HR: 1330h
AN: V42D-0382    [PDF]
TI: Hf-Nd Isotope Geochemistry of Contrasting Global Sedimentary Columns in Subduction Zones
AU: * Prytulak, J
EM: jprytulak@wsu.edu
AF: Department of Geology, Washington State University, Pullman, WA 99164 United States
AU: Vervoort, J
EM: vervoort@wsu.edu
AF: Department of Geology, Washington State University, Pullman, WA 99164 United States
AU: Plank, T
EM: tplank@bu.edu
AF: Department of Earth Sciences, Boston University, Boston, MA 02215 United States
AB: Most terrestrial Hf-Nd isotopic compositions lie along a positively correlated Hf-Nd array [1]. However, some abyssal sediments (e.g., ferromanganese nodules, crusts, and metalliferous clays; [1, 2]) have strongly decoupled Hf-Nd isotopic compositions. In order to evaluate the range of Hf-Nd isotopic compositions in subducting sediments on a global scale, we have selected contrasting drill sites outboard of active subduction systems. The selected sections differ in characteristics such as age, thickness and type of sediment, convergence rate and accretionary prism development. An example of one endmember is the metalliferous clay-rich Tonga section (DSDP sites 595 and 596). These compositions show large deviations from the Hf-Nd array by having a higher $\epsilon$Hf relative to $\epsilon$Nd, and have highly elevated REE concentrations and Lu/Hf ratios compared to normal terrigenous samples. These systems have moderately negative Hf and Nd isotopic compositions that are (at least in the case of Nd) similar to seawater. This type of signature may be seen in other slowly accumulating systems dominated by metalliferous sediments, such as the Philippine (DSDP site 291) and Ryukyu (DSDP site 294/295) sections. The Central America (ODP site 844 and DSDP site 495) subduction system has a bimodal sediment input, consisting of carbonates overlain by hemipelagic diatomaceous muds. The muds are dominated by volcanic arc input and show little variation on the Hf-Nd array ($\epsilon$Hf $^{+}$10 to $^{+}$11 and $\epsilon$Nd $^{+}$2 to $^{+}$6). Volumetrically the largest flux of sediments into subduction zones worldwide, however, are terrigenous sediments derived from active continental margins. An example of this is the Cascadia system (DSDP site 174). The Cascadia sediments all plot on the crustal portion of the Hf-Nd array, but show large variation ($\epsilon$Hf $^{-}$6.5 to $^{-}$20 and $\epsilon$Nd $^{-}$5.5 to $^{-}$14) reflecting changing sediment sources. Nd and Hf model ages for these sediments range from 0.4 Ga to 1.3 Ga. These negative epsilon values and dominantly Precambrian model ages conflict with exposed crust in the Pacific NW proximal to site 174. Instead, these sediments may be the result of Pleistocene glaciation with pulses of catastrophic flooding, which brought older cratonic material from distal sources to the site. This illustrates that Hf-Nd isotopic composition is useful not only for applications such as mantle composition and arc petrogenesis, but also for sediment provenance. [1] Vervoort et al., 1999, EPSL, Vol 168: 79-99. [2] Albar\`{e}de et al., 1998, GRL, Vol 25: 3895-3898. [3] Vervoort and Plank, 2002, EOS, Vol 83.
DE: 1010 Chemical evolution
DE: 1020 Composition of the crust
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1050 Marine geochemistry (4835, 4850)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting