Water-Rock Interaction in the Upper Seismogenic Zone in the Nankai Trough Subduction Factory

Subduction processes play a key role in global geochemical cycling and convergent margin dynamics. One of the major aims of current scientific investigations is to better understand the mechanisms and quantify the transfer of geochemical tracers from the subducted basaltic crust and overlying sediment into arc magmas. Many studies rely on the mobilization and enrichments of LILEs relative to HFSEs and REEs as well as on ratios of key trace elements (e.g., B, Ba, Th, Nb) and isotopic ratios (e.g., 10Be/9Be; 232Th/230Th; 206Pb/204Pb, delta 11B). Here, we report results of uniaxial deformation experiments under PT conditions approaching those of the upper seismogenic zone (up to 150_C and 70 MPa effective normal stress) to characterize devolatilization, cementation, and isotope fractionation in marine sediments. Pore fluids as well as muds of the main lithologies in the Nankai Trough subduction zone are continuously sampled during progressive loading during the experiments, and generally show profound geochemical enrichment in the fluids and leaching in the sediments. Clay mineral-rich sediments show freshening of some elements (e.g., Ba, Th) at low T (20_C), and then overprinted by T-driven mobilization at elevated T (tests at 100_C and 150_C). The tephra-rich turbidites do not show element depletion. Despite low-T freshening in places, volatile elements, HFSEs and REEs are generally mobilized during the experiments. These results have several implications on earlier interpretations, such as the use of HFSE enrichment in magmas as a proxy for sediment consumption. B isotopes show significantly lower delta 11B values in the heated tests in both the fluids and solids, and B depletion in the sediments.