Mineralogy of serpentine muds of ODP Leg 125 holes (Table 1)

Large serpentinite seamounts are common in the forearc regions between the trench axis and the active volcanic fronts of the Mariana and Izu-Bonin intraoceanic arcs. The seamounts apparently form both as mud volcanoes, composed of unconsolidated serpentine mud flows that have entrained metamorphosed ultramafic and mafic rocks, and as horst blocks, possibly diapirically emplaced, of serpentinized ultramafics partially draped with unconsolidated serpentine slump deposits and mud flows. The clayand silt-sized serpentine recovered from three sites on Conical Seamount on the Mariana forearc region and from two sites on Torishima Forearc Seamount on the Izu-Bonin forearc region is composed predominantly of chrysotile, brucite, chlorite, and clays. A variety of accessory minerals attest to the presence of unusual pore fluids in some of the samples. Aragonite, unstable at the depths at which the serpentine deposits were drilled, is present in many of the surficial cores from Conical Seamount. Sjogrenite minerals, commonly found as weathering products of serpentine resulting from interaction with groundwater, are found in most of the samples. The presence of aragonite and carbonate-hydroxide hydrate minerals argues for interaction of the serpentine deposits with fluids other than seawater. There are numerous examples of sedimentary serpentinite deposits exposed on land that are very similar to the deposits recovered from the serpentine seamounts drilled on ODP Leg 125. We suggest that Conical Seamount may be a type locality for the study of in situ formation of many of these sedimentary serpentinite bodies. Further, we suggest that both the deposits drilled on Conical Seamount and on Torishima Forearc Seamount demonstrate that serpentinization can continue in situ within the seamounts through interaction of the serpentine deposits with both seawater and subduction-related fluids.马里亚纳和伊豆-小笠原洋内弧的海沟轴与活动火山前缘之间的弧前区域中，大型蛇纹岩海山（serpentinite seamounts）分布广泛。这类海山既可形成于泥火山（mud volcanoes）——由裹挟变质超镁铁质岩与镁铁质岩的未固结蛇纹岩泥流构成，也可形成于可能经底辟侵位的地垒块体（horst blocks），此类块体由蛇纹岩化超镁铁质岩组成，表面局部覆盖未固结的蛇纹岩滑塌沉积与泥流。从马里亚纳弧前区域的锥形海山（Conical Seamount）3个站位，以及伊豆-小笠原弧前区域的鸟岛前缘海山（Torishima Forearc Seamount）2个站位获取的粘粒与粉粒级蛇纹岩样品，主要由纤蛇纹石（chrysotile）、水镁石（brucite）、绿泥石（chlorite）以及黏土矿物组成。多种副矿物表明，部分样品中存在异常孔隙流体。在锥形海山的多数表层岩芯中，均发现了文石（aragonite）——该矿物在蛇纹岩沉积的钻探深度下并不稳定。多数样品中还发现了水碳镁石（Sjogrenite），这类矿物通常是蛇纹岩与地下水相互作用形成的风化产物。文石与碳酸盐-氢氧化物水合物矿物的存在，证明蛇纹岩沉积曾与除海水之外的流体发生过相互作用。 陆上出露的沉积型蛇纹岩沉积体与大洋钻探计划（Ocean Drilling Program, ODP）第125航次钻获的蛇纹岩海山沉积体极为相似，此类实例不胜枚举。我们认为，锥形海山可作为研究多数此类沉积型蛇纹岩岩体原位形成过程的模式产地。进一步而言，锥形海山与鸟岛前缘海山的钻取沉积体均表明，蛇纹岩化作用可在海山内部持续进行——通过蛇纹岩沉积与海水以及俯冲相关流体的相互作用得以实现。