TRACE ELEMENT CHARACTERIZATION OF FORSTERITE CHONDRITES AND METEORITES OF SIMILAR REDOX STATE

Abstract

From a systematic mineralogic/petrologic study of nine chondritic inclusions in the Cumberland Falls aubrite, Neal and Lipschutz (1981) reported that these inclusions constituted a new primitive chondritic suite--the forsterite (F) chondrites. The authors concluded that F chondrites acquired their chemical characteristics during nebular condensation and accretion into a parent body which later collided with the enstatite meteorite parent body, thereby being shocked and mixed. We report compositional data for volatile/mobile Ag, As, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U, and Zn in: eight F chondrites studied by Neal and Lipschutz (1981) and another studied by Binns (1969); the Pontlyfni and Kakangari meteorites (putative forsterite chondrites of Graham et al., 1977); Acapulco and ALH A77081 (with similar redox state); and a chondritic inclusion from the aubrite ALH A78113. Our data show trace element contents of F chondrites to be unusually coherent (only Bi, Cd, In, and Tl vary \u3e10x) and distinct. Few elements in F chondrites vary with Fs content of low-Ca pyroxene or with the ferromagnesian silicate disequilibration parameter. Five elements (Cd, Rb, Sb, As, and Se) are present in F chondrites at ordinary chondrite levels; siderophiles are slightly lower while volatile/mobile chalcophiles are substantially lower. Lithophiles (U, Rb, and Cs) and Te are higher in F chondrites than in other type 3 material. The composition of F chondrites differs markedly from those of other meteorite populations considered, indicting \u3e2 parent populations of similar redox state. Compositionally, the chondritic inclusion from ALH A78113 is different from F chondrites, although interesting similarities exist; further mineralogic and chemical information should detail their relationship. These trace element data are consistent with the idea that F chondrites are a unique primitive chondrite group, affected negligibly by the considerable shock generated during the collision between their parent body and that of the enstatite meteorites. Therefore, these data provide the first compositional information for these primitive representatives from a hitherto-unknown portion of the early solar nebula