Light Lithophile Element Trends in Nakhlite Nwa 817 Pyroxenes: Implications for Water on Mars

Abundances of LLE (Li, B, Be) are strongly zoned in augite grains of the NWA 817 nakhlite [1,2]. Be and B consistently increase to grain rims (like Fe* and Ti), consistent with their geochemical behavior as incompatible elements. Li abundances increase outward in some rims, but decrease significantly in others. The cause of this bipolar behavior is not clear. LLE are not zoned in augites in the nakhlites Nakhla and Lafayette [3], which may suggest LLE mobility in lateand post-igneous cooling [4]. Introduction: The light lithophile elements (LLE), Li, B, and Be, can be useful as tracers of water-rock processes. They are analyzed together and behave nearly identically in igneous fractionation [4], but Li and B are soluble in aqueous solutions while Be is not [5,6]. Differences in the behavior of Be versus Li and B in pyroxenes of the shergottite Martian meteorites has suggested that the meteorites’ parent magmas lost significant quantities of water along with Li and B [3]. However, similar LLE behavior is also reported in lunar basalts and shocked Earth basalts [7]. LLE in pyroxenes of the nakhlite martian meteorites (Nakhla and Lafayette) behaved identically, but not in a manner consistent with igneous fractionations [3,4]. To help resolve these issues, we investigated LLE abundances in augite phenocrysts of the NWA817 nakhlite [1,2]. It was chosen because it cooled very rapidly; its LLE abundance patterns should reflect igneous processes only. Analytical Procedures: All measurements were made on a single thin section (554). Major and minor element profiles were measured using the Cameca sx100 electron microprobe at NASA/JSC, using a focused beam at 15kV and 20 nA. Peak counting time was 20 seconds on each point. These profiles were then examined for suitable rim/core targets for SIMS analysis. Eleven major and minor element profiles were measured. Of these, 7 (#’s 1-6 and 10) were analyzed for Li, Be, B, Ti, Ca using the Cameca ims 4f ion probe at the Univ. of New Mexico, Institute for Meteoritics. Before analyses, the thin section was washed in a 1% manitol solution and rinsed in ultra pure water to eliminate B contamination [8], then gold coated. Prior to each analysis, the sample surface was cleaned using a rastered ion beam and imaged to better define the location of the analysis. Analysis was conducted with a 10 kV, 10 nA O beam with a spot diameter of 10 to 15 mm. All mass peaks (background, Ca, Ti, Li, Be, B) were counted for 2 seconds except Si which was counted for 4 seconds. Each spot was analyzed for ten count cycles. Internal precision (counting statistics) was better than 1% for all trace elements. The LLE calibration curves ([(element / Si) * SiO2%] versus element abundance) had correlation coefficients greater than than 0.997. Results: Major element (Mg, Fe, Ca) zoning patterns are similar in all clinopyroxene profiles. Cores are generally homogeneous in Mg, Fe and Ca. Minor element abundances are also relatively constant, although in some core profiles showed variations of up to 20. The cores show some spikes in Al abundance, some of which correspond to spikes in abundances of other elements. Augite grains show two types of rim profiles: sharp and “plateau”. Figure 1 shows an augite grain, with both types of rim: the left is sharp; the right is a more gradual rim with a hump or plateau.