Lithium isotopic compositions of pore fluids and sediments in the Costa Rica subduction zone: Implications for fluid processes and sediment contribution to the arc volcanoes

Pore fluid and sediment Li concentrations and isotopic ratios provide impor tant insights on the hydrology, sediment contribution to the are volcanoes and fluid-sediment reactions at the dominantly non-accretionary Costa Pica subduction zone. Ocean Drilling Program Site 1039 in the trench axis provid es a reference section of 400 m of the incoming sediments, and Site 1040, s ituated arcward from the trench, consists of a deformed sedimentary wedge a nd apron sediments, the decollement, and the partially dewatered underthrus t sediment section. At the reference site, pore fluids show important isoto pic variations (delta Li-6 = -21.7 to -37.8 parts per thousand), reflecting the interplay of in situ alteration of Volcanic material and ion exchange with clay minerals. In the basal section, a reversal of Li concentration an d delta Li-6 toward seawater values is observed, providing supporting evide nce for a lateral seawater now system in the upper oceanic basement underly ing this sediment section. At Site 1040, pore fluid of the lower deformed w edge sediments and within the dtcollement is enriched in Li and the isotopi c compositions are relatively light, suggesting infiltration of a deep-seat ed fluid. The delta Li-6 value of -22 parts per thousand of this Li-enriche d fluid (261 muM), when compared with the delta Li-6 value of the subducted sediment section (-11 parts per thousand), suggests that the deep source f luid originates from mineral fluid dehydration and transformation reactions at temperatures of 100 to 150 degreesC, consistent with the temperature ra nge of the up-dip seismogenic zone and of transformation of smectite to ill ite. The distribution of Li and its isotopes in the underthrust section are similar to those at the reference site, indicating near complete subductio n of the incoming sediments and that early dewatering of the underthrust se diments occurs predominantly by lateral flow into the ocean. The hemipelagi c clay-rich sediment section of the subducting plate carries most of the Li into this subduction zone, and the pelagic diatomaceous and nannofossil ca lcareous oozes contain little Li. The Li isotopes of both the clay-rich hem ipelagic sediments and of the pelagic oozes are, however, similar, with del ta Li-6 Values of -9 to -12 parts per thousand. The observations that (1) t he delta Li-6 values of the underthrust sediments are distinctly lower than that of the mantle, and (2) the lavas of the Costa Rican volcanoes are enr iched in Li and Li-7, provide an approximation of the contribution of the s ubducted sediments to the are volcanoes. A first order mass balance calcula tion suggests that approximately half of the Li flux delivered by subducted sediments and altered oceanic crust into the Middle American Trench is rec ycled to the Costa Rican are and at most a quarter of sedimentary Li is ret urned into the ocean through thrust faults, primarily the decollement thrus t. (C) 2000 Elsevier Science B.V. All rights reserved.