Brachina: A new type of meteorite, not a chassignite

Abstract

The Brachina meteorite, previously classified as a chassignite, is found to be a nearly primitive and unique meteorite. It is a fine‐grained, unshocked equigranular igneous rock containing small melt inclusions. It contains 80% olivine (Fo70), 5.5% clinopyroxene (Wo38En49Fs13), 10% plagioclase (An22), 0.5% chromite, 0.5% chlorapatite, 3% Fe‐sulfide, 0.3% pentlandite, and a trace of Ni‐rich metal; orthopyroxene (En70) and anorthoclase‐rich glass are found only in melt inclusions. Siderophiles are high in the sulfides, and the oxidation state is similar to that of LL‐chondrites. Major element and mineralogic data of Brachina are similar to Chassigny, but trace element data are dramatically different. The major differences are higher FeO/MnO, Ga/Al, and K/La ratios, W depletion, high siderophile content, near‐horizontal REE pattern at about 1× chondrite level, high refractory/nonrefractory metal ratios (e.g., Ir/Au), and chalcophile element depletion. Compositionally, Brachina is close to C2 chemistry, but has undergone some segregation of (Fe,Ni)S melt. These data, in addition to its near 4.5 b.y. age, and oxygen isotopic signature within the eucritic field, suggest that Brachina is not related to Chassigny or SNC meteorites. It may be related to primitive meteorites, such as silicate inclusions in IAB irons and winonaites, but is more oxidized; reduction would make it very similar to winonaites. The assemblage may have experienced the beginning of melting in its parent body. Since it has characteristics that differ from any other meteorite, we suggest it be called a ‘brachinite’.