Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.208c&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 208
Mathematics
Logic
Scientific paper
The enstatite meteorites comprise a group of chondrites, achondrites (aubrites), stony irons, and related irons with close mineralogical and geochemical affinities: highly reduced assemblages (almost FeO-free silicates, Si-bearing metal, uncommon sulfides, etc.) and nearly identical oxygen isotopic signature. The interpretation of these similarities has been the focus of numerous studies addressing the possible genetic relationships among the different groups. None of the attempts to derive aubrites from known enstatite chondrites have been quantitatively satisfactory [1-5]. Current ideas invoke the necessity of at least four different enstatite meteorite parent bodies: EH chondrites, EL chondrites, aubrites, and the Shallowater parental material [1]. In general, troilites are richer in titanium in aubrites than in enstatite chondrites; this enrichment cannot be achieved through simple melting of enstatite chondrites and fractionation of a sulfide-rich eutectic liquid from a sulfide-poor residue. The small density contrast between Ti-rich and Ti-poor troilite, prevents the efficient fractional crystallization of the sulfide phases. Consequently, a different body with sulfides richer in Ti has been advocated for the parental material of aubrites [6]. This hypothesis, however, does not yet satisfactorily explain either the presence in several aubrites of troilite grains with "E-chondrite-like" Ti contents (e.g., Khor Temiki, Bishopville, Norton County) [7], nor that the bulk titanium abundance of enstatite chondrites and achondrites are very similar (about 600 and 400 ppm, respectively). Despite its very low abundance, the Ti content of osbornite (77.4 wt%, if stoichiometric TiN) makes this rare mineral a non-negligible contributor to the bulk Ti content of this group of meteorites. By mass balance: [Ti](sub)bulk = X(sub)FeS . [Ti](sub)FeS + X(sub)TiN . [Ti](sub)TiN where X(sub)i is the mass fraction of the indicated mineral. For simplicity, all Ti-bearing sulfides can be grouped in a single term (FeS): this approximation is justified because troilite is by far the most abundant Ti-bearing sulfide in both enstatite chondrites and aubrites [7]. Solutions to this equation show that it may be possible to produce Ti-rich, if redistribution of some titanium originally present in the form of osbornite is allowed after segregation of a Fe-FeS liquid during core formation. How can this proposed "redistribution" occur? A quantitative assessment of this problem cannot be performed at this time: although pure TiN is a very refractory mineral, its stability under conditions of partial melting of silicate+sulfide+metal magmas is not known. If all the titanium present in osbornite goes into the troilite, the maximum concentrations of this element observed in some aubritic sulfide grains (e.g., 16.3 wt% in Bustee, [7]) are easily achieved. This could imply that, at a certain temperature, osbornite decomposes and releases free N2 gas. Experimental evidence [9] suggests that the equilibrium decomposition of osbornite (TiN(sub)(s) = Ti(sub)(g)+0.5N2(8)) occurs at approximately 1800 degrees C; these temperatures are not implausible to occur during the igneous evolution of aubrites. REFERENCES: [1] Keil K. (1989) Meteoritics 24, 195-208. [2] Watters T.R. and Prinz M. (1979) Proc. Lunar Planet Sci. Conf. 10th, 1073-1093. [3] Biswas S., Walsh T., Bart G., and Lipschutz M.E. (1980) Geochim. Cosmochim. Acta 44, 2097-2109. [4] Dodd R.T. (1981) Meteorites: A Petrologic Chemical Synthesis. Cambridge Univ. Press, 368 pp. [5] Rubin A.E. (1983) J. Geophys. Res. 88 (Suppl.), B293-B300. [6] Brett R. and Keil K. (1986/87) Earth Planet. Sci. Lett. 81, 1-6. [7] Keil K. (1969) Earth Planet. Sci. Lett. 7, 243-248. [8] Keil K., Ntaflos Th., Taylor G.J., Brearley A.J., Newsom H.E., and Romig Jr. A.D. (1989) Geochim. Cosmochim. Acta 53, 3291-3307. [9] Linevsky M.J. (1965) General Electric Co. AFML-TR-64-420.
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