Physics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.233h&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 233
Physics
3
Scientific paper
There is wide agreement that chondrules were formed by melting of pre-existing minerals, but there is still controversy over how, when, and from exactly what they were formed. Much work on chondrules has emphasized magnesian granular/microporphyritic type I chondrules, but metal-rich type I chondrules are even more abundant in carbonaceous chondrites (McSween, 1977). The observation that metal is homogeneous within one chondrule but differs from chondrule to chondrule (Zanda et al., 1991) suggests some systematic relationship may exist between metal and silicates. It is the purpose of this paper to investigate those relationships for Renazzo and Semarkona. We observe a strong correlation between the silicate texture of chondrules, which falls in a fine-coarse granular-porphyritic-barred sequence related to degree of melting, and the nature of the metal. Where olivine grains are small and/or closely packed, metal occurs as tiny spherules. Where grain size and melt channels are larger, metal forms coalescing blebs or chains. With distinctly microporphyritic textures metal occurs mostly near the periphery of the chondrule and with truly porphyritic and barred chondrules it forms a rim or crown around the chondrule. Similar metal coalescence and expulsion textures have been observed for Bishunpur chondrules (Rambaldi and Wasson, 1981) and geochemical evidence shows that metal rims on Semarkona chondrules were derived from their interiors (Grossman and Wasson, 1987). There appears to be a continuous gradation between metal-rich and ordinary type I chondrules as a function of degree of melting, which suggests that many type I chondrules passed through a stage of being metal-rich during formation. If chondrules were manufactured from homogeneous interstellar dust, there is a very short time period for metal-silicate fractionation. If chondrules were formed from condensate aggregates, this constraint can be relaxed as condensates aggregated over different temperature ranges could have different metal-silicate ratios. High concentrations of Si, Cr, and P in metal in chondrules have been explained as due to precursor grains of condensation origin (Grossman and Olsen, 1974; Desnoyers, 1980; Zanda, 1992) but might alternatively be due to reduction during melting at very low oxygen fugacities (Dickinson et al., 1992). We find the highest concentrations of Cr and Si in the metal in the chondrules with the highest degree of melting associated with the most magnesian silicates. This suggests that these elements were reduced into the metal during chondrule melting. However, among these forsteritic chondrules, those that experienced higher oxygen fugacities based on Cr partitioning between olivine and metal have metal grains with silica melt droplets instead of reduced Si. This indicates oxidation during cooling at temperatures exceeding 1713 degrees C. Clearly metal, like silicate, was modified during chondrule formation and that probably accounts both for the integration of Cr and Si into metal and the formation of the round silica inclusions observed by Perron et al. (1992). References: McSween H. Jr. (1977) Geochim. Cosmochim. Acta 41, 1843-1860. Zanda B., Bourot-Denise M., and Perron C. (1991) Lunar Planet. Sci. XXII, 1543-1544. Rambaldi E.R. and Wasson J.T. (1981) Geochim. Cosmochim. Acta 45, 1001-1015. Grossman J. and Wasson J.T. (1987) Geochim. Cosmochim. Acta 51, 3003-3011. Grossman L. and Olsen E. (1974) Geochim. Cosmochim. Acta 38, 173- 187. Desnoyer C. (1980) Earth Planet. Sci. Lett. 47, 223-234. Dickinson T.L., Lofgren G.E., and Casanova I. (1992) Lunar Planet. Sci. XXII, 309-310. Perron C., Zanda B., Bourot-Denise M., and Mostefaoui S. (1992) Meteoritics, this volume.
Hewins Roger H.
Zanda Brigitte
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