The Northwest Africa 1500 meteorite: Not a ureilite, maybe a brachinite

Abstract

The Northwest Africa (NWA) 1500 meteorite is an olivine-rich achondrite containing approximately 2-3 vol% augite, 1-2 vol% plagioclase, 1 vol% chromite, and minor orthopyroxene, Cl-apatite, metal and sulfide. It was originally classified as a ureilite, but is currently ungrouped. We re-examined the oxygen three-isotope composition of NWA 1500. Results of ultra-high precision (0.03& for D 17 O) laser fluorination analyses of two bulk chips, and high precision (0.3&) secondary ion mass spectrometry (SIMS) analyses of olivine and plagioclase in a thin section, show that the oxygen isotope composition of NWA 1500 (D 17 O= )0.22& from bulk samples and )0.18 ± 0.06& from 16 mineral analyses) is within the range of brachinites. We compare petrologic and geochemical characteristics of NWA 1500 with those of brachinites and other olivine-rich primitive achondrites, including new petrographic, mineral compositional and bulk compositional data for brachinites Hughes 026, Reid 013, NWA 5191, NWA 595, and Brachina. Modal mineral abundances, texture, olivine and pyroxene major and minor element compositions, plagioclase major element compositions, rare earth element abundances, and siderophile element abundances of NWA 1500 are within the range of those in brachinites and, in most cases, well distinguished from those of winonaites ⁄ IAB silicates, acapulcoites ⁄ lodranites, ureilites, and Divnoe. NWA 1500 shows evidence of internal reduction, in the form of reversely zoned olivine (Fo 65-73 core to rim) and fine-grained intergrowths of orthopyroxene + metal along olivine grain margins. The latter also occur in Reid 013, Hughes 026, NWA 5191, and NWA 595. We argue that reduction (olivine fi enstatite + Fe 0 +O 2) is the best hypothesis for their origin in these samples as well. We suggest that NWA 1500 should be classified as a brachinite, which has implications for the petrogenesis of brachinites. Fe-Mn-Mg compositions of brachinite olivine provide evidence of redox processes among bulk samples. NWA 1500 provides evidence for redox processes on a smaller scale as well, which supports the interpretation that these processes occurred in a parent body setting. SIMS data for 26 Al- 26 Mg isotopes in plagioclase in NWA 1500 show no 26 Mg excesses beyond analytical uncertainties (1-2&). The calculated upper limit for the initial 26 Al ⁄ 27 Al ratio of the plagioclase corresponds to an age younger than 7 Ma after CAI. Compared to 53 Mn- 53 Cr data for Brachina (Wadhwa et al. 1998b), this implies either a much younger formation age or a more protracted cooling history. However, Brachina is atypical and this comparison may not extend to other brachinites.