<\/div><\/div>Brachinites are ultramafic, dunitic to wherlitic, unbrecciated and essentially unshocked rocks that are low in SiO2 (\u223c36\u201339 wt.%), high in MgO (\u223c27\u201330 wt.%) and notably high in FeO (\u223c26\u201337 wt.%), and low in Al2O3 (\u223c0.2\u20132.5 wt.%) and combined alkalis Na2O and K2O (\u223c0\u20130.7 wt.%). They consist mostly of olivine (\u223c71\u201396 vol.%; \u223cFo64\u201373), major clinopyroxene (minor to \u223c15 vol.%; \u223cEn40\u201363Wo36\u201348), with variable small amounts of plagioclase (0 to \u223c10 vol.%; \u223cAn15\u201333), and minor to trace amounts of orthopyroxene (none to \u223c20 vol.%; En69\u201373Wo2\u20134), Fe-sulfides (trace to \u223c7 vol.%), chromite (none to \u223c5 vol.%), phosphates (none to \u223c3 vol.%) and metallic Fe,Ni (trace to \u223c2 vol.%). Minerals tend to be homogeneous, and textures are medium to coarse-grained (\u223c0.1\u20131.5 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 \u223c0.01 to \u223c0.9 CI. Oxygen isotope composition (\u039417O) ranges from \u223c\u22120.09 to \u22120.39\u2030, with the mean = \u22120.23 \u00b1 0.14\u2030. 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\u201353Cr systematics, relative to the Pb\u2013Pb age of the angrite LEW 86010, and yielded an age of 4563.7 \u00b1 0.9 Ma. Thus, Brachina is at most \u223c4 Ma younger that the CAIs whose age is 4567.2 \u00b1 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 <\u223c100 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.[\/su_quote]\n","protected":false},"excerpt":{"rendered":"
Keil K (in press) Chemie der Erde – Geochemistry Volume 74, Issue 3, October 2014, Pages 311\u2013329 LINK Brachinites are ultramafic, dunitic to wherlitic, unbrecciated and essentially unshocked rocks that are low in SiO2 (\u223c36\u201339…<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[28,40,137],"tags":[671,1819,670,1884,325,1859],"_links":{"self":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/2357"}],"collection":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=2357"}],"version-history":[{"count":4,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/2357\/revisions"}],"predecessor-version":[{"id":4958,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/2357\/revisions\/4958"}],"wp:attachment":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2357"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=2357"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=2357"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}