Selective release of D and ¹³C from insoluble organic matter of the Murchison meteorite by impact shock

Abstract

Abstract-We performed shock-recovery experiments on insoluble organic matter (IOM) purified from the Murchison meteorite, and determined the abundances and isotope ratios of hydrogen and carbon in the shocked IOM sample. We also performed shock experiments on type III kerogen and compared the results of these experiments with the experimental results regarding IOM.The shock selectively released D and 13 C from the IOM, while it preferably released H and 12 C from the kerogen. The release of these elements from IOM cannot be explained in terms of the isotope effect, whereas their release from kerogen can be explained by this effect. The selective release of heavier isotopes from IOM would be due to its structure, in which D and 13 C-enriched parts are present as an inhomogeneity and are weakly attached to the main network. Shock gave rise to a high release of D even at a lower degree of dehydrogenation compared with the stepwise heating of IOM. This effective release of D is probably an inherent result of shock, in which a dynamic highpressure and high-temperature condition prevails. Thus, shock would effectively control the hydrogen isotope behavior of extraterrestrial organic matter during the evolution of the solar nebula.

Gradual and stepwise pyrolyses of insoluble organic matter from the Murchison meteorite revealing chemical structure and isotopic distribution

“…Because the dD values of the residues result from mixing of the pyrolysates and the thermally stable components, the thermally stable components should have lower dD values than the residues (e.g., +734&). Mimura et al (2007) reported that a shocked IOM sample had the low dD value of +235&. Thus, it is possible that the thermally stable parts have much smaller dD values than the bulk IOM.…”

“…Other possible reactions are irradiation and shock reactions, which occur ubiquitously in space. The irradiation of organic mixtures produces carbon isotope heterogeneity (Sephton et al, 2004), while shock reactions effectively decrease the hydrogen isotope values of the IOM (Mimura et al, 2007). It is highly plausible that the product had been affected by these reactions before forming the meteorite.…”

“…We purified a sample of the IOM from the Murchison meteorite according to the method described by Mimura et al (2007). A fresh inner portion of the Murchison meteorite was pulverized and then washed by a Soxhlet apparatus employing a mixture of dichloromethane and methanol (7/1:v/v).…”

“…The distributions and values of some isotopes in IOM from a particular chondrite have been studied using several methods: stepwise combustion - Kerridge (1983), Alexander et al (1998); stepwise pyrolysis - Yang and Epstein (1983) and Yang and Epstein (1984); hydrous pyrolysis - Sephton et al (2000); hydropyrolysis - Sephton et al (2004); chemical oxidation - Hayatsu et al (1980), Remusat et al (2006), Huang et al (2007); GC-irMS - Sephton and Gilmour (2001), Wang et al (2005); SIMS - Busemann et al (2006), Nakamura-Messenger et al (2006), Remusat et al (2010); and impact shock - Mimura et al (2007). These experiments have revealed that IOM is divided into several components characterized by different structural and isotopic features.…”

Complex aromatic hydrocarbons in Stardust samples collected from comet 81P/Wild 2

“…One way to dilute isotopic enrichments is through the partial pyrolysis of the organic carrier phase during impact induced shock heating. For example, Mimura et al (2007) recently demonstrated the selective release of deuterium from Murchison (CM2) insoluble organic matter by shock impact. This illustrates the fundamental point that all cometary particles collected by Stardust will have experienced a significant thermal pulse during impact.…”