Exposure history of shergottites

The cosmogenic nuclides 36Cl(tsub(1/2)=3.0x105 years), 26Al(7.05x105 years), 10Be(1.6x106 years) and 53Mn(3.7x106 years) were measured in three shergottites, ALHA 77005, EETA 79001, and Shergotty by accelerator mass spectrometry and neutron activation. The cosmogenic nuclide data constrain the history and origin of shergottites. ALHA 77005 was ejected from its parent body about 2.7 My ago and was exposed to cosmic rays as a body of 5-6 cm radius. Solar cosmic ray produced 26Al and 53Mn were first clearly observed in this meteorite. The exposure age of EETA 79001 was 0.6 My and the preatmospheric radius was about 15 cm. The exposure age of Shergotty was calculated to be 2.2 My and the preatmospheric size was about 10-15 cm in radius. the terrestrial age of ALHA 77005 was 0.2 My and that of EETA 79001 was less than 0.06 My. All cosmogenic nuclide data indicate that the three shergottites were never exposed to cosmic rays in their parent bodies whether that parent body was Mars or an asteroid. All shergottites were ejected from greater than a 3 m depth in their parent bodies. The ejection of EETA 79001 was a different event from that of the other shergottites. (author)