Allan Hills Icefields

Cassidy (2003) visited the area in 1978 to search for meteorites because Butcher Ridge is located close to the edge of the polar plateau. Although he did not find any meteorites, what he saw caused him to suspect that the igneous rocks exposed at Butcher Ridge are melt-rocks that formed when a large meteorite impacted on quartz-rich sandstones of the Beacon Supergroup and an interbedded sill of Ferrar Dolerite. 

This alternative explanation is based on extensive information derived from the study of meteorite impact craters (e.g., Koeberl 1994 Dressier et al. 1994 Koeberl and MacLeod 2002). Unfortunately experts familiar with hypervelocity impacts of large meteorites have not had an opportunity to examine the rocks at Butcher Ridge. 

The Allan Hills are a relatively small group of sandstone and dolerite ridges on the edge of the polar plateau at 74°43 S and 159°40 E. The East Antarctic ice sheet adjacent to the Allan Hills in Fig. 18.10 contains several areas of blue ice where meteorite specimens have accumulated which are identified by the indicated letter codes (Appendix 18.12.1)  

The Main ice field of the Allan HiUs is the site where in 1976/77 a joint American- Japanese team first collected nine meteorite specimens (ALHA76001 to ALHA76009). This collection consisted of seven ordinary chondrites, one iron, and one polymict eucrite (Score and Lindstrom 1990). In the following year, a systematic search by another American-Japanese team led by W.A. Cassidy collected 299 specimens. The next season (1978/79) yielded 260 specimens. After that collecting season, ANSMET had turned the comer and found meteorite heaven (Cassidy 2003, p. 34). 

The progress in the recovery of ALH meteorites is recorded in Fig. 18.11. The spikes in the number of specimens collected identify the field seasons during which ANSMET carried out systematic searches in the blue-ice areas of the Allan HiUs. The last major harvest during 1988/89 consisted of the 163 specimens that were collected by members of GANOVEX. The specimens collected in the 1976/77,1977/78, and 1978/79 fieldseasons on the Allan Hills icefields were split and the USA and Japan shared half of each specimen. The catalog of Score and Lindstrom (1990) Usts the original masses of these specimens, whereas Yanai and Kojima (1995) recorded the masses of the splits that were transferred to the National Institute of Polar Research (NIPR) in Tokyo, Japan. 

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A final point to be made is that the spherules must have been deposited on the surface of the East Antarctic ice sheet, which means that East Antarctica was glaciated at the time (i.e., two to three million years ago). Therefore, the terrestrial age of the meteorite-ablation spherules is consistent with the evidence that Cenozoic volcanoes in the Transantarctic Mountains (e.g., Mt. Early and Sheridan Bluff) erupted through glacial ice from which it follows that the glaciation of East Antarctica started during the late Oligocene/early Miocene perhaps as early as 25 million years ago. However, none of the ice cores that have been drilled in East or West Antarctica have encountered ice that is as old as the meteorite ablation spherules in the icefield near the Allan Hills. 

The meteorite specimens collected on the icefields of the Allan Hills include at least three rocks from Mars and one from the Moon. In addition, the collection includes rare specimens of certain achondrites and other... 

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... 

The stony meteorites collected during the 1977/78 field season on the icefields adjacent to the Allan Hills included seven specimens that contain deposits of white efflorescences on their surfaces and in cracks. Marvin (1980) reported that these deposits were composed of the hydrated carbonates and snlfates of magnesium and calcium listed in Table 18.6. In spite of the presence of these deposits, only two of the seven specimens have rust stains. The absence of heavy rust stains is difficult to reconcile with the prevalence of Mg-salts... 

The properties of meteorites collected in Antarctica that have been measured include their chemical and mineralogical compositions as well as their textures, isotopic ages, and cosmic-ray exposure histories. The scientific value of the accumulated data is exemplified in this chapter by selected specimens of Antarctic meteorites such as the Derrick Peak iron, by the first lunar meteorite recovered on the icefields of fhe Allan Hills (ALHA 81005), and by the martian rock ALH 84001. The interpretation of data derived from these and other meteorites collected in Antarctica contribute to the on-going exploration of the solar system. 

Harvey RP (1990) Terrestrial age mapping of the Allan Hills Main Icefield and implications for the Whillans-Cassidy model of meteorite concentration. In Cassidy WA, WhUlans IM (eds) Workshop on Antarctic meteorite stranding surfaces. LPI Tech. Rept. 90-03. Houston, TX, pp 88-90... 

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