HR: 09:45h
AN: V51B-06    [Abstracts]
TI: Li and B Insights into Subduction Signatures in the Mantle
AU: * Ryan, J G
EM: jryan@nsf.gov
AF: University of South Florida Department of Geology, 4202 East Fowler Ave. SCA 528, Tampa, FL 33620 United States
AU: Savov, I P
EM:
AF: National Museum of Natural History Department of Mineral Sciences, Smithsonian Institution, Washington, DC 20560 United States
AU: Tonarini, S
EM: s.tonarini@igg.cnr.it
AF: Instituto di Geoscienze e Georisorse, Area di Riserca di Pisa Via Moruzzi 1, Pisa, 1-561127 Italy
AB: Boron and lithium are powerful tracers of subducted materials in forearc and sub-arc mantle regions. Their isotopic systematics (δ¹¹B and δ⁷Li) may be useful in identifying slab-mantle exchange processes and quantifying slab outfluxes. Early results for Li and B isotopes in basaltic lavas and mantle rocks suggest complexities in the transit of Li and B from slabs into different mantle domains. B contents and δ¹¹B in forearc mantle rocks suggest early release of B from slabs and preferential removal of ¹¹B, leading to low δ¹¹B on deep or hot slabs, seen in some arc settings (i.e. Rose et al 2001; Bebout and Nakamura 2001; Leeman et al 2004). However, many arcs record δ ¹¹B and B contents too high to explain via shallow removal, necessitating inputs of forearc mantle to arc sources (i.e., Straub and Layne, 2001). Data for intraplate sources are equivocal, with some results on OIBs positing low δ ¹¹B (Chaussidon and Marty 1995), and others suggesting greater complexity. Intraplate basalts all show B depletions relative to MORBs, suggesting pervasive subduction-induced B redistribution in the Earth. While arc lavas and forearc rocks show Li enrichments, δ ⁷Li in these samples are distinct. Basalts are uniform in δ ⁷Li, varying ±2‰ from the mean value for MORBs. Mantle samples are, by contrast, diverse, ranging from +10‰ to -17‰. As Li partitions strongly into serpentine and other hydrated magnesian minerals, and solid-fluid exchanges result in strong Li isotopic fractionations, it is possible to generate rocks that are heterogeneous in Li content and δ ⁷Li via progressive fluid-rock exchange at relatively low temperatures. Magmas sample the δ ⁷Li of a large region of mantle, so fine-scale heterogeneity may be averaged out. The limited range of δ ⁷Li in lavas may also indicate that subducted materials do not transport a fractionated Li isotopic signature into the mantle.
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly