A detailed record of early solar system melting in the carbonaceous achondrites Northwest Africa 7680 and 6962

The carbonaceous achondrites Northwest Africa (NWA) 7680 and NWA 6962 have been investigated for their texture, mineralogy, geochemistry and geochronology. The major, minor and trace element compositions of the mineral components, oxygen isotope and chromium isotope compositions, along with U-Pb ratio phosphate thermochronology suggest that they were formed by similar processes on the same parent body. The achondrites have olivine compositions of Fa44.8 and Fa47.4 for NWA 7680 and NWA 6962 respectively. Replicate oxygen isotope analyses of grains and bulk powders from NWA 7680 yielded average Δ17O values of -1.04‰ ± 0.03 and -1.00 ± 0.05‰ respectively, which is identical to that reported for NWA 6962. The whole rock ɛ54Cr compositions are also equivalent for NWA 7680 and NWA 6962 (1.36 ± 0.05 and 1.30 ± 0.05 respectively). Both meteorites are plagioclase-rich, and NWA 7680 is also Fe-metal-rich, suggesting they both formed via differentiation processes that resulted in the pooling of partial melt products. Major element geochemical trends show that both rocks could be formed through melting of chondritic material on a CR chondrite-like parent body. This is consistent with oxygen isotope and chromium isotope compositions. Intrusion of a late-stage melt is evident in both meteorites and the melt products include silica-rich, alkali-deficient nepheline. The late-stage liquid has re-melted and mixed with primary plagioclase in NWA 6962. In contrast, the late-stage liquid was often restricted to grain boundaries in NWA 7680, leaving some of the primary plagioclase crystals intact. In situ dating of NWA 7680 phosphate minerals (merrillite and fluorapatite) reveals that it has not experienced long duration thermal metamorphism, or impact related Pb loss and age resetting since 4578 ± 17 Ma (207Pb/206Pb age ± 2σ, within error of solar system age). Phosphates associated with the late-stage melt in NWA 6962 yield a 207Pb/206Pb age of 4556.6 ± 8.0 Ma (2σ) within 2σ of the NWA 7680 age. Thermochronology data confirms that the observed chromium isotope signatures in these meteorites were not introduced by a later high temperature event, such as late impact accretion processes. These data are consistent with a rapid separation of inner and outer solar system chemical reservoirs, planetesimal melting, differentiation and cooling, all within several million years of calcium aluminum-rich inclusion (CAI) formation.