Chemical weathering of stony meteorites

The long residence ages of meteorites on the ice fields of the Allan Hills and elsewhere in Antarctica imply that the rates of chemical weathering reactions are 

Meteoroids of all kinds are also altered in various ways by exposure to cosmic rays and to the solar wind while they revolve around the Sun (e.g., Section 18.6.1). Some specimens of stony meteorites even contain evidence that their chemical and mineral compositions were altered before they were released into 

1983 Fig. 18.5). Specimens that fell less than 150,000 yettfs ago occur primarily west of the ice ramp on ice from higher stratigraphic levels in the ice sheet. The LL6 chondrite ALH 88019, which has the longest terrestrial age of 2.20 0.40 Ma, also lies at the ice ramp with other specimens that fell more than 300,000 years ago. The hne across the edge of the ice at the Cul de Sac is the radar-echo profile measured by Faure and Buchanan (1987) (Adapted from Nishiizumi et al. 1989 Scherer et al. 1997) 

The Antarctic meteorites are more extensively weathered and fractured than expected because of a misconception about the microclimatic conditions on the ice fields of the polar plateau. Although the air temperature remains below the freezing point of water, the temperature in the interior of a meteorite specimen exposed to solar radiation may rise sufficiently to melt snow on its surface. For example, Schultz (1990) showed in Fig. 18.18 that the internal temperature of a sample of the carbonaceous chondrite Allende placed on the ice of the Far Western ice field adjacent to the Allan Hills in December of 1985, was consistently higher than the air temperature by up to about 15°C and that the temperature dijference increased as the wind speed and cloud cover decreased. The wind apparently cools meteorite specimens exposed on the icefields while the cloud cover modulates the amount of solar energy they receive. On several occasions, when the wind speed decreased to zero, the internal temperature of the test specimen monitored by Schultz (1990) actually rose to +5°C. Similar results were reported by Harvey (2003) who measured the temperature at the 

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The evidence that stony meteorites collected in Antarctica are weathered implies that certain chemical elements are mobilized within the affected meteorite specimens. In addition, glacial meltwater and atmospheric carbon dioxide invade the affected meteorite specimens together with halogens, sulfur-bearing componnds, and organic molecules. Therefore, meteorites that fell on the East Antarctic ice sheet are altered mineralogically as well as chemically and, for that reason, their trace-element concentrations may differ from those of non-Antarctic meteorite falls. 

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