Mathematics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.197a&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 197
Mathematics
Scientific paper
Chondrules are the most abundant and arguably the most enigmatic objects in the ordinary chondrites. Simulations of chondrules textures have placed limits on the range of cooling rates (100-3000 degrees C/hr) and peak temperatures (1600-1550 degrees C) chondrules experienced during their formation (Hewins, 1988). Also bulk analyses have been used to constrain the compositions of the chondrule precursor components (e.g., Grossman and Wasson, 1982). However, little is known of the distribution of trace elements between the various chondrule components. Yet these trace element distributions, here measured in porphyritic chondrules from Semarkona, Chainpur, and Bishunpur, are potentially useful additional tools in the study of chondrule cooling rates, chondrule-rim/matrix relationships, and possibly chondrule precursors. The olivine and low-Ca pyroxene trace element contents (Figs. 1 and 2) are remarkably unfractionated compared to those expected from equilibrium distribution coefficients, but are similar to the patterns observed in synthetic chondrules cooled at 1000-2000 degrees C/hr (Kennedy et al., 1992). Such fast cooling rates as these data imply are close to the upper limit for porphyritic chondrules estimated from simulation experiments (Hewins, 1988). With the exception of Na, K, and Eu, the incompatible lithophiles are concentrated in the glass, typically by a factor of 10 relative to the bulk meteorite and in more or less CI-like proportions. The rims and matrix are generally enriched in Na, K, and Eu (Alexander, 1991), possibly because these elements have been partially volatilized from chondrules and recondensed on the fine-grained material. However, the three chondrule glass-rim pairs studied to date show no striking correlation between degree of volatile depletion of the glass and enrichment of the rims. The olivine, pyroxene, and glass compositions share some similarities with the lithophile chondrule precursor components identified by Grossman and Wasson (1982) from bulk chondrule analyses. The most significant difference between the two is the correlation of Mg with the refractory incompatible elements in the Grossman and Wasson data, whereas within the chondrules Mg is mostly in the minerals and the refractory incompatables are in the glass. However, a compilation of published chondrule analyses suggests this correlation in the bulk data may not be very strong, prompting a reexamination of the origin of the chondrule precursors. Alexander C. M. O'D. (1991) Meteoritics (abstract) 26, 312. Grossman J. N. and Wasson J. T. (1982) Geochim. Cosmochim. Acta 46, 1081-1099. Hewins R. H. (1988) In Meteorites and the early solar system (eds. J. F. Kerridge and M. S. Mathews), pp. 660-679. Kennedy A. K., Lofgren G. E. and Wasserburg G. J. (1992) Lunar Planet. Sci (abstract) 23, 679-670. Figure 1, which in the hard copy appears here, shows mean and standard error of 12 olivines from porphyritic chondrule. Figure 2, which in the hard copy apears here, shows mean and standard error of nine low-Ca pyroxenes from porphyritic chondrules.
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