THE CONTROL OF LITHIUM BUDGETS IN ISLAND ARCS

Abstract

Measurements of the Li isotopic compositions of lavas from magmatic arcs worldwide suggest common processes at work that lead to the retention of isotopically heavy Li in the mantle. Samples from this study derive from the Kurile arc, eastern Russia, the Sunda arc, Indonesia, and a segment of the Aleutian arc, western Alaska. The overall range in δ7Li is very restricted (+2.1 to+5.1±1.1, 2σ) for 34 of 36 samples. These values overlap the values of unaltered normal MORB glasses. The two samples with isotopic compositions that fall outside this range in δ7Li have B/Be <13, and hence do not bear classical ‘slab’ trace element signatures. Considering the high δ7Li in altered ocean crust, marine and terrigenous sediments, and forearc fluids, aqueous components lost by subducting slabs are expected to have similarly heavy enriched Li isotope signatures. If Li behaves similarly to a fluid-mobile element such as B, δ7Li should correlate strongly with, for example, B/Be. As such, samples with high B/Be should show elevated δ7Li. The sample set we have examined does not show such correlations and is interpreted to reflect a globally significant process. Although Li is a fluid-mobile element, its partitioning into Mg-silicates may cause it to be effectively removed during equilibration with subarc mantle peridotite. Elements with stronger fluid/mantle partitioning behavior, such as B, are not so affected. The convergence of Li isotope ratios on MORB-like values is interpreted to result from the sequestration of slab-derived Li in the subarc mantle before it reaches the zone of melting. The results indicate conditions appropriate for mantle ‘buffering’ of slab-derived Li are widespread in magmatic arcs. Alternately, some proportion of Li could be retained on the slab in high Li/B minerals. Either way, this indicates that regions of the upper mantle with δ7Li>MORB may be common, as a direct consequence of the subduction process.