Isotopic Composition of Lithium in the Allende Meteorite

Introduction: Lithium has two stable isotopes, Li and Li with a mass difference of ~16%, resulting in appreciable isotopic fractionation during chemical reactions. The variability of Li/Li in meteorites may be used as a geochemical tracer for the origin of meteoritic components and alteration processes. The data for variability of Li/Li in chondritic meteorites are few, but include relatively homogeneous compositions for bulk chondrules and chondrites [1-3], variations of a few per mil betweeen separated bulk components of carbonaceous chondrites [4], and highly variable SIMS results on intra-chondrule components [5,6]. To shed further light on the cosmochemical behavior of Li and to explore its potential as a tracer, we performed a reconnaissance study of Li isotope variations within olivine of various types in the Allende CV3 chondrite. Samples: Olivines of variable composition from a variety of petrographic associations within the Allende meteorite were analyzed. Results from a total of eleven chondrules or isolated matrix olivines are reported here. These include olivine within porphyritic and barred chondrules, and isolated euhedral and subhedral olivines in the matrix. A few analyses of included or adjacent mesostasis, and of adjacent fine-grained matrix were also performed (e.g., Fig. 1). Analytical Techniques: Li/Li and Li/Si ratios were measured on the Cameca IMS-6f secondary ion mass spectrometer (SIMS) at Arizona State University. A 15-30 nA primary O ion beam was used to sputter secondary ions from craters ~30 to 50 μm in diameter. Mass resolution was set at ∆M/M of 600 to 2000 after no evidence for LiH was detected during high resolution mass scans. For each spot analysis, 100 Li/Li ratios were determined, with a 3 to 5 minute presputter. Charge compensation was evaluated every 20 cycles, and ranged from 0 to -30 V. Each 100 cycle analysis was averaged manually, removing outliers and analyses affected by surface contamination or changes in primary ion intensity (as revealed by systematic trends or fluctuations in Li count rate). This procedure gave a standard error range of 0.4 to 2.2 ‰ per spot analysis for Li concentrations of 3 to 0.03 ppm, respectively. Li/Si was acquired prior to isotope analysis in the same crater in many cases, with each reported ratio the average of 20 cycles. Mg/Si and Fe/Si ratios were determined in the same routine. B ion intensities were monitored as a control on terrestrial contamination.