Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..570r&link_type=abstract
Meteoritics, vol. 30, no. 5, page 570
Other
2
Calcium-Aluminum-Rich Inclusions, Efremovka, Layer Growth, Leoville, Rims, Vigarano
Scientific paper
Many hypotheses have been proposed to account for the ~50 micrometer-thick layer sequences (Wark-Lovering rims) that typically surround coarse-grained Ca,Al-rich inclusions (CAIs), but to date no consensus has emerged on how these rims formed. A two-step process-- flash heating of CAIs to produce a refractory residue on the margins of CAIs [1,2,3], followed by reaction and diffusion between CAIs or the refractory residue and an external medium rich in Mg, Si and other ferromagnesian and volatile elements to form the layers [3,4,5]-- may have formed the rims. We have tested the second step of this process quantitatively, and show that many, but not all, of the layering characteristics of CAI rims in the Vigarano, Leoville, and Efremovka CV3 chondrites can be explained by steady-state reaction and diffusion between CAIs and an external medium rich in Mg and Si. Moreover, observed variations in the details of the layering from one CAI to another can be explained primarily by differences in the identity and composition of the external medium, which appears to have included vapor alone, vapor + olivine, and olivine +/- clinopyroxene +/- vapor. An idealized layer sequence for CAI rims in Vigarano, Leoville, and Efremovka can be represented as MSF|S|AM|D|O, where MSF = melilite (M) + spinel (S) + fassaite (F) in the interior of CAIs; S = spinel-rich layer; AM = a layer consisting either of anorthite (A) alone, or M alone, or both A and M; D = a clinopyroxene layer consisting mainly of aluminous diopside (D) that is zoned to fassaite towards the CAI; and O = olivine-rich layer, composed mainly of individually zoned olivine grains that apparently pre-existed layer formation [3]. A or M are absent between the S and D layers in roughly half of the rims. The O layer varies considerably in thickness (0-60 micrometers thick) and in porosity from rim to rim, with olivine grains either tightly intergrown to form a compact layer or arranged loosely on the outer surfaces of the CAIs. None of these variations in rim layers are correlated with the modal compositions of the CAIs. In our models, we investigated the reaction of CAI interiors (containing M + S + F) with various proportions of vapor (V), O, and D in the 5-component system MgO-AlO(sub)3/2- CaO-SiO2-TiO2. Representative compositions were assumed for the solids. Most likely, a vapor reacting with CAIs would have small (e.g., solar) or trivial abundances of Al, Ca, and Ti compared to Si and Mg, and such Al-, Ca-, and Ti-poor compositions were assumed for the vapor. The model zone sequence MSF|S|A|D|V can form when Mg/[Mg+Si] 0.28-0.47 in the vapor, and is consistent with rims that contain an A layer but that lack an O layer. The zone sequence MSF|S|D|VO, which can form when Mg/[Mg+Si] 0-0.47 in the vapor, may explain rims that lack an A (and M) layer and that have an porous (or poorly compacted) O layer. Finally, the model zone sequence MSF|S|A|D|O +/- D is consistent with rims that contain both an A layer and an compact O layer, but this sequence can form only if the system experienced open-system loss of Ca at the D-O contact, with Ca-poor vapor being a possible open-system sink for Ca. The occasional presence of M in a mono- or bi-mineralic layer within rims apparently cannot be explained by the models, possibly indicating that the rims did not fully attain a steady-state condition. References: [1] Boynton W. V. and Wark D. A. (1985) Meteoritics, 20, 117-118. [2] Murrell M. T. and Burnett D. S. (1987) GCA, 51, 985-999. [3] Ruzicka A. and Boynton W. V. (1994) Meteoritics, 29, 529. [4] MacPherson G. J. et al. (1981) Proc. LPS 12B, 1079-1091. [5] Wark D. A. et al. (1988) LPS XIX, 1230-1231.
Boynton Willam V.
Ruzicka Adam
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