Hydrogen isotope abundances in the solar system. Part I: Unequilibrated chondrites☆

Abstract Concentrations and isotopic compositions of H extracted by pyrolysis under a He flow were determined in three unequilibrated type 3 ordinary chondrites: Chainpur, Bishunpur and Ngawi. In Chainpur, a D-depleted component ( δD = −500‰ ) is removed at low temperature (200 to 400°C), whereas at temperatures ranging between 450°C and 900°C, a D-rich component is evolved with a bimodal release pattern. The isotopic composition patterns reported as a function of temperature are qualitatively similar in the three meteorites, with minimum and maximum values at 300–400°C and 700–900°C, respectively. Consequently, the bulk isotopic composition of each meteorite is interpreted as resulting from the mixing of two components, the low-δD fraction constituting 80 to 90% of the total H. The assumption that a carbonaceous chondrite-like polymer is the carrier of the D-rich H is supported by two observations: i) the D-rich H is released with bimodal patterns, and ii) oxidation of whole-rock samples with H 2 O 2 removes this D-rich hydrogen (as well as a significant fraction of the D-depleted com ponent). For each meteorite the peak in H concentration associated with the marked increase in the δD pattern permits us to calculate an isotopic composition for the D-rich H. The calculated δD values never exceed ca . + 5500%. and no experimental indication of the presence of any D-richer H was found. Uncertainties on these determinations do not permit us to establish the presence of a unique D-rich component, common to these three meteorites. In Chainpur the low-δD hydrogen is concentrated in the amorphous matrix surrounding the chondrules, while the D-rich component is located inside the chondrules or at their surfaces.