Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.426r&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 426
Computer Science
1
Clasts, Igneous, Igneous Differentiation, L Chondrites, Ordinary Chondrites, Partial Melting, Petrology, Silica
Scientific paper
The discovery of a ~4 X 4.5 X 7 mm^3, igneous-textured, silica-rich clast in the Bovedy chondrite [1] may have important implications regarding igneous processes that occurred on chondritic parent bodies [1,2]. This clast, designated Bo-1, is comprised of orthopyroxene, a silica polymorph, two feldspars, pigeonite, and minor chromite and trace metal and sulfide [1]. Bulk SEM/EMPA analyses of the clast indicated superchondritic Si/Mg and Si/Fe ratios, which Ruzicka and Boynton [1] proposed was produced by extensive olivine fractionation from a melted L-chondrite precursor. The low Fe/Mn ratio and low metal and sulfide abundances also suggest that the clast is largely missing a chondritic complement of metal and sulfide. To test these hypotheses, we measured the bulk composition of the clast using INAA techniques and found that the siderophile elements were lost in a two-step process and that partial melting also depleted incompatible lithophile elements. Lithophile Elements: Two splits (2.94 and 2.39 mg) of Bo-1 were analyzed. The concentrations of major elements (Ca, Fe, Cr, K, Na) bracket those previously determined by SEM/EMPA [1], suggesting that the two splits are reasonably representative of the bulk clast. If olivine and metal had been removed from an ordinary chondrite melt to produce the clast, then incompatible lithophile trace elements should have been enriched. Contrary to this expectation, however, the REE, Zr, Hf, Th, Sr, Rb, Cs and Br are consistently depleted to a level of 0.5-1.0 X CI abundances, while all of them (except the highly volatile Cs and Br) have concentrations of ~1.0-2.0 X CI abundances in ordinary chondrites. If the clast had been derived from melted ordinary chondrite material, then an additional step that removed incompatible elements, such as the loss of a partial melt, must have occurred. Siderophile Elements: Unlike lithophile trace elements, which are relatively unfractionated, the siderophiles Ni, Co, and Au are dramatically fractionated from Re, Os, Ir, and Ru. Nickel, Co, Au, and the chalcophile element Se are present at approximately 0.004-0.015 X CI abundances, compared to Re, Os, Ir, and Ru at 0.1-0.3 X CI. All the former elements have high affinities for S- rich metallic liquids, while the latter elements prefer solid metal [3], implying that the petrogenesis of Bo-1 included the loss of a S-rich metallic liquid. An equilibrium batch melting model of these trace siderophile elements, using the partition coefficients of [3], was constructed assuming an ordinary chondrite precursor. In these models, the proportion of liquid to solid silicate or of silicate to metal is unimportant, because the siderophile elements partition almost entirely into the metallic phases. The model results suggest that the siderophile trace elements can be adequately accounted for by a two-step process: (1) loss of a S-rich metallic liquid at high degrees of melting; and (2) subsequent loss of much of the remaining solid metal fraction. For an L-chondrite precursor, an optimal model involves the complete removal of metallic liquid generated by 90% partial melting of the metal + sulfide system, followed by the loss of 80% of the remaining 10% of the solids. Together, these two steps remove all but 2% of the initial metal + sulfide complement, consistent with the presence of only trace metal and sulfide in Bo-1. The large fraction of metallic melt involved in the first step implies that metallic liquid segregated from the remainder of the system at relatively high temperatures (~1325 degrees C for an L-chondrite precursor, based on the Fe-S phase diagram of [4]). References: [1] Ruzicka A. and Boynton W. V. (1992) Meteoritics, 27, 283. [2] Ruzicka A. and Boynton W. V. (1992) Meteoritics, 27, 284. [3] Jones J. H. and Drake M. J. (1986) Nature, 322, 221-228. [4] Kellerud G. and Yoder H. (1959) Econ. Geol., 54, 533-572.
Boynton Willam V.
Hill Dolores H.
Kring David A.
Ruzicka Adam
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