The Hf-Nd isotopic diversity of subducting oceanic sediments

We report Lu-Hf and Sm-Nd isotopic data for sediments from several DSDP and ODP sites from the Atlantic (South Sandwich, ODP 801), Indian (Nicobar Fan, DSDP 211) and Pacific Oceans (Aleutian site 183; W. Alaskan abyssal plain, DSDP 178; Astoria Fan, DSDP 174; Central America, DSDP 495, ODP 844; S. American, DSDP 321; Tonga, 595/6; Marianas, DSDP 801; Phillippines/Ryuku, DSDP 291, 294/295). These sediments span the compositional variation that exists in the ocean basins and that are actively being transported to subduction zones on the present-day Earth. The compositions of these sediments lie between three end-member groups. One group of sediments (Central America; Aleutians) has radiogenic Hf and Nd isotopic compositions (ɛ _Hf =⁺10; ɛ _Nd =⁺5) that are largely controlled by ash inputs from proximal volcanic arcs. A second group of sediments (Astoria and Nicobar fans) has very unradiogenic Hf and Nd isotopic compositions (ɛ _Hf =^-20; ɛ _Nd =^-15) that reflect their derivation from mature continental crust. Both these end-members, and intermediate compositions between them, lie within the crust-mantle Hf-Nd array and have Lu/Hf and Sm/Nd ratios typical of crustal rocks. This variation in isotopic composition mirrors to a large extent the range that exists in continental sediments. The third end-member group of sediments, composed of slowly accumulating metalliferous clays, is highly anomalous in terms of both its trace element and isotopic composition. These pelagic clays are the dominant components of the Tonga and Ryuku sites and are also important in the Philippine site and, locally, the Marianas site. The clays are highly enriched in REEs (many with Nd concentrations 100 to 400 ppm), but have average concentrations of Hf. This results in extremely high, superchondritic Lu/Hf ratios (¹⁷⁶Lu/¹⁷⁷Hf up to 0.18), but Sm/Nd ratios that are typically subchondritic. In addition, nearly all of these sediments have anomalously radiogenic Hf compared to Nd (ɛ _Hf =⁺10 to ⁺15; ɛ _Nd =^-5), which causes them to plot distinctly above the crust-mantle array. In summary, there is a wide range of isotopic compositions being delivered to subduction zones that is a function of the diversity of sediment types in the world's oceans. Although the REE rich pelagic clays have unique Hf-Nd isotopic compositions, their restricted global distribution will limit their widespread application as an isotopic tracer.