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

The presence of primary troilite in chondrules requires that nebular temper atures were <650 K (the 50% condensation temperature of S) at the time of c hondrule formation and that chondrules were molten for periods short enough (less than or equal to 10 s) to avoid significant volatilization of S. We examined 226 intact chondrules of all textural types from eight unshocked t o weakly shocked ordinary chondrite falls of low petrologic type to determi ne the origin of troilite in chondrules; 68 chondrules are from LL3.0 Semar kona. There is a high probability that troilite is primary (i.e., was prese nt among the chondrule precursors) if it is completely embedded in a mafic silicate phenocryst, located within one-half radius of the apparent chondru le center and is part of an opaque assemblage with an igneous texture. Base d on these criteria, 13% of the chondrules in Semarkona and in the set as a whole contain primary troilite. Most of the remaining chondnrules contain troilite that is probably primary, but does not meet all three criteria. Tr oilite occurs next to tetrataenite in some opaque spherules within low-FeO chondrules in Semarkona, implying that the Ni required to form the tetratae nite came from the troilite. Troilite can accommodate 5 mg/g Ni at high tem peratures (>1170 K) but much less Ni at lower temperatures; because this is far higher than the metamorphic temperature inferred for Semarkona (simila r to 670 K), the troilite must be primary. Primary troilite fitting the thr ee criteria occurs in a smaller fraction of low-FeO [FeO/(FeO + MgO) in oli vine and/or low-Ca pyroxene not greater than 0.075] than high-FeO porphyrit ic 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 th at contain Na2O-bearing augite and Na2O- and K2O-rich mesostasis; these cho ndrules were probably formed after ambient nebular temperatures cooled belo w 910 K (the 50% condensation temperature of Na) and before they reached 65 0 K. Literature reports of rare fayalitic microchondrules in the rims aroun d a few normal-size chondrules suggest that chondrule formation persisted u ntil 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 s ome chondrules occur within opaque assemblages containing magnetite, carbid e, Ni-rich sulfide, awaruite, and Go-rich kamacite produced by parent-body hydrothermal alteration. Copyright (C) 1999 Elsevier Science Ltd.