Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.344e&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 344
Computer Science
Anorthite, Cais, Chondrites, Carbonaceous, Condensation, Fassaite, Forsterite, Perovskite, Rare-Earth Elements, Refractory Lithophile Elements
Scientific paper
A fragment (30 mg) consisting of two inclusions (101.1 and 101.2) was separated from the Efremovka (CV3) meteorite. 101.1 is an unusual Type A CAI, whereas 101.2 consists of Cr-spinel and fassaite. INAA of the whole fragment revealed 16% MgO reflecting significant contributions from 101.2. Refractory lithophile elements in the bulk fragment have CI-enrichment factors of ~14 with two times enrichment factors for Ca, Eu, and Yb. CAI 101.1 (1.6 mm) contains more than 90% gehlenitic melilite (Ak(sub)1- Ak(sub)32) in its core. It is surrounded by a 5 layer rim sequence (~40 micrometers thick) consisting of spinel -->Al- diopside + fassaite (<= 0.7% Sc2O3) -->forsterite (Fo(sub)97- Fo(sub)100) --> diopside --> forsterite. Two small complete CAIs with a two layer sequence (diopside + anorthite) are contained within the core. Numerous layered sinuous inclusions presumably rim sequence fragments also consisting of diopside + anorthite, are locally crowded in the core. The melilite core is sprinkled with fassaite, perovskite, FeNi, and OsRu-rich metal blebs. Fassaite grains (<= 30 micrometers) contain enormous concentrations in Sc (up to 12.9% Sc2O3) and Zr (up to 5.4% ZrO2). Fassaite rims around FeNi blebs are rich in V (up to 5.4% V2O3) and are zoned with decreasing Sc-, Zr-, and V-concents from the metal cores to the outer fassaite rims. Sc2O3 and ZrO2 concentrations in fassaite display a positive correlation with a correlation coeffient of 0.88. This coherent behavior is a result of a complex cation substitution involving Mg, Ti, Sc, Zr, and V. A coupled substitution is demonstrated by the excellent linear correlation between Mg^2++Ti^4+(y) and Sc^3++Zr^4++Ti^3++V^3+(x) satisfying the equation y = 0.70-0.66x and having a linear regression coefficient of 0.84. Ti^3+/Ti^tot varies between 0.27 and 1. In contrast to fassaites, perovskites are generally depleted in Sc and Zr and enriched in Y (<=1.4% Y2O3). The assemblage andradite+wollastonite+ Fe^degree/or FeNi metal was encountered inside the sinuous fragments and in the diopside-anorthite rim sequence of one of the captured CAIs. The texture is strongly suggestive of the reduction of andradite to wollastonite+Fe^degree (via Fe3O4 and FeO). We consider this texture as evidence that andradite was formed in an oxidizing solar gas before capture of the sinuous inclusions by the host CAI and that andradite reduction took place after capture in the highly reduced host CAI refractory liquid. REE concentrations in various minerals reveal a distinct superrefractory pattern with depletions in Tm and Yb. Zr, Y, and Sc abundances of individual phases in the CAI core are indicative of crystal-liquid fractionation during crystallization: fassaite is relatively depleted in Y and enriched in Sc and Zr. In contrast, perovskite displays a complimentary abundance pattern for these elements. The individual mineral layers of the captured CAIs, sinuous fragments, and the rim sequence of the host CAI have similar superrefractory REE patterns but do not show fractionation between Sc, Y, and Zr. This indicates that the rim sequences did not crystallize from the same liquid of the CAI, but condensed from a common gas reservoir with a distinct superrefractory REE signature. Melilite contains excess ^26Mg* with an inferred initial ^26Al/^27Al ratio of (4.4 x 10^-5). However, the Mg-Al system in the anorthites of the captured sinuous fragments and the rim layers of the small CAIs is disturbed. This strongly suggests that oxidation and alteration processes took place in the earliest stage of the solar system.
El Goresy Ahmed
Lin Yangtin
Matsunami Satoshi
Nazarov Maxim
Palme Herbert
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