Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..340d&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 340
Computer Science
1
Basaltic Achondrites, Eucrites, Iron-Manganese, Oxygen Isotopes, Parent Body
Scientific paper
Introduction: The Fe/Mn ratios of basaltic achondrites (~30-40) do not match any chondrites [1]. The oxygen isotope ratios of these meteorites fall to the ^16O rich side of the terrestrial oxygen mass fractionation line and do not match any known chondrites [2]. Fe/Mn ratios of the achondrite precursors were low and reflect reflect oxygen fugacity driven variations in the Fe^2+/Fe^O ratio of the nebular precursor [3] instead of the volatility controlled depletion of Mn seen in C-chondrites [3,4]. Oxygen constraints favor a C-chondrite precursor to the basaltic achondrites, but the Fe-Mn-Mg constraints suggest an H-chondrite like precursor is more likely. Both constraints cannot be reconciled simultaneously with any known precursor nebular material and require either an unidentified (untestable) preplanetary precursor or a mixture of precursors that satisfies both constraints in combination. To test this later model, the oxygen isotopic ratios of basaltic achondrites are assumed to lie on the same mass fractionation line as their parent body bulk composition, and mixtures of different nebular materials were created using the oxygen isotope ratios of known potential precursors [5]. The mixing lines between a variety of Ochondrite and C-chondrite precursors intersect the basaltic achondrite mass fractionation line at the potential starting compositions from which basaltic achondrites fractionated. Several different mixtures of O- and C-chondrites produce potential compositions on the basaltic achondrite mass fractionation line. Mixing H and CM Chondrite Precursors: A mixture of average H chondrite and CM chondrites (represented by bulk Murchison) [6,7] illustrates this model. Mixing ~35 wt% Murchison and 65% average H chondrite gives an oxygen isotope ratio for the achondrite planetoid with delta^17O < 1 per mil heavier than the average basaltic achondrite. Although there is no data for solid-gas reactions on the parent body, this value may be a reasonable starting composition. With this H/CM ratio, the major and minor element bulk composition of the achondrite precursor can be calculated. This bulk composition strongly resembles the eucrite parent body generated by [8] with three exceptions: (i) The mixture contains ~19% FeO instead of the 14.4% [8]; (ii) The Fe/Mn ratio is 53 instead of 43; (iii) alkali elements Na and K are one order of magnitude too abundant. If the C content (~0.7 wt%) reacted with the FeO, it would reduce both the FeO content and Fe/Mn ratio (silicate) to values similar to Dreibus et al. Only the alkalis remain unexplained. Alkalis in the achondrite precursors cannot be depleted by simple volatility effects as Mn is undepleted and K isotopic systematics are presently incompatible with volatile depletion of K in basaltic achondrites [9]. The alkali elements remain unexplained by this model. Summary: Using oxygen isotopic ratios to constrain their ratio, at least one mix of H-chondrite (65 wt%) and carbonaceous- chondrite (35 wt%) precursors can provide a plausible basaltic achondrite parent body composition. This bulk composition contains sufficient C to reduce 19 wt% FeO in the mixture to 15%, a level compatible with reasonable estimates of the eucrite parent body. and also satisfies the Fe-Mn constraint on the parent body. This model predicts that the basaltic achondrite parent body contains about 16 wt% Fe metal and 6 wt% FeS that possibly formed a core. Trace element constraints can be applied to test this aspect of the model. References: [1] Delaney J. S. (1991) Meteoritics, 26. [2] Clayton et al. (1983) EPSL, 62. [3] Delaney J. S., this volume. [4] Kallemeyn and Wasson (1981) GCA, 45. [5] Clayton and Mayeda, (1992) Ann Rev. Earth Planet Sci. [6] Jarosewich (1990) Meteoritics, 25. [7] Wasson and Kallemeyn (1988) Trans. R. Soc. London, A325. [8] Dreibus et al (1977) Proc. LPSC, 8th. [9] Humayan and Clayton (1993) LPS XXIV. Acknowledgments: NASA Grant: NAG9-304.
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