Granulometry, mineralogy and geochemistry of sediments from ODP Hole 169-1037B and Site 169-1038

Central Hill is in the northern part of the Escanaba Trough, which is a sediment-filled rift of southern Gorda Ridge. Central Hill is oriented north-south and is associated with extensive sulfide deposits. Hydrothermal alteration of sediment from Site 1038 was studied through analyses of mineralogy and the chemistry and oxygen isotopic compositions of one nearly pure clay sample. In addition, Site 1037 was drilled to establish the character of the unaltered sedimentary sequence away from the hydrothermal centers of the Northern Escanaba Trough Study Area (NESCA). Mineralogy of the clay-size fraction of turbiditic and hemipelagic sediments of Hole 1037B are predominantly quartz, feldspar, pyroxene, illite, chlorite, and smectite, representing continental-derived material. Cores from Hole 1038I, located within the area of Central Hill but away from known active vent areas, recovered minor amounts of chlorite/smectite mixed-layer clay in the fine fraction, indicating a low-temperature hydrothermal alteration. The 137.4-m-thick sediment section of Hole 1038G is located in an area of low-temperature venting. The uppermost sample is classified as chlorite/smectite mixed layer, which is underlain by chlorite as the dominant mineral. The lowermost deposits of Hole 1038G are also characterized by chlorite/smectite mixed-layer clay. In comparison to Hole 1038I, the mineralogic sequence of Hole 1038G reflects increased chloritization. Intensely altered sediment is almost completely replaced by hydrothermal chlorite in subsurface sediments of Hole 1038H. Alteration to chlorite is characterized by depletion in Na, K, Ti, Ca, Sr, Cs, and Tl and enrichment in Ba. Further, Eu depletion reflects a high-temperature plagioclase alteration. A chlorite 18O value of 2.6 indicates formation at a temperature of ~190°C. It is concluded that the authigenic chlorite in Hole 1038H formed by an active high-temperature fluid flow in the shallow subsurface.中央海丘（Central Hill）位于埃斯卡诺巴海槽（Escanaba Trough）北部，该海槽是戈尔达海岭（Gorda Ridge）南部的充填沉积物裂谷。中央海丘呈南北走向，与广泛分布的硫化物矿床伴生。研究人员通过对1038站位沉积物的矿物组成、化学特征及1件近乎纯黏土样品的氧同位素组成开展分析，对该站位沉积物的热液蚀变（hydrothermal alteration）特征进行了研究。此外，为明确北埃斯卡诺巴海槽研究区（NESCA）热液活动中心外围未蚀变沉积层序的特征，科研人员钻探了1037站位。1037B钻孔的浊积与半远洋沉积物黏土粒级组分的矿物组成以石英、长石、辉石、伊利石、绿泥石及蒙脱石为主，指示其为陆源物质（continental-derived material）成因。 采自中央海丘区域内但远离已知活跃喷口区域的1038I钻孔岩心，其细粒组分中仅发现少量绿泥石/蒙脱石混合层黏土，表明此处发生过低温热液蚀变。1038G钻孔的137.4米厚沉积段位于低温喷溢活动区域，其最顶部样品被归为绿泥石/蒙脱石混合层，下方以绿泥石为主要矿物；该钻孔最底部的沉积层同样以绿泥石/蒙脱石混合层黏土为特征。相较于1038I钻孔，1038G钻孔的矿物序列反映出更强的绿泥石化作用。1038H钻孔的地下沉积物中，强烈蚀变的沉积物几乎完全被热液成因绿泥石交代。绿泥石化蚀变的地球化学特征表现为Na、K、Ti、Ca、Sr、Cs及Tl元素的亏损，以及Ba元素的富集；此外，Eu元素亏损反映了高温斜长石蚀变过程。绿泥石的δ¹⁸O值为2.6，指示其形成温度约为190℃。综上可知，1038H钻孔中的自生绿泥石（authigenic chlorite）形成于浅表层活跃的高温流体活动过程中。