The Rb and Cs concentrations of pore fluids recovered during Integrated Ocean Drilling Program (IODP) Costa Rica Seismogenesis Project Expedition 344 are reported. Five sites were drilled during IODP Expedition 344, three in the overriding plate (Sites U1380, U1413, U1412) and two on the incoming Cocos plate (Sites U1381, U1414). One of the sites drilled in the overriding plate, Site U1380, is a complementary site to the adjacent Site U1378 drilled during expedition 334. The Rb and Cs systematics were analyzed to unravel the behavior of the highly incompatible alkali metal elements at the different sites in order to better understand the fractionation between them and K during fluid-rock reactions at greater depths in forearc basins. The pore fluids at the sites drilled on the upper plate suggest an important role of ash (or tephra) alteration for K concentrations; however, this role is less apparent for Rb and particularly for Cs concentrations. At the three overriding plate sites drilled, communication with a deeper Cs-enriched fluid is observed, suggesting that the most incompatible alkali metal element, Cs, is not involved in the overriding plate sediment diagenetic reactions and hence is expelled into the fluid phase that is circulating at great depths. Some of the Rb is, however, involved in certain clay and possibly zeolite diagenetic reactions but not in carbonate or silica recrystallization reactions. At the two incoming plate sites cored during this expedition, Sites U1381 and U1414, the pore fluids are enriched in both Rb and Cs, as discussed below.
|ScienceOpen disciplines:||Earth & Environmental sciences, Oceanography & Hydrology, Geophysics, Chemistry, Geosciences|