Tracking along-arc sediment inputs to the Aleutian arc using thallium isotopes

Abstract

Sediment transport from the subducted slab to the mantle wedge is an important process in understanding the chemical and physical conditions of arc magma generation. The Aleutian arc offers an excellent opportunity to study sediment transport processes because the subducted sediment flux varies systematically along strike (Kelemen et al., 2003) and many lavas exhibit unambiguous signatures of sediment addition to the sub-arc mantle (Morris et al., 1990). However, the exact sediment contribution to Aleutian lavas and how these sediments are transported from the slab to the surface are still debated. Thallium (Tl) isotope ratios have great potential to distinguish sediment fluxes in subduction zones because pelagic sediments and low-temperature altered oceanic crust are highly enriched in Tl and display heavy and light Tl isotope compositions, respectively, compared with the upper mantle and continental crust.Here, we investigate the Tl isotope composition of lavas covering almost the entire Aleutian arc a well as sediments outboard of both the eastern (DSDP Sites 178 and 183) and central (ODP Hole 886C) portions of the arc. Sediment Tl isotope compositions change systematically from lighter in the Eastern to heavier in the Central Aleutians reflecting a larger proportion of pelagic sediments when distal from the North American continent. Lavas in the Eastern and Central Aleutians mirror this systematic change to heavier Tl isotope compositions to the west, which shows that the subducted sediment composition is directly translated to the arc east of Kanaga Island. Moreover, quantitative mixing models of Tl and Pb, Sr and Nd isotopes reveal that bulk sediment transfer of ∼0.6–1.0% by weight in the Eastern Aleutians and ∼0.2–0.6% by weight in the Central Aleutians can account for all four isotope systems. Bulk mixing models, however, require that fractionation of trace element ratios like Ce/Pb, Cs/Tl, and Sr/Nd in the Central and Eastern Aleutians occurs after the sediment component was mixed with the mantle wedge. Models of Sr and Nd isotopes that involve sediment melting require either high degrees of sediment melting (>50%), in which case trace element ratios like Ce/Pb, Cs/Tl, and Sr/Nd of Aleutian lavas need to be produced after mixing with the mantle, or significant fluid additions from the underlying oceanic crust with Sr and Nd isotope compositions indistinguishable from the mantle wedge as well as high Sr/Nd ratios similar to that of low (<20%) degree sediment melts.Thallium isotope data from Western Aleutian lavas exhibit compositions slightly lighter than the upper mantle, which implies a negligible sediment flux at this location and probably involvement of low-temperature altered oceanic crust in the generation of these lavas. In general, the lightest Tl isotope compositions are observed for the highest Sr/Y ratios and most unradiogenic Sr and Pb isotope compositions, which is broadly consistent with derivation of these lavas via melting of eclogitized altered oceanic crust.