New Data on Carbon Isotopic Compositions of Some Ureilites

Introduction: The ureilites are an unusual class of carbon-rich, ultramafic, achondritic meteorites, which display both primitive and evolved characteristics. For example, their noble gases (which are carried largely in the diamond phase) display a planetary or fractionated signature, and their O isotopes retain a heterogeneity imparted from the solar nebula [1,2]. Paradoxically, though, the mineralogical and textural characteristics displayed by the ureilites are seemingly indicative of extensive igneous processing on the ureilite parent body (UPB), a fact which is supported by chemical evidence such as depleted lithophile element abundance [3]. It is clearly apparent that models aiming to resolve the question of ureilite petrogenesis must satisfy the seemingly inconsistent chemical, mineralogical and isotopic evidence. We have previously suggested that studying carbon, and the role it plays in the formation of the ureilites, may be a step forward in the understanding of these perplexing meteorites [4]. High precision O isotope analyses by Franchi et al. [5,6] indicated that the group 1 ureilites [7] may be resolved into four distinct sub-groups (A, B, C, D) on the basis of ∆O values. The consistency of ∆O within each sub-group implies homogenisation, occurring either in discrete local regions on a single ureilite parent body, or perhaps, on four separate ureilite parent bodies. A pathfinder study [6] utilising bulk carbon analyses (samples pre-combusted at 400°C for two hours then combusted at 1000°C overnight) indicated a clear co-variation of δC with ∆O. The present study expands on this work by undertaking a more detailed analysis of carbon within samples from sub-groups B and sub-group D (∆O = -0.633 ± 0.025and –0.989 ± 0.025 respectively). Samples and Method: Five ureilites from subgroups B and D have been analysed for their carbon stable isotopic compositions and overall carbon distribution using a combination of mass spectrometry and stepped combustion. The five samples are; RKPA 80239, PCA 82506, Sahara 99201 (provisional name), META 78008 and LEW 85328. High resolution stepped combustion experiments are a powerful tool when investigating carbon distributions and isotopic compositions. These high resolution experiments allow different species of carbon to be determined e.g. organic contaminant, graphite, carbide, according to characteristic combustion temperatures, from a whole rock sample. Although low temperature organic contaminants can be removed by pre-combusting samples at 400°C prior to a bulk analysis, the resolution of distinct carbon components and isotopic compositions is impossible. Importantly, carbonates are not removed by pre-combustion, which can skew both carbon yields and isotopic compositions, if carbonate is present in the sample. The high resolution stepped combustion analyses on samples from sub-groups B and D will better constrain carbon yields and isotopic compositions and, allow both interand intragroup relationships to be better determined. Detailed mineralogical and petrographic examination of these samples is also being carried out which, in combination with our carbon investigation, may shed light on the formation conditions and processes occurring on the ureilite parent body(ies). Chips were taken from internal portions of the samples (to reduce the effects of contamination and weathering and also to exclude fusion crust) and were crushed in an agate pestle and mortar. Small fractions of the powdered sample were then loaded into Pt buckets and high resolution stepped combustion experiments were carried out. The stepped combustion experiments were carried out on MS86, a fully automated, high-sensitivity, static-vacuum mass spectrometer at the Open University [8]. Experiments were carried out from 300°C to 1400°C with the 500°C to 900°C temperature range studied at high resolution (25°C steps). Results and Discussion: The results of the carbon analyses are shown below in table 1. In all the ureilites studied there is a common phase liberated at temperatures below ~500°C with a weighted average δC(PDB) of -20 to -30‰, which is indicative of terrestrial organic carbon [9]. Considering the presence of this low temperature organic contaminant the values given in table 1 below for δC(PDB) and wt % C are weighted averages for temperature steps between 500°C and 1300°C. Sample Sample wt. (mg) Wt % C δC