Tracking along-arc sediment inputs to the Aleutian arc using thallium isotopes
Abstract:The Aleutian arc provides an opportunity to study subducted sediment fluxes because such material unambiguously contributes to many Aleutian lavas. The sediment subduction flux is highest in the east and almost absent in the westernmost portions of the arc. However, it is unknown if the sedimentary flux beneath the arc is reflected in the magnitude of the sediment component in the arc lavas. In addition, it has been suggested that the chemical characteristics of the subducted sediment is likely to change along the arc from continental detrital in the east to abyssal pelagic clays in the central Aleutians, which should be manifested in the isotopic characteristics of Aleutian lavas. However, no direct measurements of pelagic sediments outboard of the Central Aleutians have been conducted to date.
Here we use the novel tool of thallium (Tl) isotope ratios to assess sediment fluxes to the Aleutian arc. Thallium isotopes can be used to quantify sediment fluxes from the slab because pelagic sediments are highly enriched in Tl and display isotopic compositions that are heavier than the upper mantle. We have measured Tl isotope ratios of ~40 lavas covering the entire Aleutian arc as well as ~50 sediment samples from DSDP 178 and 183 as well as ODP 886C outboard of the Central Aleutians. We find that the Tl isotope compositions of the sediments subducted along the Aleutian arc change from values corresponding to continental crust material in the east to heavier values in the Central Aleutians, reflecting increased ferromanganese components in the pelagic clays going from east to west. This trend is mirrored by the lavas in the Eastern and Central Aleutians, which confirms that the composition of subducted sediment changes along the Aleutian arc. Lavas from the Western Aleutians, however, show no evidence of any sediment involvement, but indicate that a component from altered oceanic crust dominates the Tl isotope budget in this portion of the arc.
We conclude that Tl isotopes clearly reflect the change in sediment provenance outboard of the Aleutian arc and support previous studies that have inferred almost absent sediment inputs to the Western Aleutians.