Lithium and Carbon Isotopic Fractionations between the Alteration Assemblages of Nakhla and Lafayette

The nakhlites experienced a fluid alteration event on Mars leading to the rapid crystallization at low temperature (e.g. 25-150C) of siderite, clay and other salts in fractures within olivine and interstitial sites [1-2]. In order to help constrain the origin and evolution of the brine we present new Li-isotopic, C and Thermal Gas Analysis data for Nakhla and Lafayette samples. Techniques: Whole rock samples of Nakhla (BM 1911, 369) and Lafayette (BM 1959, 755) were powdered using an agate mortar and pestle. We also used Nakhla pyroxene separates (>95% pure) and olivineenriched samples of lower purity. Samples for Li-isotope analysis were dissolved in HF/HNO3, and passed through a cation exchange column to separate Li [3]. Li-isotope ratios were measured using a Nu instruments multicollector ICP-MS [4]. A sub-sample of Nakhla was subjected to sequential leaching procedure: the sample was ultrasonicated in water to dissolve soluble salts, and the residue was then leached with weak (1N) acetic acid. The residue from this procedure was dissolved in HF/HNO3. Li isotope ratios are reported as δLi, where δLi ‰={[(Li/Li)SAMPLE/(Li/Li)STANDARD] -1}*1000. Carbon yield and isotopes of whole samples of Nakhla (7.85 mg) and Lafayette (6.63 mg) were analysed by stepped-combustion mass spectrometry [5]. Samples were heated to 1300oC in 100oC steps from 0500oC and 1000-1300oC. Heating between 5001000oC was in 50oC steps. The same procedure was applied to Nakhla olivine-rich (10.44 mg) and pyroxene (10.55 mg) samples. Lafayette was heated in 50oC steps from 300-500oC and 25oC steps from 500-700oC. In order to remove traces of carbonates in the pyroxene sample an aliquot (9.53 mg), was washed in 0.1M HCl prior to combustion. Powders of both meteorites were pyrolysed using a combined thermogravimetric and evolved gas analysis system in a He carrier gas from 20-1000C, with CO2 release monitored. Results: Li-Isotopes. Table 1. shows our results which include the first bulk δLi values for martian meteorites. The δLi value for bulk Nakhla is within error of that for bulk Lafayette, and similar to those of terrestrial ultramafic rocks [6]. The Nakhla waterleach (+14.2 ‰) reflects the δLi of soluble salts known to be in this meteorite. However, the Nakhla acid-leach has a similar composition to the igneous phases implying that the siderite does not contain fractionated Li-isotopes. The detectable fractionation between Nakhla pyroxene and olivine-rich samples (0.5‰) suggests that the olivine contains a Li-rich alteration phase, which may be the nontronite-like clay present in cracks e.g. [1] or intramineral fractionation [6].