HIGH CARBON CONCENTRATIONS IN METEORITIC CHONDRULES: A RECORD OF METAL-SILICATE DIFFERENTIATION

Abstract

Both (reduced) type I and (oxidized) type II chondrules from the primitive chondrites Allende, Semarkona, and Bishunpur, were analysed with ion microprobe for the carbon and hydrogen concentrations of their silicates. The high and variable H and C concentrations found, i.e., from ≈100 to ≈1500 ppm H2O and from ≈100 to ≈3900 ppm C, show that these volatile elements were present in chondrule precursors as solid grains. Mean C concentrations are ≈ 3 times higher in type I chondrules than in type II chondrules. Therefore, it seems likely that metal-silicate oxydo-reduction reactions were promoted by C-rich grains whose heterogeneous distribution in the protosolar nebula caused the variation in the redox states of chondrules.Based on the present results, an in situ reduction of the silicates during chondrule melting seems plausible although reduction in the gas phase, before condensation, cannot be ruled out. Calculations of the devolatilisation rate of CO from hot chondrules account for the presently measured high C concentrations of type I chondrules, if 2–6 wt% C grains were originally present among chondrule precursors. Such a result is in agreement with the C concentration in C1 chondrites and with laboratory simulations which have demonstrated that up to ≈4 wt% C-grains were necessary to yield type I chondrules by in situ reduction (Connolly et al 1994). Assuming a solar C/O ratio in the chondrule forming regions of the protosolar nebula, it is likely that the high density of C grains in the gas resulted from presolar chemistry. This interpretation is supported by the corresponding C-grain/atom-gas ratio (2.7 × 10−13) which is ≈ 10 times below the ratio measured in the prestellar clouds.