Project Title: Collaborative Research: Hf-Nd Isotopic and Trace-Element Geochemistry of Globally Subducting Sediments
PI's: J.Vervoort * J.Patchett * T.Plank
Project Summary (NSF Funded Project)
Ferromanganese nodules, crusts, and associated metaliferrous clays have been long known to contain a highly radiogenic Hf component relative to Nd (White et al., 1986). These oceanic sediments are of great interest because they represent some of the very few terrestrial materials that diverge systematically off of the terrestrial Hf-Nd isotopic array. This distinctive isotopic signature, therefore, has the potential to be a powerful tool tracing the fate of oceanic sediments. It has recently been suggested, for example, that pelagic sediments can be detected in some Hawaiian basalts (Blichert-Toft et al., 1999) and in volcanic rocks from the Luzon arc (Marini et al., 2000) based on their Hf-Nd isotopic compositions. However, it is not well known how widespread this radiogenic Hf signature is or how volumetrically important it is in the global subducting sediment flux. In fact, terriginous and other continentally derived sediments, which dominate the sediment flux in many subduction zones, have normal Hf-Nd isotopic compositions indistinguishable from the terrestrial array. However, most marine sediments analyzed thus far are samples collected on or near the ocean floor and constitute an incomplete and unrepresentative inventory of the sediment column bound for the subduction zone.
The aim of this proposal, therefore, is to constrain the Hf-Nd isotopic composition of the oceanic sediment flux into subduction zones. We will accomplish this by analyzing Lu/Hf and Sm/Nd isotopes and major and trace element concentrations of sedimentary samples from drilled sections in front of 12 subduction zones, which represent 92% of the global subducting sediment flux that has been drilled. Geochemical analysis will involve MC-ICP-MS, ICP-MS and ICP-ES techniques that have already been developed by the PIs. We have divided these trenches into two categories: Those with the highest sediment mass flux (Andaman/Sumatra, Makran, Vanuatu, Cascadia, Alaska/Aleutians, Peru/Columbia) and those that exhibit the widest range of predicted Lu/Hf and Hf isotopic compositions (Tonga, Kamchatka/Kuriles, Central America, South Sandwich, Lesser Antilles, and Ryuku/Philippines).
Our proposed work has three primary goals. First, we will be evaluating the potential of the Hf- Nd isotopic composition of marine sediments as a signature that can be used to trace their presence in the subduction zone or in the mantle. In this regard we will address the following questions: How volumetrically important is the radiogenic Hf isotopic signature in marine sediments? Is this a widespread isotopic signature or one limited to specific subduction zones or regions? Second, the Lu/Hf and Sm/Nd elemental and isotopic decoupling has been proposed to occur by several different mechanisms. Detailing the Hf-Nd isotopic composition of the different components of marine sediments will allow us to distinguish between the role of different mechanisms. Third, the isotopic and trace-element composition of oceanic sediments and their flux into the mantle will be important in constraining terrestrial Hf and Nd isotopic mass balance.