HR: 0800h
AN: V51C-0591    [Abstracts]
TI: Li Isotope Heterogeneity in Mantle-derived Xenoliths.
AU: * Bell, D R
EM: David.R.Bell@asu.edu
AF: Dept of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604 United States
AU: * Bell, D R
EM: David.R.Bell@asu.edu
AF: Dept. of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Hervig, R L
EM: Richard.Hervig@asu.edu
AF: Dept. of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Buseck, P R
EM: pbuseck@asu.edu
AF: Dept of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604 United States
AU: Buseck, P R
EM: pbuseck@asu.edu
AF: Dept. of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AB: Variations in 7/6Li of mantle-derived rocks are a potentially powerful tracer of mantle geodynamic processes. Analysis of 7/6Li by secondary ion mass spectrometry (SIMS) avoids surface contamination and permits assessment of sample heterogeneity at high spatial resolution. We performed Li abundance and isotope analysis by SIMS of mantle-derived olivine and cpx in order to assess intragranular isotopic variability and explore the Li isotope effects accompanying contrasting types of metasomatism of Archean mantle lithosphere. Analytical techniques are described in the accompanying abstract by Hervig et al., with further investigations into matrix effects on instrumental fractionation in progress. The mineral grains were extracted from peridotites and megacrysts erupted in southern African kimberlites and basalts from the SW USA. Samples from southern Africa selected to represent varying degrees of reaction between refractory Archean lithosphere and deep-seated kimberlitic precursor melts revealed a spread in $\delta$7Li of ~20 per mil, correlated with indices of metasomatism. Assuming a calibration based upon San Carlos olivine (Seitz et al., Lithos, in press), Archean mantle olivine is characterized by 7/6Li below the MORB range. Within this group, high Li concentrations occur in olivines from pyroxenites and low Li in those from refractory dunites, consistent with Li introduction during ancient metasomatism. In contrast, Fe-rich olivine and cpx megacrysts crystallized from Mesozoic kimberlite precursor melts are comparatively Li-rich, with $\delta$7Li up to +16 per mil. Metasomatized, sheared peridotites span an intermediate range of compositions, consistent with the introduction of Li with high $\delta$7Li during reaction with asthenospheric megacryst magmas. Some megacrysts show extreme internal isotopic heterogeneity, up to 15 per mil, and olivines from peridotites are commonly zoned towards grain margins and internal cracks. We suspect these effects are due to recent metasomatism as well as late-stage Li diffusion during entrainment.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting