Klaus Keil

Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, Honolulu, HI 96822, USA

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.%; ~Fo_64–73), major clinopyroxene (minor to ~15 vol.%; ~En_40–63Wo_36–48), with variable small amounts of plagioclase (0 to ~10 vol.%; ~An_15–33), and minor to trace amounts of orthopyroxene (none to ~20 vol.%; En_69–73Wo_2–4), 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.

Reference
Keil K (in press) Brachinite meteorites: Partial melt residues from an FeO-rich asteroid. Chemie der Erde
[doi:10.1016/j.chemer.2014.02.001]
Copyright Elsevier

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Sebastiano Padovan¹, Jean-Luc Margot^1,2, Steven A. Hauck II³, William B. Moore^4,5 and Sean C. Solomon^6,7

¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California, USA
²Department of Physics and Astronomy, University of California, Los Angeles, California, USA
³Department of Earth, Environmental, and Planetary Sciences, Case Western Reserve University, Cleveland, Ohio, USA
⁴Department of Atmospheric and Planetary Sciences, Hampton University, Hampton, Virginia, USA
⁵National Institute of Aerospace, Hampton, Virginia, USA
⁶Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA
⁷Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, District of Columbia, USA

The combination of the radio tracking of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft and Earth-based radar measurements of the planet’s spin state gives three fundamental quantities for the determination of the interior structure of Mercury: mean density ρ, moment of inertia C, and moment of inertia of the outer solid shell C_m. This work focuses on the additional information that can be gained by a determination of the change in gravitational potential due to planetary tides, as parameterized by the tidal potential Love number k₂. We investigate the tidal response for sets of interior models that are compatible with the available constraints (ρ, C, and C_m). We show that the tidal response correlates with the size of the liquid core and the mean density of material below the outer solid shell and that it is affected by the rheology of the outer solid shell of the planet, which depends on its temperature and mineralogy. For a mantle grain size of 1 cm, we calculate that the tidal k₂ of Mercury is in the range 0.45 to 0.52. Some of the current models for the interior structure of Mercury are compatible with the existence of a solid FeS layer at the top of the core. Such a layer, if present, would increase the tidal response of the planet.

Reference
Padovan S, Jean-Luc Margot J-L, Hauck II SA, Moore WB and Sean C. Solomon SC (in press) The tides of Mercury and possible implications for its interior structure. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004459]
Published by arrangement with John Wiley & Sons

Link to Article