HR: 08:00h
AN: V51A-01    [PDF]
TI: Li isotope composition of the upper mantle
AU: * Elliott, T
EM: tim.elliott@bris.ac.uk
AF: Department of Earth Sciences, University of Bristol, Will Memorial Building, Queens Road, Bristol, BS8 1RJ United Kingdom
AU: Thomas, A
EM: Alexander.Thomas@earth.ox.ac.uk
AF: Department of Earth Sciences, University of Bristol, Will Memorial Building, Queens Road, Bristol, BS8 1RJ United Kingdom
AU: Thomas, A
EM: Alexander.Thomas@earth.ox.ac.uk
AF: Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR United Kingdom
AU: Jeffcoate, A
EM: A.Jeffcoate@bristol.ac.uk
AF: Department of Earth Sciences, University of Bristol, Will Memorial Building, Queens Road, Bristol, BS8 1RJ United Kingdom
AU: Niu, Y
EM: yniu@mail.uh.edu
AF: Deaprtment of Geosciences, University of Houston, 4800 Calhoun Road, Houston, TX 77204 United States
AB: Subduction returns altered oceanic crust to the mantle as an integral part of plate tectonics. Yet evidence for recycled material in the mantle remains scarce. Li isotope ratio measurements have the potential to be a highly effective tracer of recycled material. Recent development of measurement procedures on our Finnigan Neptune plasma ionisation mass spectrometer allows us to measure small samples (~1ng Li) with a reproducibility (2 sigma standard deviation) of $\pm$0.2$\permil$. This considerably improves our ability to detect small variations in the Li isotope ratios and thus our sensitivity in identifying recycled contributions in mantle derived material. We have used our new technique to analyse the Li isotope ratios of a suite of MORB. The current Li isotope database for MORB is small, but nevertheless ranges from 1.5 to 6.5$\permil$ in samples from normal ridge depth. We wanted to assess the variability of MORB Li isotope ratios at high precision ($\pm$0.2 rather than ~$\pm$1$\permil$) in a suite of spatially closely related lavas. Thus we analysed samples from two small study areas around $10\deg$30'N and $11\deg$30'N East Pacific Rise (EPR) which comprise a range of compositions from normal (e.g. $\epsilon$(Nd) $\sim$ 10) to enriched (e.g. $\epsilon$(Nd) $\sim$ 8). Our $\delta^{7}$Li results vary from 3.1-5.2$\permil$ and correlate with other geochemical indices- positively with $^{87}$Sr/$^{86}$Sr and La/Sm and negatively with $^{143}$Nd/$^{144}$Nd. These correlations suggest shallow contamination is an unlikely cause of Li isotope variations. Thus the sources of these lavas appear to be variability enriched with recycled material with a heavy Li isotope signature. Recent work has suggested that after subduction zone processing the slab is isotopically light and thus is not a likely candidate as this recycled component. We suggest instead that the fluid-fluxed mantle wedge is mixed back into the upper mantle and forms the enriched component in the sources of these MORB and probably other mantle derived magmas.
DE: 1010 Chemical evolution
DE: 1025 Composition of the mantle
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting