Oxygen Isotopes in Ungrouped Achondrite Nwa 1500 and Comparison to Brachinites

Introduction: North West Africa (NWA) 1500 is an olivine-rich achondrite that is currently classified as ungrouped, based on petrography and oxygen isotope ratios [1-2]. Although some petrographic characteristics of NWA 1500 suggest a relationship to the augitebearing ureilites, it shows a distinct thermal history (equilibration to much lower T) that rules out derivation from the ureilite parent body [2]. Nevertheless, evidence of internal reduction processes suggest that it may have been derived from a parent body with a similar differentiation history. The first oxygen isotope data reported for this meteorite [3] showed a bulk composition intermediate between the IAB/winonaite trend and the region of acapulcoites/lodranites. A more recent report indicates a similarity to brachinites [4]. In order to clarify this discrepancy and further check its homogeneity in oxygen isotope ratios, we performed both ultra-high precision (~0.03‰) laser fluorination analyses of bulk chips and high precision (~0.3‰) SIMS analyses of minerals in a thin section of NWA 1500. Method: Two bulk analyses of ~2mg chips of NWA 1500 were performed using laser fluorination mass spectrometer at the University of Wisconsin [5]. Reproducibility of the UWG-2 standard is 0.05‰, 0.02‰ and 0.03‰ for δO, δO and Δ1O (=δO 0.52 × δO), respectively (in 2SE). Because of low degree of weathering [2], we did not perform acid leaching. The SIMS analyses were made using the IMS-1280 at the University of Wisconsin (Wisc-SIMS Laboratory). The method is similar to that described previously [6-7]. The primary Cs+ ion beam was focused to ~15μm diameter with 6nA current and secondary O signal was ~ 5×10 cps. The precision of single analysis was ~0.3‰ for δO and δO. We analyzed a thin section of NWA 1500 containing a large (~3 mm) poikilitic plagioclase (~An37) grain [2]. We used a thin section of San-Carlos olivine as a running standard. Three plagioclase standards (An22An49) were used to calibrate instrumental bias between analyses of olivine and plagioclase. Results: SIMS analyses of olivine (n=11) and plagioclase (n=5) are shown in Fig. 1. Oxygen isotope ratios in individual minerals are homogeneous within analytical uncertainty, though the average δO in plagioclase is higher than that of olivine by 1.7‰ along a slope 0.52 mass dependent fractionation line. The two laser fluorination bulk chip analyses and the average of SIMS olivine and plagioclase analyses are shown in Fig. 2 along with to previously reported bulk analyses by [3-4]. Our new bulk chip data plot slightly above IAB/Winonaite trend and nearly on the same mass dependent fractionation line as brachinites, which are quite different from the value reported by [3], but very similar to that of [4]. Our new bulk chip data yield a Δ1O value of -0.20‰, identical to the brachinite group.