Petrology and Oxygen Isotopic Compositions of Anomalous Achondrite NWA 011

Introduction: Northwest Africa (NWA) 011 is a single stone of 40 g and has been discovered in the Moroccan Sahara desert in 1999 [1]. This pyroxeneplagioclase meteorite was classified as noncumulate eucrite, on the basis of petrography and geochemistry [1]. However, certain characteristics of this meteorite such as the high Fe/Mn values in pyroxene (~65) and particularly the bulk-rock oxygen isotopic compositions (δO = +2.54, δO = -0.48) are not of typical eucrites and may suggest a new type of basaltic meteorite [2]. In an attempt to resolve this issue, we have investigated mineral chemistry and oxygen isotopic compositions on portions of NWA 011. Trace-element chemistry and Ar-Ar age is presented in a companion paper [3]. Petrographic Description: Petrographic study of 2 polished thin-sections reveals that the sample is finegrained monolithic, unbrecciated achondrite [1]. The absence of primary breccia texture (i.e., relic plagioclase and augite grains [2]), even in our larger section (3 cm), suggests that NWA 011 may not be a recrystallized breccia. Modally, the sample consists mainly of pigeonite (58.50%) and plagioclase (39.56%), with minor amounts of opaque (0.71%), silica (0.67), and phosphate (0.54) minerals. Pigeonite (0.2-0.8 mm) is commonly subhedral and often has curve boundaries at the contacts with plagioclase (Fig. 1). All pigeonite grains contain exsolution lamellae of augite, usually a few microns wide but sometimes reach ~10 μm. Plagioclase (up to 0.1 mm) is anhedral and usually forms crystal aggregates (up to 2 mm) or straddles along grain boundaries or fractures of pigeonite grains. This texture may indicate recrystallization, possibly as a result of thermal metamorphism. Opaque minerals, occurring as small clusters, include ulvospinel, ilmenite, and small Fe metal-sulfide intergrowths (2-5 μm). Baddeleyite inclusions have also been found in some ilmenties. Olivine usually occurs in the same aggregates as oxides (Fig. 1). Ca-phosphates (whitlockite and Clapatite) are common but not evenly distributed. Mineral Chemistry: Compositions of pigeonite (Wo5.7-6.5 Fs63.2-65.1; Mg# = 32-33) and its lamella augite (Wo36.1-39.2 Fs35.2-38.3 ; Mg# = 38-43) are similar to those of Nuevo Laredo Trend eucrites (Fig. 2) [4] and some lunar metabasalts [5]. However, the Fe/Mn ratios (Pig = 67, Aug = 65) are significantly higher than those of typical eucrites (Fig. 3). Plagioclase is bytownite and low in K2O contents (An80.2-87.7Or0.7-0.2). Olivine is Ferich and its composition varies in a restricted range (Fa79.5-81.4). In ulvospinel, FeO and TiO2 contents are 55.3 and 24.2 wt%, resp.; ilmenite contains 45.1 and 52.7 wt%, resp.