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  • Role of recycled oceanic basalt and sediment in generating the Hf-Nd mantle array RID A-1762-2011

    Chauvel, Catherine   Lewin, Eric   Carpentier, Marion   Arndt, Nicholas T.   Marini, Jean-Christophe  

    Following its subduction, oceanic crust either contributes to the source of island-arc volcanic rocks or it is recycled into the mantle(1). Most(2,3), but not all authors(4) believe that recycled crust is incorporated into the plume source of oceanic basalts. The hafnium (Hf) and neodymium (Nd) isotopic compositions of basalts from oceanic islands and mid-ocean ridges exhibit a linear relationship - the mantle array - which is thought to result from mixing between material from the depleted mantle and an enriched recycled component. Here, we model the Hf - Nd isotopic composition of oceanic basalts as a mixture of recycled oceanic crust and depleted mantle and find that recycling of basalt alone is not sufficient to reproduce the mantle array. We conclude that oceanic sediments, which have a relatively high Hf-176/Hf-177 ratio, must also be recycled. Combining oceanic sediments with recycled oceanic basalts and subsequent mixing with depleted mantle peridotite produces Hf and Nd isotopic compositions that coincide with the mantle array. The composition of bulk continental crust requires the existence of a complementary low Hf-176/Hf-177 reservoir, which we suggest is zircon-rich sediment.
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  • Hf-Nd input flux in the Izu-Mariana subduction zone and recycling of subducted material in the mantle RID A-1762-2011

    Chauvel, Catherine   Marini, Jean-Christophe   Plank, Terry   Ludden, John N.  

    In subduction zones, two major mass fluxes compete: the input flux of altered oceanic crust and sediments subducted into the mantle and the output flux of magma that forms the volcanic arc. While the composition and the amount of material erupted along volcanic arcs are relatively well known, the chemical and isotopic composition of the subducted material (altered oceanic crust and sediments) is poorly constrained and is an important factor in the mass balance calculation. Ocean Drilling Program Leg 185 in the Western Pacific used systematic sampling of the altered basaltic basement and sediment pile and the creation of composite mixtures to quantify the total chemical flux subducted at the Izu-Mariana margin. Here, we report Hf and Nd isotopic compositions of materials recovered from this Leg. The Hf and Nd isotopic compositions of altered basalts from Hole 801C are indistinguishable from those of recent unaltered Pacific mid-ocean ridge basalt, suggesting that hydrothermal alteration had no effect on either isotopic systems. The complete Site 1149 sedimentary pile has a weighted average epsilon(Nd) of -5.9 and epsilon(Hf) of +4.4, values similar to those of Fe-Mn crusts and nodules. Therefore, the Hf and Nd isotopic compositions of the sediments collected at Site 1149 indicate minimal contributions from continental detrital material to the rare earth elements and high field strength elements. However, the Hf isotopic budget of the oldest sediments is more influenced by continental material than the younger sediments, despite the large distances to continental masses 130 Ma ago. In the Izu subduction zone, we calculate a sedimentary input of less than about 2% in the volcanic lava source. In contrast, at least 85% of the sedimentary Nd and Hf are recycled into the mantle to affect its general composition. Assuming that sediments have been recycled in a similar manner into the mantle for millions of years, large chemical heterogeneities must be produced in the mantle. In addition, the depletion of the mantle due to the extraction of continental crust must be partly counterbalanced by the injection of vast quantities of enriched sedimentary material.
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