Lithium Isotopic Analyses of Chondrites and Chondrules

Introduction: Light stable isotopes provide insights and constraints on nebular and parent body processes. Lithium isotope studies of chondrites offer such potential, with some specifically unique aspects. Lithium is monovalent, the lightest of nongaseous elements, it has a 15% mass difference between its 2 isotopes (Li and Li), it is fluid mobile, and it is one of the fastest diffusing elements (after H, He). In this study we consider two goals. First, the extent of Li isotopic fractionation in chondrules and bulk chondrite as a proxy of nebular and parent body processes, and second, accuracy and precision in Li isotopic analyses. We find that the lithium isotopic composition of chondrites and their chondrules are relatively homogeneous and reflect limited mass fractionation during condensation, accretion and postaccretion evolution. Analytical Methods: For the chondrite analyses, bulk rock and chondrule fragments were ground to a fine powder with an agate mortar, dissolved in high pressure bombs fluxed with a mixture of HF-HNO3 followed by a HClO4 fusion. All samples were monitored for complete dissolution of solids. Chromatographic separation of Li was accomplished using a modified published procedure, the details of the method and our modifications are reported in [1-2]. Total procedural blanks were analyzed for concentration and isotopic composition and determined to be negligible relative to the mass of sample (1-25 mg) analyzed. The Li isotopic measurements were performed on a Nu Plasma multi-collector ICP-MS at the University of Maryland. The δLi values for all samples are determined by comparison to the standard L-SVEC, measured before and after each sample analysis [1-2]. The time-dependent monitoring of the absolute variation in Li/Li values of L-SVEC throughout the analysis session provides a further check on precision and accuracy of analyses, which are better than ±1‰ (2-sigma). During all analytical sessions we measured the δLi values of 2 or more reference materials (this includes processed rock standards) before each batch of samples and, where possible, after each suite of samples. Purified metals of isotopically enriched Li (>99% pure) and enriched Li (>99% pure) were prepared as solutions and mixed to produce solutions with an appropriate range of δLi values to calibrate the range of compositions encountered. These spikes were also added to selected in-house standard materials and these mixtures were analyzed and compared with their predicted compositions. Results: In total, 19 chondrites were analyzed in this study; all but 2 (CV3 Efremovka and the EH3 Kota-Kota) are falls. The falls were selected to minimize earth-derived contamination. In addition, samples selected were chosen to obtain a representative spectrum of chondrites. The average δLi value for all of the chondrites is 1.3 ± 1.3 (1σ) Twelve carbonaceous chondrites, including the groups CI, CM, CO, CV and CK, have an average δLi value of 1.7 ± 1.3. Six Allende chondrule fragments were analyzed and have an average δLi value of 1.1 ± 0.9, which is identical to the bulk Allende composition (δLi = 1.6). The 6 ordinary chondrites, including LL, H/L and H groups, have an average δLi value of 0.4 ± 2.0. Discussion: There is a unimodal distribution of δLi values for all of the chondrites with considerable overlap in compostions for the carbonaceous and ordinary chondrites, although the ordinary chondrite Krymka has a markedly lower δLi value (Fig. 1). The 3 samples plotting at a δLi value of +4 include the CI chondrites Ivuna and Orgueil and the CK4 chondrite Karoonda. The higher δLi values for the CI chondrites are unlikely to be a pristine nebular signature, but is suggested to be the result of parent body aqueous alteration processes. It is also likely that the higher δLi value of Karoonda is due to aqueous alteration processes on its parent body.