Rare earth elements in Angra dos Reis and Lewis Cliff 86010, two meteorites with similar but distinct magma evolutions

Abstract

Rare earth element (REE) and selected trace element abundances were measured, with an ion microprobe, in individual grains of merrillite, fassaite, olivine, kirschsteinite and plagioclase of LEW 86010, the second known angrite, and compared with measurements in merrillite and fassaite of Angra dos Reis (ADOR). The rare merrillite is highly enriched in REE; the REE patterns show steep decreases from La ∼ 3,300 × Cl to Tm ∼ 100 × Cl in LEW 86010, and from La ∼ 1,500 × Cl to Tm ∼ 130 × Cl in ADOR. REE patterns in fassaite of both meteorites are similar although concentrations are higher in ADOR. Whereas ADOR pyroxenes show a high degree of equilibration, the LEW 86010 pyroxenes show much variation in composition. The abundances of the REE, Zr, Al and Ti correlate positively with Fe/Fe + Mg and those of Cr, Sc and V correlate negatively, consistent with an igneous origin in a closed system. Pyroxene and plagioclase cores in LEW 86010 formed in equilibrium with a melt whose composition is close to that of the meteorite bulk composition. The ADOR melt had higher REE concentrations but a lower Fe/Mg ratio than that of LEW 86010. Despite numerous common characteristics, the two meteorites were produced in separate magmatic events involving similar but distinct processes and parent melts. The unusual angrite composition is not due to purely nebular processes.