Porewater chemistry of younger volcanic sediments of the Mariana back-arc region

Porewaters in volcanogenic sediments from the Mariana Back-arc riftzone near 18°N were investigated in order to characterize their chemical composition, reveal the reaction processes which determine their composition and examine differences in fluid chemistry in relation to their distance from the spreading axis. Extensive hydrothermal activity has recently been observed along the spreading axis near 18°N, therefore an important aim of this investigation was to identify any hydrothermal component in the porewaters. The samples are from 11 sediment cores taken along two profiles crossing the NNW-SSE orientated spreading axis at 18°02'N and 18°13'N. Sediment samples were investigated with regard to their lithological, mineralogical and chemical composition. The fluid samples were analyzed for dissolved nutrients ([PO4]3·, NH 3, [SO4]2-, SiO2), trace metals (Fe, Mn, Cu, Cd, Zn), chloride, alkali and alkali earth elements (Li+, K+, Mg2+, Ca2+), total alkalinity, pH, dD, d180 and 87/86Sr. From their crystalline mineral composition, high proportion of volcanic glass, and bulk geochemistry, we conclude that the young 520,000 year old sediments of the Mariana Trough near 18°N are primarily a product of local volcanic activity. The mineralogical composition (mainly plagioclase and clinopyroxene) reflects the phenocrysts of the Mariana Back-arc and Island-arc Basalts. Only minor amounts of detrital and biogenic components are present in the sediments. The minor content of authigenic minerals, as smectites and analcite, and the abundant well-preserved glass particles show that the sediment is primarily unaltered, with alteration processes having only a minor effect on the mineral composition. The degree of sediment alteration increases with the distance from the spreading axis. Statistical evaluation of the chemical composition of the leached surface sediments show that bathymetry, distance from the Mariana island-arc, and distance from the spreading axis all have a strong influence on recent sedimentation processes in the area investigated. These parameters are principally responsible for differences in the chemical composition of the surface sediments. Only diagenetic reactions were found to control the chemical composition of the porewaters in the investigation area. These reactions include the alteration of volcanoclastic material, forming authigenic minerals (smectite and zeolite) and hydrated glass and clay minerals, the dissolution of siliceous and carbonaceous biogenic particles, and the decomposition of organic matter. The chemical compositions of the pore fluids are influenced by the following processes: * hydration of volcanic ash, causing a passive increase in elemental concentrations in the porewater; * alteration of volcanic material, causing a decrease in the 8180-value and the 87/86 Sr-ratio of the seawater-derived fluid; * dissolution of volcanic minerals such as plagioclase and clinopyroxene, releasing Ca into the porewaters; * formation of Mg- and Fe-rich silicates such as saponite, nontronite and zeolites, causing a decrease in Mg-, Fe- and Si-concentrations in the porewaters; * decomposition of organic matter dividing the sediment column into oxic (upper -10 cm) and into suboxic (below -10 cm) zones. In the oxic zone trace metals such as Cd, Cu and Zn are released from the organic matter into the porewater. In the suboxic zone trace metals are removed from the porewater, probably being precipitated as sulfides or adsorbed onto particulate matter; * formation of anoxic environments in areas of formerly high organic matter content such as station 35 GKG. At this location the decomposition of organic matter leads to a significant [PO4]3- increase in the porewater; and * dissolution of siliceous minerals, siliceous biogenic particles such as diatoms and radiolaria, and carbonaceous biogenic particles such as foraminiferas, leading to an increase in the Ca- and Si-concentrations of the porewaters. Although the intensive hydrothermal activity observed in 1985 may have been responsible for the formation of authigenic quartz and nontronite in the sediment samples, at the time of sampling in 1990 there was no evidence for the influence of hydrothermal activity on porewater compositions in the investigated sediment cores