Fluids in convergent margins: what do we know about their composition, origin, role in diagenesis and importance for oceanic chemical fluxes?

Abstract

The nature and origin of fluids in convergent margins can be inferred from geochemical and isotopic studies of the venting and pore fluids, and is attempted here for the Barbados Ridge, Nankai Trough and the convergent margin off Peru. Venting and pore fluids with lower than seawater Cl- concentrations characterize all these margins. Fluids have two types of source: internal and external. The three most important internal sources are: (1) porosity reduction; (2) diagenetic and metamorphic dehydration ; and (3) the breakdown of hydrous minerals. Gas hydrate formation and dissociation, authigenesis of hydrous minerals and the alteration of volcanic ash and/or the upper oceanic crust lead to a redistribution of the internal fluids and gases in vertical and lateral directions. The maximum amount of expelled water calculated can be ca. 7 m3 a-1 m-1, which is much less than the tens to more than 100 m3 a-1 m-1 of fluid expulsion which has been observed. The difference between these figures must be attributed to external fluid sources, mainly by transport of meteoric water enhanced by mixing with seawater. The most important diagenetic reactions which modify the fluid compositions, and concurrently the physical and even the thermal properties of the solids through which they flow are: (1) carbonate recrystallization, and more importantly precipitation; (2) bacterial and thermal degradation of organic matter; (3) formation and dissociation of gas hydrates; (4) dehydration and transformation of hydrous minerals, especially of clay minerals and opal-A; and (5) alteration, principally zeolitization and clay mineral formation, of volcanic ash and the upper oceanic crust.