An Integrated Mineralogical, Petrographic, Light Stable Isotope and Noble Gas Investigation of Sahara 99201 Ureilite

Introduction: Sahara 99201 (provisional name) is a new ureilite weighing 91g collected from the Sahara desert during 1999. The sample has been analysed to determine both mineralogy and petrography and also its light element stable isotopic composition. Methodology: Mineralogical analyses were carried out using a Cameca SX100 electron microprobe, with a beam current of 20nA, accelerating voltage of 20kV and a 10μm spot size in WDS analytical mode. At least 6 analyses were taken from each mineral grain to ensure good experimental precision. A ~200mg chip was taken from an internal portion of a 35g sample (to reduce the effect of contamination and weathering) and crushed in an agate pestle and mortar. Small fractions of powdered sample were loaded into Pt buckets and high resolution stepped combustion experiments were performed from 300°C to 1400°C to determine the yield and isotopic composition of C, N and Ar. C stable isotope analyses were carried out using MS86, a fully automated highsensitivity static-vacuum mass spectrometer, whilst a separate aliquot of material was analysed for N and noble gases using Finesse, a multi-element staticvacuum mass spectrometer [1,2]. O isotope analyses were carried out on a dual inlet mass spectrometer (VG PRISM) following the method of Miller et al. [3]. Results: Mineralogy and petrography. The sample is a coarse-grained (average grain size 12mm) olivine pigeonite ureilite displaying typical monomict texture. The sample is relatively unweathered compared with other desert ureilites with a weathering grade of W0-W1[4]. The sample is also of low shock with undulatory extinction in olivine implying a shock classification of S2 [5]. Olivine grains within the sample are quite fractured although one prominent grain displays very little fracturing. Reduction rims occur in olivines in two forms: as typical 50-100μm rims at the grain boundaries of the unfractured olivine, and also as “rosettes” which mainly occur in the more fractured olivine (figure 1). The rosettes consist of globular masses of metal blebs approximately 100μm in diameter. The rosettes occur in groups adjacent to the grain boundaries of the olivine and can be up to 800μm in thickness. These rosettes are apparently a primary texture and not related to weathering. The pigeonite grains do not display reduction rims as seen in the olivine although they do contain trails of metallic inclusions. These inclusions appear to follow both cleavage planes in the pigeonite and also fracture zones. A single blade of graphite can be seen at the boundary of a fractured olivine which is surrounded by a ~100μm thick zone of reduction. Interestingly, Crich matrix material is apparently lacking in the section. Mean mg# from olivine cores is Fo78.2 ± 0.5 (1σ) with mean pyroxene core compositions of Wo 9.6 ± 0.1, Εn 72.4 ± 0.4.