Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.426r&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 426
Mathematics
Logic
Cais, Bulk Rim Composition, Cv Chondrites, Mineralogy, Rims, Vigarano
Scientific paper
A striking feature of Ca,Al-rich inclusions (CAIs) in chondrites is the presence of mineralogical layers that typically form rim sequences up to 50 micrometers thick [1]. Many ideas regarding the origin of CAI rims have been proposed, but none are entirely satisfactory. The detailed mineralogy and bulk compositions of relatively unaltered CAI rims in the Vigarano (CV3) chondrite described here provide constraints on hypotheses of rim formation. Rim Mineralogy: CAIs in Vigarano consist of melilite (mel)- and spinel (sp)- rich varieties, both of which are rimmed [2]. Around mel-rich objects, the layer sequence is CAI interior --> sp-rich layer (sometimes absent) --> mel/anorthite (anor) layer --> Ti-Al-rich clinopyroxene (Tpx) layer --> Al- diopside (Al-diop) layer --> olivine (ol) +/- Al-diop layer --> host matrix. The sequence around sp-rich objects differs from this in that the mel/anor layer is absent. Both the sp-rich layer around mel-cored CAIs and the cores of sp-rich CAIs in Vigarano are largely comprised of a fine-grained (<=1 micrometer) intergrowth of sp, Tpx, and minor mel and perovskite. These intergrowths are typically so fine grained that little internal texture is discernible. Mixing calculations suggest the presence of ~10 vol% Tpx in the sp-rich layer of two mel-cored CAIs, and the presence of ~35 vol% Tpx within one sp-cored CAI. The mel/anor layer is sometimes monomineralic, consisting of mel alone, or bimineralic, consisting of both mel and anor. Where bimineralic, anor typically occurs in the outer part of the layer. In places, anor (An(sub)99-100) has partially altered to nepheline and voids. Rim mel is systematically less gehlenitic than mel in the CAI interiors, especially compared to mel in the interior adjacent to the rims. The Tpx layer (>2 and up to 15 wt% TiO2) and Al-diop layer (<2 wt% TiO2) are monomineralic and show chemical zoning trends radial to the CAIs. Moving outward, TiO2 and Al2O3 generally decrease, while SiO2 and MgO increase, although Al2O3 shows a small concentration maximum in the Al-diop layer. High-quality EMPA data suggest that Ti^3+/Ti^4+ decreases outward in the Tpx layer, and that Fe^3+ is present in the Al-diop layer, implying that a steep gradient in oxidation state occurs across these two layers. The ol layer is comprised of individually zoned grains (<=5 micrometers across) that have forsteritic cores and thin (<=1 micrometer) rims of more ferrous ol. The ol grains often form triple-grain junctions and occasionally form clusters that are enclosed by Al-diop. The texture of this polycrystalline layer suggests that it formed by the attachment of preexisting ol grains onto the surfaces of CAIs, and the triple-junctions and steep zoning profiles of the ol grains suggest that they were annealed in a short-lived heating event. Bulk Rim Composition: Bulk rim compositions for several mel-rich CAIs were determined by using EMPA traverses across representative portions of the rims. These compositions plot within the sp + forsterite (fo) field of the gehlenite-anorthite-forsterite ternary diagram of Stolper [3], and are unattainable by the igneous crystallization of a mel-rich CAI composition. Moreover, a hypothetical melt with the composition of the rims has a predicted crystallization sequence (sp --> sp + fo --> sp + fo + anor or mel or Tpx) that does not correspond to observed rim sequences. It thus appears that (1) the rim region did not form through crystallization of molten CAIs; and (2) rim layers did not originate solely by the crystallization of a melt layer present on a solid CAI core [4,5]. References: [1] Wark D. A. and Lovering J. F. (1977) Proc. LSC 8th, 95-112. [2] Ruzicka A. and Boynton W. V. (1991) Meteoritics, 26, 390-391. [3] Stolper E. (1982) GCA, 46, 2159-2180. [4] Korina M. I. et al. (1982) LPS XIII, 399- 400. [5] Bunch T. E. and Chang S. (1980) Meteoritics, 15, 270- 271.
Boynton Willam V.
Ruzicka Adam
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