Other
Scientific paper
Mar 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996lpi....27..467g&link_type=abstract
Lunar and Planetary Science, volume 27, page 467
Other
11
Alteration: Aqueous, Alteration: Of Chondrules, Chondrules, Sodium: In Chondrules, Volatiles: In Chondrules
Scientific paper
Sodium is the most easily measured volatile element in chondrules, accessible to nearly all commonly used micro- and bulk-analytical techniques. Experiments show that Na is easily lost from chondrule melts under certain conditions, making it one of the key elements for understanding chondrule thermal histories, precursor materials, and the environment in which chondrules formed. The apparent paradox that chondrules contain abundant Na, whereas theory and experiment seem to require that it should have been evaporated during the melting event have led to speculation that, (a) the experiments are not applicable, (b) chondrules formed in peculiar environments, or (c) the distribution of Na and other volatiles, particularly sulfur, in chondrules may be secondary. The latter suggestion has been particularly fueled by the recent discovery of a chondrule with zoned mesostasis in Semarkona, which is consistent with late entry of volatiles. I report three lines of evidence that support speculation (c). First, a significant fraction of Semarkona chondrules appear enriched in Na within mesostasis near the chondrule surface. Second, fine-grained chondrules almost always show evidence for significant secondary loss of Na from their surfaces. And third, all types of olivine-bearing chondrules contain glass inclusions in the olivine in which the inclusions are strongly depleted in volatiles compared to the intergranular mesostasis. I attribute these effects to subsolidus alteration of chondrules, probably by more than one process; candidates include very light metamorphism and aqueous alteration, changing the outer portions of chondrules, and an earlier, more pervasive metasomatism of chondrule glass. The setting for each process remains uncertain.
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