Pb (and Hf, Nd) Isotope Composition of Subducting Marine Sediments
Abstract
Sediments represent one of the most variable components in a subduction zone system and, therefore, their detailed chemical characterization is necessary to accurately constrain subduction zone processes. In this study, we report Pb isotope data for over 80 samples from 9 DSDP sites, complementing our existing Hf and Nd data, in order to examine their coupled Hf-Nd-Pb isotopic relationships. These include samples from the North Pacific (Aleutians, Cascadia, Kamchatka; DSDP sites 183, 174, and ODP site 881), Western Pacific (Tonga, Ryuku, Philippines, Vanuatu; DSDP sites 595/6, 294/5, 291, 286), Eastern Pacific (Central America; DSDP site 495); and Indian Ocean (Nicobar, DSDP site 211). These compositions cover the full range of marine sediment types and define positively correlated Pb isotope arrays from approximately 207Pb/204Pb and 206Pb/204Pb (hereafter simply 7/4 and 6/4) of 15.5 and 18.3 to 15.75 and 19.3. The Pb isotopic data for these sediments fall into 3 broad groups. Samples from the Philippine, Vanuatu, Ryuku, and Tonga sites, which are principally composed of metal-rich pelagic clays, lie on the primitive end of the array with 7/4 and 6/4 ratios that range from 15.50-15.65 and 18.3-18.7. These overlap with the high 6/4 end of Pacific MORB but are uniformly higher in 7/4. Samples from Kamchatka and Central America occupy tightly defined isotopic fields, with 7/4 and 6/4 ratios that range from 15.60-15.65 and 18.6-18.7. Both these sites have Pb likely influenced by ash inputs from their respective arcs. The most striking trends are formed by the Pb isotope data from the Cascadia and Nicobar sites and to a lesser extent by the Aleutian site. These data define highly linear arrays with large ranges in Pb isotope ratios. These arrays presumably represent mixing lines between juvenile and evolved crustal end members. The highly linear nature of the Cascadia and Nicobar sites indicates there was a dominant radiogenic endmember that controlled the Pb isotopic budget of these sediments. Both of these sites consist of terrigenous sediments derived from older cratons. In terms of overall Hf-Nd-Pb systematics, most sediments plot along broad, negatively correlated Hf-Pb and Nd-Pb, and positively correlated Hf-Nd, isotopic arrays. Notable exceptions include metal-rich pelagic clays from the Philippine, Vanuatu, Ryuku, and Tonga sites, which have highly anomalous Hf-Nd isotopic compositions that plot distinctly above the terrestrial Hf-Nd array. In terms of Hf and Pb, however, these samples lie within our Hf-Pb array, showing no obvious deviation. Intriguing and unexpected systematics are revealed when comparing Pb and Nd isotopes. The metal-rich clays, distinctive in their anomalous Hf-Nd isotopic compositions and unremarkable in their Hf-Pb systematics, are displaced to the unradiogenic Nd side of the Nd-Pb array. These relationships emphasize the complex interplay of Hf, Nd and Pb inputs to slowly accumulating, metal rich clays. These new data suggest, somewhat counter-intuitively, that the anomalous Hf-Nd compositions of metal-rich pelagic sediments are not controlled by radiogenic Hf, but rather by their unradiogenic Nd isotopic compositions.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2005
- Bibcode:
- 2005AGUFM.V41D1499V
- Keywords:
-
- 1031 Subduction zone processes (3060;
- 3613;
- 8170;
- 8413);
- 1040 Radiogenic isotope geochemistry;
- 1050 Marine geochemistry (4835;
- 4845;
- 4850);
- 1051 Sedimentary geochemistry;
- 3022 Marine sediments: processes and transport