INTERSTELLAR HYDROXYL IN METEORITIC CHONDRULES: IMPLICATIONS FOR THE ORIGIN OF WATER IN THE INNER SOLAR SYSTEM - I. A NEW APPROACH FOR THE DETERMINATION OF HYDROGEN ISOTOPE FRACTIONATION AT MODERATE TEMPERATURES

Abstract

Ion probe determinations of water concentrations and D/H ratios have been performed on different phases (pyroxene, olivine, and mesostasis) of chondrules from the Bishunpur and Semarkona LL3 chondrites. Mean water concentrations average 1290 ppm in olivine (500 to 2100 ppm), 2400 ppm in pyroxene (400 to 9800 ppm), and up to 3950 ppm in the mesostasis (1000 to 16500 ppm). In one chondrule an iron oxide phase exhibits a mean water concentration of 1.70 wt% (1.21 to 2.15 wt%).Hydrogen isotopic measurements reveal the presence of two water sources with high and low D/H ratios: pyroxenes range from 74 to 479 x 10-6, with a mean value of 186 x 10-6 (n = 54); the mesostasis from 124 to 203 x 10-6, with a mean value of 152 x 10-6 (n = 8); olivines from 63 to 209 x 10-6, with a mean value of 130 x 10-6 (n = 13); the iron oxide phase from 157 to 360 x 10-6, with a mean value of 219 x 10-6 (n = 7).Hydroxyls in pyroxenes have been identified as submicroscopic, poorly crystallized amphibole lamellae, through high resolution transmission electron microscopy. Considering the broad range of D/H ratios in pyroxene chondrules, alteration in a unique meteoritic parent body seems to be excluded. An early hydroxylation of the pyroxene occurring during chondrule formation due to the presence of water-bearing minerals among chondrule precursors seems the only possible interpretation of the data. Accordingly, the distribution of D/H ratios in chondrules reflects that of their precursors. Such a distribution is quite similar to that found in matrix minerals.As recorded by water concentration and D/H ratio, the mesostasis has been subjected to hydrothermal alteration after the incorporation of the chondrules into the meteoritic parent body. This alteration took place with water having a mean D/H ratio reflecting the mixing of the D-depleted and D-rich sources. The D-rich component is interpreted as a preserved interstellar source, while the low D/H ratio reflects a water component that underwent isotopic exchange with protosolar molecular hydrogen.