Pore-fluid trace metal concentrations: implications for fluid/rock interactions in the Barbados accretionary prism

Abstract

Pore-fluid samples across the decollement zone at the northern Barbados accretionary prism were simultaneously analyzed at high resolution for their minor and trace element (Rb, Ba, Co, Zn, Mo) concentrations by inductively coupled plasma-mass spectrometry. Elevated Rb concentrations below the decollement zone suggest exchange with the oceanic basalt layer. The very different K/Rb concentration ratios across the decollement zone suggest that the fluids above and below this zone are geochemically distinct. Highly elevated concentrations of Mo, Zn, and Co, associated with a distinct Cl minimum depth interval at Site 948, imply that the source of this fluid is organic-rich sediment layers below the decollement zone, possibly the organic-rich turbidite layer encountered at Site 948. Ba concentrations are controlled largely by barite solubility equilibrium. The presenc e of Ba supersaturation within and immediately above the decollement zone at Site 948 and its absence at Site 949 is consistent with enhanced overpressuring at the latter site.