1Bidong Zhang,2Nancy L.Chabot,1,3Alan E.Rubin4Munir Humayun,5Joseph S.Boesenberg,6Deonvan Niekerk
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.02.004]
1Department of Earth, Planetary & Space Sciences, University of California, Los Angeles, CA 90095-1567, USA
2Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
3Maine Mineral & Gem Museum, 99 Main Street, P.O. Box 500, Bethel, ME 04217, USA
4National High Magnetic Field Laboratory and Department of Earth, Ocean & Atmospheric Science, Florida State University, Tallahassee, FL 32310, USA
5Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02912, USA
6Department of Geology, Rhodes University, Makhanda 6140, South Africa
Group IIIF was established as a magmatic iron-meteorite group based on similar Ga and Ge abundances, unusually high Ga/Ge ratios, and the IIIAB-like interelement trends in its members; recent Mo and Ru isotopic data indicate that three of its members exhibit the isotopic signature of carbonaceous-chondrite (CC) irons. Here we report the elemental chemistry of this group and model its crystallization history. Included are new elemental data for IIIF irons acquired by both instrumental neutron activation analysis (INAA) and laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). A fractional-crystallization model was used to evaluate the IIIF compositional trends for 19 elements and was unable to explain the observed fractionation trends for several key elements (Co, Ga, Ge). In particular, the inability of this model to match Co in the IIIF irons is striking because (1) group IIIF has the widest Co variation among all magmatic iron groups and (2) none of the tested initial S contents (0−20 wt.%) explains both the wide Co variation and steep Co-As slope. Attempts to fit subsets of the IIIF irons were also unsuccessful. In addition, group IIIF has the greatest variety of structural classes and kamacite bandwidths among all established magmatic iron groups. If the IIIF irons constitute a coherent group, they were derived from a parent body that experienced more complex processes than simple fractional crystallization of the core.
The Zinder and Northwest Africa (NWA) 1911 pyroxene-bearing pallasites were recently suggested to be related to group IIIF based on their Ga and Ge contents, and we completed a petrographic study of the pallasite silicates and LA-ICP-MS analyses of their metal fractions. The two pallasites are related to one another: they have nearly identical mineralogical, elemental and O-isotopic compositions in their silicates and metals. Their metallic compositions resemble those of the IIIF irons Moonbi, St. Genevieve County, and Cerro del Inca, but their O-isotopic compositions resemble those of non-carbonaceous (NC) achondrites. Additional isotopic measurements are needed to test the potential genetic relationship to group IIIF.