Results of experiments of hydrothermal alteration of Middle Valley sediment from ODP Hole 139-855A

The presence of sedimentary organic matter blanketing midocean ridge crests has a potentially strong impact on metal transport in hydrothermal vent fluids. To constrain the role of organic matter in metal mobility during hydrothermal sediment alteration, we reacted organic-rich diatomaceous ooze from 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° to 400°C, 350 to 500 bars, and initial fluid: sediment mass ratios ranging from 1.6 to 9.8. Reaction of these fluids with both sediment types released CO2 and high concentrations of ore-forming metals (Fe, Mn, Zn, Pb) to solution. Relatively low concentrations of Cu were observed in solution and likely reflect the reducing 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 sediment types, metal concentrations varied strongly as a function of temperature, increasing by up to an order of magnitude over the 75°C range of each experiment. Major element fluid chemistry and observed alteration assemblages suggest that during hydrothermal alteration of organic-lean sediment from Middle Valley a feldspar-quartz-illite mineral assemblage buffered in situ pH. In contrast, data from the experimental alteration of organic-rich Guaymas Basin sediment suggest that a calcite-plagioclase-quartz assemblage regulated in situ pH. Fluid speciation calculations suggest that in situ pH during Guaymas Basin sediment alteration was lower than during alteration of Middle Valley sediment and accounts for the substantially greater metal mobility at a given temperature and pressure during the former experiment. Comparison of our results with the results of basalt alteration experiments indicate that except for Cu, hydrothermal sediment alteration results in equal or greater concentrations of ore-forming metals at a given temperature and pressure. Accordingly, the presence of ore-forming metals in fluids currently venting from sediment-covered hydrothermal systems at concentrations substantially lower than in fluids from bare-rock systems may reflect chemical reequilibration during subsurface cooling within the sediment pile.覆盖于大洋中脊峰顶的沉积有机质，对热液喷口流体（hydrothermal vent fluids）中的金属运移具有潜在的显著影响。为明确有机质在热液沉积物蚀变（hydrothermal sediment alteration）过程中对金属迁移性的调控作用，我们开展了两组水热实验：分别使用取自加利福尼亚湾瓜伊马斯盆地的富有机质硅藻软泥（diatomaceous ooze）以及北胡安德富卡洋脊中部山谷的贫有机质半远洋泥（hemipelagic mud），将二者分别与海水以及盐度与海水一致的Na-Ca-K-Cl流体，在275℃至400℃、350至500巴的条件下进行反应，初始流体与沉积物的质量比介于1.6至9.8之间。两类沉积物与上述流体反应后，均向溶液中释放了CO₂以及高浓度的成矿金属（ore-forming metals，Fe、Mn、Zn、Pb）。溶液中铜（Cu）的浓度相对较低，这大概率反映了沉积有机质存在所导致的还原环境。与瓜伊马斯盆地沉积物反应后的流体中，CO₂与溶解金属的浓度均低于中部山谷沉积物反应后的流体。在两类沉积物的蚀变过程中，金属浓度随温度变化显著，在单次实验的75℃温度区间内，浓度最高可提升一个数量级。主量元素流体化学特征与观测到的蚀变矿物组合（alteration assemblages）表明，在中部山谷贫有机质沉积物的热液蚀变过程中，长石-石英-伊利石矿物组合对原位pH（in situ pH）起到缓冲作用。与之相反，针对富有机质瓜伊马斯盆地沉积物的实验蚀变数据显示，方解石-斜长石-石英组合调控了原位pH。流体组分形态计算（fluid speciation calculations）表明，瓜伊马斯盆地沉积物蚀变过程中的原位pH低于中部山谷沉积物，这解释了前者在给定温压条件下金属迁移性显著更强的原因。将本研究结果与玄武岩蚀变实验（basalt alteration experiments）的结果对比可知，除铜之外，在给定温压条件下，热液沉积物蚀变所产生的成矿金属浓度与裸岩系统（bare-rock systems）流体中的浓度相当，甚至更高。因此，目前在覆盖有沉积物的热液系统喷流流体中观测到的成矿金属浓度，显著低于裸岩热液系统的流体浓度，这一现象可能反映了沉积物柱（sediment pile）内地下冷却过程中的化学再平衡（chemical reequilibration）作用。