Brachinite meteorites: Partial melt residues from an FeO-rich asteroid |
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| Institution: | 1. Institute of Planetary Research, German Aerospace Center (DLR), Rutherfordstraße 2, 12489 Berlin, Germany;2. Institute of Planetology, Westfälische Wilhelm-University Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany;1. Damascus University, Faculty of Sciences, Department of Geology, Damascus, Syria;2. Sorbonne Universités, UPMC Univ Paris 06, CNRS, Institut des Sciences de la Terre de Paris (iSTeP), 4 place Jussieu 75005 Paris, France;3. Université de Brest, CNRS UMR 6538 (Domaines Océaniques), I.U.E.M., Place Nicolas Copernic, 29280 Plouzané Cedex, France;1. Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA;2. Geosciences Research Division, Scripps Institution of Oceanography, La Jolla, CA 92093-0244, USA;3. Astromaterials Research and Explorations Science Directorate, Acquisition and Curation, NASA Johnson Space Center, 2101 NASA Road 1, Houston, TX 77058, USA;4. Institut de Physique du Globe de Paris, Université Paris Diderot, Sorbonne Paris Cité, 1 rue Jussieu, 75238, Paris Cedex 05, France;1. U.B.O.-I.U.E.M., CNRS UMR 66538 (Domaines Océaniques), Place Nicolas Copernic, 29280 Plouzané, France;2. Planetary and Space Sciences, Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK76AA, United Kingdom;3. Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, HI 96822, USA;4. CNRS UMS 3113, I.U.E.M., Place Nicolas Copernic, 29280 Plouzané Cedex, France;5. Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA;6. Geological Sciences, Brown University, Providence, RI 02912, USA;7. Muséum National d’Histoire Naturelle, Laboratoire de Minéralogie et de Cosmochimie du Muséum, CNRS UMR7202, 61 rue Buffon, 75005 Paris, France;1. Department of Mineral Sciences, Smithsonian Institution, National Museum of Natural History, Washington, DC 20560, USA;2. Department of Geosciences, The University of Tulsa, Keplinger Hall L101, 441 South Gary Avenue, Tulsa, OK 74104, USA;3. Department of Geology and Environmental Geosciences, Lafayette College, 116 Van Wickle Hall, 4 South College Dr., Easton, PA 18042, USA;4. Geophysical Laboratory, Carnegie Institution for Science, 5251 Broad Branch Road NW, Washington, DC 20015, USA;1. Laboratoire G-Time, Université Libre de Bruxelles, CP 160/02, 50, Av. F.D. Roosevelt, 1050 Brussels, Belgium;2. Department of Earth and Planetary Sciences, University of California at Davis, Davis, CA 95616, USA |
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| Abstract: | Brachinites are ultramafic, dunitic to wherlitic, unbrecciated and essentially unshocked rocks that are low in SiO2 (～36–39 wt.%), high in MgO (～27–30 wt.%) and notably high in FeO (～26–37 wt.%), and low in Al2O3 (～0.2–2.5 wt.%) and combined alkalis Na2O and K2O (～0–0.7 wt.%). They consist mostly of olivine (～71–96 vol.%; ～Fo64–73), major clinopyroxene (minor to ～15 vol.%; ～En40–63Wo36–48), with variable small amounts of plagioclase (0 to ～10 vol.%; ～An15–33), and minor to trace amounts of orthopyroxene (none to ～20 vol.%; En69–73Wo2–4), Fe-sulfides (trace to ～7 vol.%), chromite (none to ～5 vol.%), phosphates (none to ～3 vol.%) and metallic Fe,Ni (trace to ～2 vol.%). Minerals tend to be homogeneous, and textures are medium to coarse-grained (～0.1–1.5 mm), with olivine commonly displaying triple junctions. Brachina has near-chondritic lithophile element abundances, whereas other brachinites show variable depletions in Al, Ca, Rb, K, Na, and LREE. Siderophile element abundance patterns vary and range from ～0.01 to ～0.9 CI. Oxygen isotope composition (Δ17O) ranges from ～?0.09 to ?0.39‰, with the mean = ?0.23 ± 0.14‰. Brachinites are ancient rocks, as was recognized early by the detection, in some brachinites, of excess 129Xe from the decay of short-lived 129I (half-life 17 Ma) and of fission tracks from the decay of 244Pu (half-life 82 Ma) in phosphate, high-Ca clinopyroxene and olivine. The first precise crystallization age was determined for Brachina using 53Mn–53Cr systematics, relative to the Pb–Pb age of the angrite LEW 86010, and yielded an age of 4563.7 ± 0.9 Ma. Thus, Brachina is at most ～4 Ma younger that the CAIs whose age is 4567.2 ± 0.6 Ma. There is no consensus on the origin of brachinites, but they most likely are primitive achondrites, i.e., ultra-mafic residues from various low degrees of partial melting. Partial melting experiments suggest that they possibly formed from a parent lithology chemically similar but not identical to the Rumuruti (R) chondrites, although the different oxygen isotopic compositions of the R chondrites and the brachinites put a serious constraint on this hypothesis. The apparent lack of abundant rocks representing the partial melts suggests that brachinites may have formed on a parent body <～100 km in radius, where early partial melts were removed from the parent body by explosive pyroclastic volcanism. Graves Nunataks 06128 and 06129 are felsic, andesitic basalts which have properties that suggest a relationship to brachinites and thus, may be samples of the elusive partial melts. |
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| Keywords: | Ultramafic igneous rocks Differentiated parent asteroid Partial melt residue Size of parent body Very old age |
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