Noble gas content and isotope abundances in phases of the Saint-Aubin (UNGR) iron meteorite

Abstract

We analyzed the noble gas isotopes in the Fe-Ni metal and inclusions of the Saint-Aubin iron meteorite, utilizing the stepwise heating technique to separate the various components of noble gases. The light noble gases in all samples are mostly cosmogenic, with some admixture from the terrestrial atmosphere. Total abundances of noble gases in metal are one of the lowest found so far in iron meteorites and the 4He/21Ne ratio is as high as 503, suggesting that the Saint-Aubin iron meteorite was derived from a very large meteoroid in space. The exposure ages obtained from cosmogenic 3He were 916 Ma. Saint-Aubin is very peculiar because it contains very large chromite crystals, whichlike the metalcontain only cosmogenic and atmospheric noble gases. The noble gases in all the samples do not reveal any primordial components. The only exception is the 1000 °C fraction of schreibersite which contained about 5% of the Xe-HL component. The Xe-Q and the El Taco Xe components were not found and only the Xe-HL is present in this fraction. Some presolar diamond, the only carrier for the HL component known today, must have been available during growth of the schreibersite. However, it is also possible that this excess is due to the addition of cosmogenic and fission components. In this case, all the primordial components are masked (or lost) by the later events such as cosmic-ray irradiation, heating, and radioactive decay.