Evidence for Extremely-High-Temperature Melting in the Solar Nebula from a CaAl4O7-bearing Spherule from Murchison

Details Evidence for Extremely-High-Temperature Melting in the Solar Nebula from a CaAl4O7-bearing Spherule from Murchison Evidence for Extremely-High-Temperature Melting in the Solar Nebula from a CaAl4O7-bearing Spherule from Murchison

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..437s&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 437

Computer Science

Cais, Hibonite, Murchison, Refractory Inclusions, Spinel

Scientific paper

We have recovered a unique refractory spherule (B6) from the Murchison C2 chondrite. Approximately 140 micrometers in diameter, it is concentrically zoned, with an outer rim sequence, from outermost to innermost, of aluminous diopside (10 micrometers thick), anorthite (3 micrometers) and melilite (3 micrometers). Inside the melilite layer is a 7-micrometer-thick, nearly pure (except for a single, diverging-inward spray of hibonite crystals) layer of spinel. Inward from this layer is a 22-micrometer-wide zone of hibonite (~5.5 wt% TiO2) + spinel, in which hibonite laths, 1-4 micrometers across and up to 10 micrometers wide, are predominantly radially oriented and enclosed in spinel. Inward from this zone, presumably at the core of the inclusion, are CaAl4O7, occurring as anhedral grains ~10 micrometers across, and minor perovskite. Some of the hibonite laths protrude into the CaAl4O7. The sequence of mineral assemblages from the spinel shell inward parallels that expected for fractional crystallization of a melt of the composition of B6. Based on this, the inclusion's spherical shape, and its texture (radially oriented hibonite laths, including a diverging-inward spray; laths enclosed in spinel and protruding into CaAl4O7), we conclude that the oxide phases in B6 crystallized from a liquid. The spinel layer indicates that at least some of the spinel was molten; from the bulk composition, calculated liquidus phase relations in the system Al2O3-MgO-CaO [1], and the amount of spinel contained in the layer, we infer a melting temperature >2000 degrees C. This is >500 degrees higher than the maximum temperature at which any condensed major phase is stable at 10-3 atm in a gas of solar composition, but we see no evidence of evaporation. First, the inclusion has a Group II REE pattern, rather than a Group III or an ultrarefractory pattern, which could reflect devolatilization. Second, although evaporation of molten (but not solid) Mg2SiO4 leads to Mg isotopic mass fractionation [2], we found the Mg isotopic composition of spinel and hibonite in B6 to be essentially normal (DELTA 25Mg = 0 +- 2.5 permil). This means that no more than ~15% of the Mg could have evaporated, which, by analogy with experiments with forsterite at 2050 degrees C [2], suggests that the melt was exposed to the solar nebula for a very short time, perhaps as little as two minutes. This could indicate rapid formation of the spinel shell in B6, sealing off the molten interior from the solar nebula. Evaporation of solid spinel could have occurred, but would probably not fractionate Mg isotopes significantly. Evidence of an unusually high temperature history is preserved in the spinel of B6. It averages 1.7 +- 0.4 mol% excess Al2O3 relative to MgAl2O4, unlike the stoichiometric (within analytical error) spinel found in most CAIs. Much larger Al2O3 solubilities than observed in B6 spinel have been produced in synthetic systems at temperatures as low as 1300 degrees C [3]. In our crystallization experiments, excess Al2O3 ranges from 2 mol% in spinel equilibrated with melilite + hibonite + liquid at 1400 degrees C to 30 mol% in spinel equilibrated with liquid at 1499 degrees C. In corundum-bearing runs, excess Al2O3 in spinel increases from 12 mol% at 1349 degrees C to 24 mol% at 1450 degrees C, consistent with [3]. Excess Al2O3 in spinel is directly correlated with aAl2O3/aMgO based on experiments with solids [4]; it should also be correlated with aAl2O3/aMgO of coexisting liquids, and with temperature at constant aAl2O3/aMgO [1]. Spinels in our experiments have large excess Al2O3 contents because coexisting liquids have aAl2O3/aMgO >6 [1]. The bulk composition of B6 and residual liquids produced by crystallization of spinel from this composition have aAl2O3/aMgO ~1 [1], resulting in lower excess Al2O3 in B6 spinel than in our synthetic spinel. In type B inclusions, liquids with which spinel equilibrated also had aAl2O3/aMgO ratios ~1, but because equilibration temperatures were <~1500 degrees C, this spinel has negligible excess Al2O3, consistent with the results of [4]. The larger amounts of excess Al2O3 in B6 spinel indicate that its equilibration temperature was substantially higher than in type Bs (i.e., >~ 1500 degrees C), consistent with the above observations. References: [1] Berman R. G. (1983) Ph.D. thesis, U. British Columbia. [2] Davis A. M. et al. (1990) Nature, 347, 655-658. [3] Viertel H. U. and Seifert F. (1980) N. Jb. Miner. Abh., 140, 89-101. [4] Chamberlin L. et al. (1992) GSA Abs. with Prog., 24, A257.

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