Hydrothermal alteration of hemi-pelagic sediments: experimental evaluation of geochemical processes in shallow subduction zones

Laboratory hydrothermal experiments provide unique information regarding the fate of volatile and/or incompatibles (e.g. B, Li, and As) during oceanic crust subduction. Examination of chemical redistribution between the subducted slab, mantle wedge, arc volcanics and overlying ocean water during subduction is critical to gain further insight into arc volcanism and chemical oceanic budgets. For instance, efficient mobilization of B at shallow depths may be a key aspect of its oceanic budget, and can help to explain the systematics of B-δ¹¹B and B-¹⁰Be in arc lavas. Fluid–rock interactions at elevated temperatures and pressures in accretionary prisms were studied using a rocking autoclave hydrothermal apparatus to monitor sediment–porewater interaction over the range of 25 to 350°C, at 800 bars. Clay-rich hemi-pelagic sediments from the décollement zone of Ocean Drilling Program Site 808, Nankai Trough, were reacted with NaCl–CaCl₂ solutions at water/rock ratios of ∼3.5 to 1.5 (w/w) to mimic alteration processes in the shallow subduction zone. Fluids were extracted at 25–50°C intervals and were analyzed for major and trace chemical constituents. The fluid chemistry changed significantly during the course of these experiments, but there was generally only minor modification of the solid phase; only B, Li, As, Br, and Pb are significantly depleted. During the heating cycle, dissolved Na, Mg and SO₄ decreased sharply and NH₄, SiO₂, K, B, and Li increased at T⩾300°C. Calcium drops gradually at low temperatures, but concentrations rise sharply at T⩾300°C. Decomposition of organic matter, SO₄²⁻ depletion, and Mg-fixation dominate at low temperatures; however, albitization of calcic plagioclase leads to marked Na depletions and Ca enrichments at T⩾300°C. Dissolved SiO₂ remained below saturation with respect to quartz and amorphous silica throughout the entire experiment. B and Li mobilization with large isotopic fractionations occurred at low temperature. Exchangeable B (δ¹¹B=∼15‰) is completely leached before reaching 150°C. Substantial O₂ exchange between fluids and the solid phase occurred at T⩾200°C in the spiked experiment, where δ¹⁸O varies more than 100‰ in the fluids. During retrograde cooling, dissolved Mg, SO₄, Ca, Si, K and Sr are released as a result of carbonate or anhydrite dissolution, and marked B re-adsorption occurred at temperatures below 60°C.