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Hydrogen isotope abundances in the solar system. Part II: Meteorites with terrestrial-like ratio

1987; Elsevier BV; Volume: 51; Issue: 7 Linguagem: Inglês

10.1016/0016-7037(87)90171-2

1872-9533

F. Robert, M. Javoy, Jérôme Halbout, Bernard Dimon, Liliane Merlivat,

Methane Hydrates and Related Phenomena

Abstract Hydrogen isotopic compositions were determined by progressive pyrolysis in type 6 to type 3 ordinary chondrites. A marked decrease in the isotopic composition patterns was observed at intermediate temperatures (250–300°C) and results from the pyrolysis of a D-depleted component. A δD value of−400‰ for this component can be inferred from a mathematical treatment of the H concentration release pattern. At higher temperatures (600 to 900°C) the bimodal δD pattern was observed in Hedjaz (L3–L6) with negative δD values, suggesting the presence of a carbonaceous chondrite-like organic polymer in this meteorite. A peak in the δD pattern was observed at high temperature in all the analyzed samples, suggesting that D-rich H is widespread among meteorites. A minimum of 50% of the total H is concentrated in the fine-grained particles (the “holy smoke”) of equilibrated chondrites, reinforcing the idea that H is associated with the C. An internal correlation between the bulk isotopic composition of HT H2 and the maximum measured D H ratio is interpreted as the result of either the mixing of two components (Model 1), namely a D-depleted H at −400‰ and a D-rich H at +5000‰, or a progressive isotopic fractionation of a D-depleted reservoir (Model 2). The first model (the favored one) implies that the two components were present in different proportions at the time of the formation of each meteorite, and that metamorphism has homogeneized the two phases. The uniqueness of the two isotopic end-members for all meteorites is not demonstrated. The second model relates the isotopic fractionation of the D-rich phase to the degree of equilibration of chondrites. All the isotopic variations reported in this work for ordinary chondrites can be explained quantitatively by either one of these two models. The upper and lower limits for the D H ratios of the D-rich and of the D-depleted H in meteorites are calculated to be 1.1 × 10−3 and 9 × 10−5, respectively.