Lithium Isotopic Compositions of Marine Sediments: Implication for the Compositions of Volcanic Arcs and the Deep Mantle
Abstract
To understand the role of subducted sediment in arc magmatism and mantle evolution, we have determined the Li isotopic composition of marine sediments subducting at several trenches, as well as from non-convergent margin settings. The bulk isotopic composition (δ7Li) of global sediments varies widely (from -4.3 to 14.5%0) reflecting sediment type, provenance, and diagenetic processes. Among detrital sediments (-1.5 to 5%0), clay-rich variants are generally isotopically lighter than sands owing to isotopic fractionation during continental weathering. Sediments derived from mature continental crust are especially light. Volcanogenic sediments can have either heavier or lighter isotopic compositions than the mantle depending on alteration effects. Biogenic carbonate and silica are susceptible to recrystallization which results in heavier isotopic compositions (6 to 14.5%0). The lightest composition was observed in hydrothermally leached sediments (-4.3%0). Based on this and previous studies, integrated δ7Li values for subducted sediment vary from arc to arc, ranging from -0.5 to 8%0 with a flux-weighted global mean of 3%0. Calculated fluxes of subducted sediment-derived Li also vary, from 1.4 to 18.2 g/yr per cm arc length. Although Li isotopic compositions of marine sediments largely overlap with those of the upper mantle and altered oceanic crust, controls on Li isotopic compositions of arc lavas are complex and must be understood in terms of the primary compositions and relative contributions of subducted sediments and oceanic crust, fractionation during metamorphic dehydration, and equilibration of slab-derived fluids with the subarc mantle. Existing data for ocean island basalts display a δ7Li range from 2.5 to 7.4%0 reflecting the influence of recycled crustal material, but extremely light component that could result from dehydrated sediment on the residual slab has not been observed.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2006
- Bibcode:
- 2006AGUSM.V33A..03C
- Keywords:
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- 8104 Continental margins: convergent;
- 1025 Composition of the mantle;
- 1041 Stable isotope geochemistry (0454;
- 4870);
- 1051 Sedimentary geochemistry;
- 3060 Subduction zone processes (1031;
- 3613;
- 8170;
- 8413)