Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.253m&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 253
Mathematics
Logic
6
Scientific paper
Forsterite-bearing inclusions (FIs) are a small subset of Ca-Al-rich inclusions (CAI), most notable for the fact that 5 out of the 9 reported examples have unusual isotopic properties (Davis et al., 1991) relative to other CAIs. The origin of their unusual Mg-rich bulk compositions and their relation to other CAIs are not well understood (Wark et al., 1987), and little is known about their ^26Al-^26Mg isotopic systematics because of a general absence of Al-rich phases. A new FI from Vigarano (CV3) contains anorthite, and unusual petrologic features suggest that it evolved from a bulk composition more refractory than the present one. Vigarano 3137 is a crudely round, ~5-mm-diameter inclusion with some matrix embayments. It consists of fassaite (~45%; <=350 micrometers), melilite (~25%; <=950 micrometers), anorthite (~10%; <=225, micrometers), olivine (~10%; <=70 micrometers) and spinel (~10%; some up to 20 micrometers, most <=5 micrometers, with some framboidal clusters up to ~40 micrometers). Olivine and spinel grains are poikilitically enclosed within melilite and fassaite, but the distribution of olivine is very heterogeneous. Most olivines are contained only within fassaite, and the rare grains enclosed within melilite are smaller and rounder. Melilite crystals are zoned from broad cores of Angstrom k(sub)86-88 to rims of Angstrom k(sub)44-54. The primary anorthite in 3137 occurs mostly near the margin of the inclusion, some crystals being undeformed and others highly strained and deformed, and generally crosscut by thin lamellae of nepheline. Spinel grains within some of the anorthite crystals have thin rims of Al-rich melilite, ~Angstrom k(sub)11. Ion microprobe isotopic analysis of the anorthite shows no detectable excess ^26Mg at ^27Al/^24Mg ratios up to 1133, implying (^26Al/^27Al)(sub)0 <~1 x 10^-6. F(sub)Mg in anorthite, melilite, olivine, and fassaite is ~10 o/oo/amu, slightly lower than the "F" inclusion TE (Clayton et al., 1984) but heavier than most normal CV3 inclusions (Niederer and Papanastassiou, 1984). Trace element enrichments in 3137 are (all relative to C1 chondrites): fassaite - La ~4.6x, Sm ~20x, Eu ~4.4x, Yb ~22x, Sc ~70x, Sr ~3.1x; melilite - La ~3.9x, Sm ~1.5x, Eu ~33x, Yb ~0.7x, Sc <0.4x, ~Sr 40x; anorthite - La ~11x, Sm ~2.5x, Eu ~8.7x, Yb ~4.2x, Sc ~0.4x, Sr ~98x. The REE patterns are smooth, indicating that the bulk trace element pattern of 3137 is not volatility fractionated. Many features of 3137 are puzzling, and point to a complex history. Relicts of aluminous melilite within some anorthite grains cannot have crystallized from the same Mg-rich melt as the olivine and Mg-rich melilite; they may represent an Al-rich precursor to the bulk composition that was eventually melted. Conversely, the concentration of anorthite on the outside of the inclusion is difficult to reconcile with its crystallization from a melt in which olivine is a near-liquidus phase and anorthite should be relatively late. The range of compositions in zoned melilite crystals crosses the minimum melt composition of the gehlenite-akermanite binary and cannot be explained by fractional crystallization in a closed system. The overall Mg-isotopic mass fractionation in favor of heavier isotopes in 3137 suggests that some volatilization may have occurred prior to or during melting. In a general way the features of 3137 suggest multiple stages of melting coupled with two competing processes that tended to modify the bulk composition, namely addition of Mg-rich material before melting and minor volatilization of Mg during melting episodes. References: Clayton R.N., MacPherson G.J., Hutcheon I.D., Davis A.M., Grossman L., Mayeda T.K., Molini-Velsko C., and Allen J.M. (1984) Geochim. Cosmochim. Acta, 48, 535-548. Davis A.M., MacPherson G.J., Clayton R.N., Mayeda T.K., Sylvester P., Grossman L., Hinton R.W., and Laughlin J.R. (1991) Geochim. Cosmochim. Acta, 55, 621-638. Niederer F.R. and Papanastassiou D.A. (1984) Geochim. Cosmochim. Acta, 48, 1279-1294. Wark D.A., Boynton W.V., Keays R.R., and Palme H. (1987) Geochim. Cosmochim. Acta, 51, 607-622.
Davis Aileen M.
MacPherson Glenn J.
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