HR: 0800h
AN: V51C-0593    [Abstracts]
TI: SIMS Analyses for Li Isotope Ratios: From Olivine to Clay Minerals.
AU: * Hervig, R L
EM: hervig@asu.edu
AF: Dept. Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Bell, D R
EM: david.r.bell@asu.edu
AF: Dept. Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Bell, D R
EM: david.r.bell@asu.edu
AF: Dept. Chemistry & Biochemistry, Arizona State University, Tempe, AZ 85287-1604 United States
AU: Moore, G
EM: gordon.moore@asu.edu
AF: Dept. Chemistry & Biochemistry, Arizona State University, Tempe, AZ 85287-1604 United States
AU: Williams, L B
EM: lynda.williams@asu.edu
AF: Dept. Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Yamamoto, J
EM: jyama@bep.vgs.kyoto-u.ac.jp
AF: Inst. Geothermal Sciences, Kyoto University, Beppu, 874-0903 Japan
AU: Buseck, P R
EM: pbuseck@asu.edu
AF: Dept. Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Buseck, P R
EM: pbuseck@asu.edu
AF: Dept. Chemistry & Biochemistry, Arizona State University, Tempe, AZ 85287-1604 United States
AB: Secondary ion mass spectrometry (SIMS) is an extremely sensitive instrument for Li (up to ~4% of the atoms of Li sputtered from minerals are detected). While precision is high, the accuracy of Li isotope analyses by SIMS is critically dependent on the homogeneity of bulk-analyzed standards. We have used the Cameca ims 3f and 6f SIMS at Arizona State University to analyze crystalline and glassy bulk-analyzed materials and report on their homogeneity and how well the samples compare. We used a 5-20 nA (Ip) primary beam of O- and detected positive secondary ions with 0$\pm$20 eV excess kinetic energy. Mass resolving power was sufficient to separate 6LiH from 7Li, but the hydride species was never observed. 7/6 ratios on samples containing 1 ppm Li show precisions of $\sim$1.3$\permil$ (2 $\sigma$) in a 1 hour analysis. There is a strong positive correlation between Ip and 7/6 ratio in hydrous rhyolite glasses, indicating possible charge-driven Li diffusion. This relation is subtle in low-H basaltic glasses and not observed in olivines and clay minerals. The latter phases show minimal variation in the absolute 7/6 ratio measured by both SIMS instruments over a period of months. Intra-granular Li isotope heterogeneity, apparently related to microstructural imperfections, was observed in some cpx samples, and some olivines from mantle peridotites show strong zoning ($>$10$\permil$). These observations underscore the importance of developing microanalytical techniques of Li isotope analysis. Comparing bulk analyses of San Carlos olivine (Seitz et al., Lithos, in press) with our microanalyses provides a preliminary calibration of the SIMS for 7/6 ratios in olivine. Indirect evidence suggests this calibration applies between Fo80-95. When applied to SIMS analyses of MORB glasses (average $\delta$7Li $\sim$+3), the olivine calibration gives a 7/6 ratio $\sim$4$\permil$ heavier than expected. Using a homogeneous cpx as a standard produces MORB analyses near +3$\permil$. Whereas studies of more samples are required to understand matrix effects, these appear to be minor for many materials of interest.
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting