Hydrogen concentrations (expressed as H2O wt. %) and their corresponding isotopic compositions have been determined with the CRPG-Nancy ion microprobe in twelve chondrules from the Bishunpur deuterium-rich LL3 meteorite. This meteorite is known to exhibit - as a whole rock - a marked enrichment in its D/H ratio relative to terrestrial values. It is therefore an ideal sample to address the problem of the origin of hydrogen in chondrules since the terrestrial contamination has clearly different isotopic signatures (with D/H ratios around 120-160 x10-6) from those of indigeneous phases (D/H up to 450 x10-6). Results of isotopic analyses obtained on individual chondrules have been reported in the literature (Robert et al., 1979; Robert et al. 1987; Sears et al., 1995). In some cases it has been shown that hydrogen with non-terrestrial D/H ratios, was outgassed from chondrules (same refs.). However, no firm conclusions were derived from such pyrolysis experiments, since LL3 matrix is known to contain compounds (organic macromolecules and water) which have extremely high D/H ratios and are probably of interstellar origin. Therefore, matrix contamination at the surface of the chondrules or matrix inclusions embedded into the chondrules, were possible sources for these high D/H ratios. In our analyses, all the data have been acquired from well-identified olivine or pyroxene crystals and in the glass. Therefore the current data truly represent hydrogen within the main constituents of chondrules.
Based on detailed calibrations on terrestrial olivines and glasses, the relative precision in water concentration is estimated to lie between ±50 and ±7% for concentrations ranging between 500 to 10,000 ppm, respectively. The precision on the D/H ratios is ¾±10 % for corresponding concentrations „ 1,000 ppm. The analysed chondrules belong to type I and type II (after McSween's classification; type I stands for highly reduced olivine i.e. Mg/Fe+Mg close to unity while type II have Mg/Fe+Mg ¾ 0.85). Five to ten spots were analysed in each chondrule in order to test their internal homogeneity in concentration and in isotopic composition. No systematic gradients were found in these profiles. The range of water concentrations is 800 to 20,000 ppm, although most of the data lie between 1000 and 5000 ppm. In an individual chondrule, the distribution of both water concentration and its isotopic composition could be either quite homogeneus, either extremely heterogeneous. For example, in chondrule Ch.8, 7 spots exhibit dD values between -370 and +1540” (that is D/H ratios between 58 and 395 x10-6) while in chondrule Ch.9 they are restricted bewteen -63 and -5 ” (that is D/H ratios between 146 and 155 x10-6) for
3 spots.
The isotopic data demonstrate that water in chondrules results neither from terrestrial contamination nor by diffusion into the chondrule during hydrothermal alteration, since both processes would have produced a homogeneous isotopic composition within each chondrule. In addition, since low and high end members of the D/H distribution (D/H from 58 to 456 x10-6) are typically extraterrestrial, the intermediate terrestrial like ratios result from the mixing of these two end members and not from terrestrial contamination. This is a central conclusion as far as the origin of water in the solar system is concerned.
These ion microprobe measurements demonstrate that hydrogen bearing components (water and organics) were present in chondrule precursors. The sources for these components are 1) the interstellar medium for the deuterium rich phases (with D/H > 150 x10-6; Deloule and Robert, 1995) and 2) local oxidation of the protosolar nebula hydrogen resulting in the formation of water-bearing minerals (Lécluse and Robert, 1994). The large differences between the isotopic composition of these phases were not homogeneised during the flash heating and rapid cooling of the chondrules.
Deloule & Robert, Geochim. Cosmochim. Acta (1995, in press).
Lécluse & Robert, Geochim. Cosmochim. Acta 58, 2927-2939 (1994).
Robert et al., Nature 282, 785-789 (1979).
Robert et al., Geochim. Cosmochim. Acta 51, 1787-1805 (1987).
Sears et al., Meteoritics 30, 169-181 (1995).