Na-, Cl-rich mesostases in Chainpur (LL3) and Parnallee (LL3) chondrules

Details Na-, Cl-rich mesostases in Chainpur (LL3) and Parnallee (LL3) chondrules Na-, Cl-rich mesostases in Chainpur (LL3) and Parnallee (LL3) chondrules

Jul 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997m%26ps...32..555b&link_type=abstract

Meteoritics, vol. 32, pages 555-566

Other

10

Scientific paper

Forty six chondrules from Chainpur (LL3.4) and 39 chondrules and clasts from Parnallee (LL3.6) have been sectioned and searched for Na-, Cl-rich phases by EPMA. Oxygen isotopic compositions, I-Xe ages and ion probe data were also obtained on some of these chondrules. Sodium-, Cl-rich glass and microcrystalline sodalite (Na4Al3Si3O12Cl), nepheline (NaAlSiO4), scapolite (Na4Al3Si9O24Cl) have been identified in 7% of the Chainpur and 8% of the Parnallee samples. These phases are present in chondrule mesostases or, in one case, the plagioclase of a barred-olivine chondrule. None of the chondrules contain more than 5% by volume of Na-, Cl-rich phases. In the Chainpur chondrules, they originated through partial devitrification of silica-undersaturated, REE-, Na- and Cl-rich mesostases. Two processes have been identified which led to the formation of these mesostases. In two of the chondrules, which consist mainly of low-Ca pyroxene, the extended, metastable crystallization of low-Ca pyroxene created silica-undersaturated, REE-rich residua. Ba- and Cl-enrichments in nepheline and scapolite of one chondrule suggest that there was also an influx of alkalis and Cl during crystallization of the low-Ca pyroxene. Similarly, another one of the Chainpur chondrules - mainly composed of olivine phenocrysts - is markedly enriched in Cl (10 x OC). As there is no evidence of corrosive metasomatism in any of the chondrules, Cl- (and alkali) enrichment is believed to have occurred when they were still partially molten. The chondrules were derived from normal oxygen isotopic reservoirs, so the postulated influx of Ba, Na and Cl did not occur on an exotic parent body. Trace amounts of nepheline and sodalite, present in two Parnallee chondrules, crystallized from small Na-, Cl-, REE-rich residua following extended crystallization of anorthite. An I-Xe age of 5.0 Ma. post-Bjurbole obtained on one of these Parnallee chondrules dates the crystallization of feldspathoid and thus formation of the chondrule.

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