Formation of unequilibrated R chondrite chondrules and opaque phases

1,2K.E. Miller, 1,2D.S. Lauretta, 2,3,4,5H.C. Connolly Jr., 6E.L. Berger, 7K. Nagashima, 2K. Domanik
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://doi.org/10.1016/j.gca.2017.04.009]
1Space Sciences Division, Southwest Research Institute, San Antonio, TX 78238
2Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA
3Department of Geology, School of Earth and the Environment, Rowan University, 201 Mullica Hill Road, Glassboro, N. J. 08028 USA
4Earth and Environmental Sciences, The Graduate Center of the City University of New York, Brooklyn, NY 10016, USA
5Earth and Planetary Science, American Museum of Natural History, Central Park West, New York, NY 10024, USA
6GeoControl Systems Inc. – Jacobs JETS – NASA Johnson Space Center, Houston, TX 77058, USA
7Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Manoa, Honolulu, HI 96822, USA
Copyright Elsevier

Sulfide assemblages are commonly found in chondritic meteorites as small inclusions in the matrix or in association with chondrules. These assemblages are widely hypothesized to form through pre-accretionary corrosion of metal by H2S gas or through parent body processes. We report here on two unequilibrated R chondrite samples that contain large, chondrule-sized sulfide nodules in the matrix. Both samples are from Mount Prestrud (PRE) 95404. Chemical maps and spot and broad-beam electron microprobe analyses (EMPA) were used to assess the distribution, stoichiometry, and bulk composition of sulfide nodules and silicate chondrules in the clasts. Oxygen isotope data were collected via secondary ion mass spectrometry (SIMS) to assess the relationship of chondrules to other chondrite groups. Scanning electron microscopy (SEM), focused ion beam (FIB), and transmission electron microscopy (TEM) analyses were used to assess fine-scale features and identify crystal structures in sulfide assemblages. Thermodynamic models were used to assess the temperature, sulfur fugacity (fS2), total pressure, dust-to-gas ratio, and oxygen fugacity (fO2) conditions during sulfide nodule and chondrule formation.

The unequilibrated clasts include a mixture of type I and type II chondrules, as well as non-porphyritic chondrules. Chondrule oxygen isotopes overlap with ordinary-chondrite chondrules. Sulfide nodules average 200 µm in diameter, have rounded shapes, and are primarily composed of pyrrhotite, pentlandite, and magnetite. Some are deformed around chondrules in a petrologic relationship similar in appearance to compound chondrules. Both nodules and sulfides in chondrules include phosphate inclusions and Cu-rich lamellae, which suggests a genetic relationship between sulfides in chondrules and in the matrix. Ni/Co ratios for matrix and chondrule sulfides are solar, while Fe and Ni are non-solar and inversely related.

We hypothesize that sulfide nodules formed via pre-accretionary melt processes. During chondrule formation, precursors composed of a mixture of silicate and sulfide material were heated to form immiscible melt droplets, which separated and cooled to form Si-rich chondrules and S-rich nodules. Sulfide melt was stabilized by a high total pressure (∼1 atm) in a dust- or ice-enriched environment. Heating of this material contributed to a high fS2 (2 × 10-3 atm at 1138 °C), and high fO2 (IW – 1 to IW – 4), in an environment with peak temperatures between 1539 °C and 1750 °C. Oxygen isotopic compositions in this region were similar to those recorded by the LL-chondrite chondrules.

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