Origins and compositions of fluids at convergent margins

At convergent margins, fluids are critical for structural deformation, heat flow, material fluxes, and diagenesis, and when vented may influence seawater chemistry. Diagenesis depends on temperatures, pressures, fluid and solid compositions, and fluid/solid ratios. Fluid compositions often reflect their sources and important diagenetic reactions.; Although vastly different sediment compositions characterize the Peru, New Hebrides, and Barbados margins, similar diagenetic reactions are common at each of them, including regeneration of organic matter, carbonate diagenesis, volcanic ash alteration, and clay mineral dehydration and transformation. At the New Hebrides margin, volcanic ash alteration overwhelmingly controls the pore fluid compositions. During formation of minerals such as smectite and zeolites, hydration reactions elevate Cl{dollar}sp-{dollar} concentrations to over twice seawater value and decrease {dollar}deltasp{lcub}18{rcub}{dollar}O values to {dollar}-{dollar}9.5% (SMOW). Although gas hydrates, typically with {dollar}deltasp{lcub}18{rcub}{dollar}O values {dollar}>{dollar}2% are widespread at the Peru slope, {dollar}deltasp{lcub}18{rcub}{dollar}O values of the water decrease to {dollar}-{dollar}1.1%, due to diagenetic reactions. Through compaction at convergent margins, most pore fluids ultimately vent to the oceans. The common diagenetic reactions typically elevate Ca{dollar}sp{lcub}2+{rcub}{dollar} and Li{dollar}sp+{dollar} concentrations and fractionate {dollar}sp6{dollar}Li/{dollar}sp7{dollar}Li ratios, and thus convergent margin fluids could be important contributors to these, as well as other, global cycles.; Seaward of the Barbados deformation front, fluids venting from four mud volcanoes exhibit Cl{dollar}sp-{dollar} concentrations depleted to a minimum of 211 mM. In the mud volcanoes, focused flow, at rates {dollar}sim{dollar}10 m/yr, inhibit diffusive mixing with seawater. In the surrounding sediments, porous flow is calculated to be only {dollar}sim{dollar}0.2 cm/yr. The {dollar}deltasp{lcub}18{rcub}{dollar}O values of water in the mud volcanoes correlate inversely with Cl{dollar}sp-{dollar} concentrations, reflecting dilution caused by gas hydrate dissociation. Gas hydrates are unstable below {dollar}sim{dollar}500 mbsf, where temperatures are {dollar}sim{dollar}25{dollar}spcirc{dollar}C. Their dissociation liberates free gas that probably triggers eruption of the mud volcanoes. Based on chemical compositions of fluids in the mud volcanoes, several geothermometers indicate reaction temperatures of 75{dollar}spcirc{dollar} to 116{dollar}spcirc{dollar}C, corresponding to a depth of {dollar}>{dollar}2.5 km. This deep fluid originates within the accretionary wedge and flows laterally tens of kilometers where it mixes with water released from gas hydrates and vents.