Metal mobility in sediment-covered ridge-crest hydrothermal systems: Experimental and theoretical constraints

The presence of sedimentary organic matter blanketing midocean ridge crests has a potentially strong impact on metal transport in hydrothermal vent fl uids. To constrain the. role of organic matter in metal mobility during hyd rothermal sediment alteration, we reacted organic-rich diatomaceous ooze fr om Guaymas Basin, Gulf of California, and organic-poor hemipelagic mud from Middle Valley, northern Juan de Fuca Ridge, with seawater and a Na-Ca-K-Cl fluid of seawater chlorinity, at 275 degrees to 400 degreesC, 350 to 500 b ars, and initial fluid: sediment mass ratios ranging from 1.6 to 9.8. React ion of these fluids with both sediment types released CO2 and high concentr ations of ore-forming metals (Fe, Mn, Zn, Pb) to solution. Relatively low c oncentrations of Cu were observed in solution and likely reflect the reduci ng conditions that resulted from the presence of sedimentary organic matter . Both the concentrations of CO2 and dissolved metals were lower in fluids reacted with Middle Valley sediment compared with aqueous concentrations in fluids reacted with Guaymas Basin sediment. During alteration of both sedi ment types, metal concentrations varied strongly as a function of temperatu re, increasing by up to an order of magnitude over the 75 degreesC range of each experiment. Major element fluid chemistry and observed alteration ass emblages suggest that during hydrothermal alteration of organic-lean sedime nt from Middle Valley a feldspar-quartz-illite mineral assemblage buffered in situ pH. In contrast, data from the experimental alteration of organic-r ich Guaymas Basin sediment suggest that a calcite-plagioclase-quartz assemb lage regulated in situ pH. Fluid speciation calculations suggest that in si tu pH during Guaymas Basin sediment alteration was lower than during altera tion of Middle Valley sediment and accounts for the substantially greater m etal mobility at a given temperature and pressure during the former experim ent. Comparison of our results with the results of basalt alteration experi ments indicate that except for Cu, hydrothermal sediment alteration results in equal or greater concentrations of ore-forming metals at a given temper ature and pressure. Accordingly, the presence of ore-forming metals in flui ds currently venting from sediment-covered hydrothermal systems at concentr ations substantially lower than in fluids from bare-rock systems may reflec t chemical reequilibration during subsurface cooling within the sediment pi le. Copyright (C) 2001 Elsevier Science Ltd.