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Sulfide/Silicate Melt Partitioning During Enstatite Chondrite Melting

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Sulfide/Silicate Melt Partitioning During Enstatite Chondrite Melting 61st Annual Meteoritical Society Meeting 5255.pdf SULFIDE/SILICATE MELT PARTITIONING DURING ENSTATITE CHONDRITE MELTING. C. Floss1, R. A. Fogel2, G. Crozaz1, M. Weisberg2, and M. Prinz2, 1McDonnell Center for the Space Sciences and Department of Earth and Planetary Sciences, Washington University, St. Louis MO 63130, USA ([email protected]), 2American Museum of Natural History, Department of Earth and Planetary Sciences, New York NY 10024, USA. Introduction: Aubrites are igneous rocks thought (present only in CaS-saturated charges) has a flat REE to have formed from an enstatite chondrite-like pattern with abundances of about 0.5 x CI. precursor [1]. Yet their origin remains poorly Discussion: Oldhamite/glass D values are close to understood, at least partly because of confusion over 1 for most REE, consistent with previous results the role played by sulfides, particularly oldhamite. [4,5,6], and significantly lower than those expected CaS in aubrites contains high rare earth element based on REE abundances in aubritic oldhamite. In (REE) abundances and exhibits a variety of patterns contrast, D values for FeS are surprisingly high (from [2] that reflect, to some extent, those seen in about 0.1 to 1), given the low REE abundances of oldhamite from enstatite chondrites [3]. However, natural troilite. FeS/silicate partition coefficients experimental work suggests that CaS/silicate melt reported by [5] are somewhat lower, but exhibit a REE partition coefficients are too low to account for similar pattern. Alkali loss from the charges, the high abundances observed [4,5] and do not explain resulting in Ca-enriched glass, might provide a partial the variable patterns. A related problem concerns the explanation for low oldhamite REE abundances, if the solubility of oldhamite under conditions of aubrite REE follow Ca, but cannot account for the elevated formation, as a relict origin has been proposed for REE abundances in troilite. Sulfides in the Fe-Mg- some aubritic CaS [2,4]: phase equilibria experiments Mn-Ca-S system can exhibit extensive re-equilibration suggest that oldhamite readily dissolves in enstatite to low temperatures unless cooled rapidly [8]; low chondrite silicate melts [6]. In this initial work we re- temperature REE re-equilibration between troilite and examine the question of REE partitioning between oldhamite in aubrites may be a possible mechanism sulfides and silicates during melting of natural for enriching CaS in the REE. It is interesting to note enstatite chondrite material. that the REE pattern for troilite seen here is similar to Results: Powdered samples of Indarch and one of the patterns observed in aubritic oldhamite [2]. Indarch + synthetic CaS were melted at temperatures However this mechanism cannot account for all between 1250°C and 1400°C [6]. We report here the oldhamite REE patterns and occurrences (e.g., [7]). first in situ (ion microprobe) determinations of Furthermore, FeS/CaS ratios in many aubrites may not sulfide/silicate melt partition coefficients using natural be high enough to provide the strong enrichments levels of the REE (i.e., those initially present in observed in aubritic oldhamite. Additional work is Indarch). clearly needed to resolve this problem, as well as the REE concentrations in enstatite, forsterite and question of oldhamite solubility. metal are below detection limits. Glass has uniform REE concentrations and a flat pattern in each sample; References: [1] Keil K. (1989) Meteoritics, 24, abundances range from 0.5 to 10 x CI. Alabandite has 195. [2] Floss C. and Crozaz G. (1993) GCA, 57, a HREE-enriched pattern (Yb ~ 6 x CI) with a 4039. [3] Crozaz G. and Lundberg L. L. (1995) GCA, negative Eu anomaly. Natural alabandite exhibits a 59, 3817. [4] Lodders K. (1996) Meteoritics & Planet. similar but steeper pattern [7], with LREE typically Sci., 31, 749. [5] Dickinson T. L. and McCoy T. J. below detection limits [2,3]; this may be because our (1997) Meteoritics & Planet. Sci., 32, 395. [6] Fogel experimentally produced alabandite contains several R. A. et al. (1996) LPS XXVII, 371. [7] Wheelock M. wt.% Ca. Troilite is LREE-enriched (Ce ~ 5 x CI) M. et al. (1994) GCA, 58, 449. [8] Skinner B. J. and with a small positive Eu anomaly. However, in Luce F. D. (1971) Amer. Mineral., 56, 1269. aubritic troilite the REE are below detection limits [2,7], with concentrations more than two orders of magnitude lower than those observed here. Oldhamite.
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