Brachinite

Abstract: The results of a study of the small achondrite found in 1974 near Brachina, Australia are presented and discussed. Trace element data for Brachina were found to be quite different from that for Chassigny. The major differences are higher FeO/MnO, Ge/Al, and K/La ratios, W depletion, high siderophile content, near-horizontal REE pattern at about 1 x chondrite level, high refractory/nonrefractory metal ratios (e.g., Ir/Au), and chalcophile element depletion. Brachina's 4.5 billion year age and oxygen isotropic signature within the eucritic field further suggest that it is not related to Chassigny or SNC meteorites, though it may be related to primitive meteorites, such as silicate inclusions in IAB irons and winonaites.

Abstract: Brachinites are ultramafic, dunitic to wherlitic, unbrecciated and essentially unshocked rocks that are low in SiO 2 (∼36–39 wt.%), high in MgO (∼27–30 wt.%) and notably high in FeO (∼26–37 wt.%), and low in Al 2 O 3 (∼0.2–2.5 wt.%) and combined alkalis Na 2 O and K 2 O (∼0–0.7 wt.%). They consist mostly of olivine (∼71–96 vol.%; ∼Fo 64–73 ), major clinopyroxene (minor to ∼15 vol.%; ∼En 40–63 Wo 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 69–73 Wo 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 (Δ 17 O) 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 129 Xe from the decay of short-lived 129 I (half-life 17 Ma) and of fission tracks from the decay of 244 Pu (half-life 82 Ma) in phosphate, high-Ca clinopyroxene and olivine. The first precise crystallization age was determined for Brachina using 53 Mn– 53 Cr 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