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

Abstract

available online at http://meteoritics.org 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. Noble gas content and isotope abundances in phases of the Saint-Aubin (UNGR) iron meteoriteThe 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 4 He/ 21 Ne 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 3 He were 9-16 Ma.Saint-Aubin is very peculiar because it contains very large chromite crystals, which-like the metal-contain 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.

An attempt to characterize phase Q: Noble gas, Raman spectroscopy and transmission electron microscopy in residues prepared from the Allende meteorite

“…We have previously published the details of the gas purification and the measurements (Matsuda et al, 2005;Nishimura et al, 2008). All the sample data were blank-corrected based on hot blanks measured prior to the sample analysis.…”

The effect of pyridine treatment on phase Q: Orgueil and Allende

“…The gas purification and the details of the noble gas measurements are given in our previous works (Matsuda et al. 2005; Nishimura et al. 2008).…”

Multiple carriers of Q noble gases in primitive meteorites

“…129 Xe/ 132 Xe ratios show more important variations with a clear excess of radiogenic 129 Xe in all except the final extraction steps that lies close to the atmospheric composition (Table ). These results are consistent with previous reports of Kr and Xe concentrations within troilites from iron meteorites [ Mathew and Marti , ; Nishimura et al ., ]. No significant mass variation was observed following the HF/HCl treatment, but the FeS‐HCl residues displayed a significant gas loss compared to the initial material, both for Kr (≈53%) and Xe (≈38%; Figure and Table ).…”

“…In contrast, the release pattern of noble gases trapped within pyrrhotite closely resembles that of the Q gases, with a major release of Kr and Xe occurring between 900 and 1200°C (Figure ). Similar results were obtained for each of the five noble gases released from troilite from the Saint Aubin iron meteorite (ungrouped) [ Nishimura et al ., ], demonstrating that nickel‐poor iron sulfides match the thermal characteristics of phase Q. The high‐temperature release is directly linked to the incongruent dissociation of pyrrhotite, which occurs in the range 1000–1200°C, depending on the stoichiometry [ Kellerud , ].…”

“…However, recent studies report striking results that also suggest that sulfides may have been underestimated as a potential subcarrier of Q gases. Troilite (FeS) from iron meteorites reproduces the thermal behavior of phase Q well, with a common release temperature of 1000–1200°C for all noble gases [ Nishimura et al ., ]. In addition, stepped combustion measurements on Renazzo‐type chondrites have revealed that very little carbon is associated with Q gases, suggesting that phase Q might not be solely carbonaceous [ Verchovsky et al ., ].…”