Troilite in the chondrules of type-3 ordinary chondrites: Implications for chondrule formation

摘要

The presence of primary troilite in chondrules requires that nebular temperatures were <650 K (the 50% condensation temperature of S) at the time of chondrule formation and that chondrules were molten for periods short enough (<=10 s) to avoid significant volatilization of S. We examined 226 intact chondrules of all textural types from eight unshocked to weakly shocked ordinary chondrite falls of low petrologic type to determine the origin of troilite in chondrules; 68 chondrules are from LL3.0 Semarkona. There is a high probability that troilite is primary (i.e., was present among the chondrule precursors) if it is completely embedded in a mafic silicate phenocryst, located within one-half radius of the apparent chondrule center and is part of an opaque assemblage with an igneous texture. Based on these criteria, 13% of the chondrules in Semarkona and in the set as a whole contain primary troilite. Most of the remaining chondrules contain troilite that is probably primary, but does not meet all three criteria. Troilite occurs next to tetrataenite in some opaque spherules within low-FeO chondrules in Semarkona, implying that the Ni required to form the tetrataenite came from the troilite. Troilite can accommodate 5 mg/g Ni at high temperatures (>1170 K) but much less Ni at lower temperatures; because this is far higher than the metamorphic temperature inferred for Semarkona (approx 670 K), the troilite must be primary. Primary troilite fitting the three criteria occurs in a smaller fraction of low-FeO [FeO/(FeO+MgO) in olivine and/or low-Ca pyroxene not greater than 0.075] than high-FeO porphyritic chondrules in Semarkona (9% vs. 33%). Coarse-grained low-FeO porphyritic chondrules appear to contain somewhat more troilite on average than those of medium grain size. We found a few troilite-free, metallic-Fe-Ni-bearing, low-FeO chondrules that contain Na_2O-bearing augite and Na_2O- and K_2O-rich mesostasis; these chondrules were probably formed after ambient nebular temperatures cooled below 910 K (the 50% condensation temperature of Na) and before they reached 650 K. Literature reports of rare fayalitic microchondrules in the rims around a few normal-size chondrules suggest that chondrule formation persisted until nebular temperatures cooled below 500 K. Secondary troilite occurs in a few percent of Semarkona chondrules as thin veins mobilized by shock; troilite or pyrrhotite in the outer portions of some chondrules occur within opaque assemblages containing magnetite, carbide, Ni-rich sulfide, awaruite, and Co-rich kamacite produced by parent-body hydrothermal alteration.