Lithium isotopic composition of marine sediments

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Geochemistry Geophysics Geosystems
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Jun 2
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[ 1] To understand the role of subducted sediment in arc magmatism and the formation of the continental crust, we have determined the Li isotopic composition of marine sediments subducting at several trenches, as well as from nonconvergent margin settings. The bulk isotopic composition (delta Li-7) of global sediments varies widely ( from - 4.3 to 14.5 parts per thousand), reflecting sediment type, provenance, and diagenetic processes. Among detrital sediments ( - 1.5 to 5 parts per thousand), 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 parts per thousand). The lightest composition was observed in hydrothermally leached sediments ( - 4.3 parts per thousand). Slowly accumulated metalliferous sediments display anomalously high delta Li-7 ( 6 to 10 parts per thousand) values due to incorporation of seawater-derived Li. On the basis of this and previous studies, integrated delta Li-7 values for subducted sediment vary from arc to arc, ranging from - 0.4 to 9 parts per thousand. 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 isotope composition 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. Finally, although clay-rich sediments can contribute to the light isotopic composition of continental crustal rocks, our results suggest diverse compositions of sedimentary rocks. Incorporation of sedimentary components in the formation of granitic rocks may give rise to greater heterogeneity in the composition of the upper continental crust than is currently estimated.


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Doi 10.1029/2005gc001202