Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.516h&link_type=abstract
Meteoritics, vol. 30, no. 5, page 516
Mathematics
Logic
Scientific paper
C and H contents (reported hereafter as [C] and [H]) of 33 chondrules of all petrological types and sizes, and belonging to some of the least altered and metamorphosed chondrites were determined with the CRPG Nancy ion-microprobe. Special care was taken in order to efficiently get rid of the terrestrial contamination. Before analysis, each sampling area (25mm and 50mm for the smaller chondrules) was sputtered by the O2- primary beam (20nA) for 5 minutes. Precise chemical concentrations for H and C were obtained for a -60V offset applied to the sample, along with an energy filtering of 10V. Mass resolution (M/DM) of 1800 is sufficient to discriminate the 24Mg++ signal from the 12C+. Mid-ocean ridge basalts were used for calibration of C and H [1]. Major element concentrations in phases were obtained by electron probe analysis. Then, in each chondrule, phase proportions were visually estimated allowing the calculation of a bulk concentration for the major elements. [C] and [H] were obtained by two methods : 1) using internal chondrule correlations between individual ion-probe spots for carbon (or H) and major elements contents or 2) by averaging all ion-probe [C] and [H] determinations. Chondrule contents range from100 to 8000 ppm H2O, and from 100 to7000 ppm C. For comparison purposes, although the speciation of H in chondrules is unknown, [H] is expressed as H2O. These concentrations are surprisingly high since they are well above their equilibrium solubilities between silicate melts and protosolar gaseous species at any pressure. Based on their [C] and [H], two groups of chondrites can be distinguished (see Fig. 1): Group I (unequilibrated chondrites) has high [C] and low [H] while Group II is more equilibrated (types 3.6 and 3.7) with low [C] and high [H]. Correlations between chondrules in a given meteorite are also clearly defined : Group I exhibits a positive correlation [Na] vs. [H] (not shown here) and Group II a positive correlation between [C] and [H] (see Fig. 1). Finally, in nearly all individual chondrules, [C] is concentrated in the glass relative to minerals but with no counterpart for [H], the concentrations of which vary by one order of magnitude (see Clovis in Fig. 1). It seems unlikely that matrix contamination occurred during the local and mild metamorphism of type 3 chondrites. For example, in Group I, smaller chondrules are systematically enriched in Na but not in H while in Group II, the glass is enriched in C with no counterpart for [H]. If [C] and [H] were established at the time of the formation of the chondrules, the possible local co-volatilization of C, H and Na has been limited since [H] is quite heterogeneous within the same chondrule. Large scale chemical variations in solar nebula grains, pre-dating the formation of chondrules, is actually a plausible model to account for these volatile element concentrations. Moreover, such high contents of chondrules could strongly modify the H and C budgets of the primitive Earth. References: [1] Chaussidon M. et al. (1991) Stable Isotope Geochemistry: A Tribute to Samuel Epstein (H. P. Taylor et al., eds.), pp. 325-337.
Chaussidon Marc
Hanon Pascal
Robert Frédéric
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