y. later than IIIAB irons, raising doubts about a IIIAB core–mantle origin for main-group pallasites (E. Scott, 2007). Moreover, pallasites have a much younger range of CRE ages than the IIIAB irons (Huber et al., 2011). In a study of the close O-isotopic relationship between Main Group pallasites, mesosiderites, and the HED clan, Ziegler and Young (2007) discovered that non-homogenized samples of main-group pallasite olivines exhibit a bimodality in 17O values, which would distinguish their origin from that of the mesosiderites and the HED clan. Although refined O-isotopic studies have not supported a bimodality in 17O values, they definitively indicate that the parental source of main-group pallasites was different from that of mesosiderites and the HED clan (Greenwood et al., 2008).

The olivine grains in Theil Mountains have been significantly rounded, consistent with a moderate burial depth on the parent body, perhaps following silicate mantle excavation by impact (Saiki et al., 2003). The timescales calculated for this rounding are on the order of 10 to 100 m.y. The specimen of Theil Mountains shown above is a 16.1 g partial slice.

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