Geochemistry of felsic rocks and minerals of ODP Hole 135-841B

A felsic volcanic series (605-825 mbsf) overlain by upper Eocene shallow-water sediments (500-605 mbsf) and basalticandesitic sills that intruded into sediments of Holocene to Miocene age (0-500 mbsf) was drilled in the forearc region of the Lau Basin at a water depth of 4810 m. The volcanic sequence at Site 841 includes altered and mineralized calc-alkaline rhyolites and dacites, dacitic tuffs, lapilli tuffs, flow breccias, and welded tuffs. These rocks formed subaerially or in a very shallow-water environment suffering a subsidence of >5000 m since Eocene times. Calculations of gains and losses of the major components during alteration show most pronounced changes in the uppermost 70 m of the volcanic sequence. Here, Al, Fe, Mg, and K are enriched, whereas Si and Na are strongly depleted. Illite, vermiculite, chlorite, and hematite predominate in this part of the hole. Throughout the section, quartz, plagioclase, kaolinite, and calcite are present. Sulfide mineralization (up to 10 vol%) consisting mainly of disseminated pyrite (with minor pyrrhotite inclusions) and marcasite together with minor amounts of chalcopyrite is pervasive throughout. Locally, a few sulfide-bearing quartz-carbonate veins as well as Ti-amphibole replacement by rutile and then by pyrite were observed. Strong variations in the As content of sulfides (from 0 to 0.69 wt%) from the same depth interval and local enrichments of Co, Ni, and Cu in pyrite are interpreted to result from fluctuations in fluid composition. Calculations of oxygen and sulfur fugacities indicate that fO2 and fS2 were high at the top and lower at the bottom of the sequence. Sulfur isotope determinations on separated pyrite grains from two samples give d34S values of +6.4ë and +8.4ë, which are close to those reported from Kuroko and Okinawa Trough massive sulfide deposits and calc-alkaline volcanic rocks of the Japanese Ryukyu Island Arc. Calculated chlorite formation temperatures of 265°-290°C at the top of the sequence are consistent with minimum formation temperatures of fluid inclusions in secondary quartz, revealing a narrow range of 270°-297°C. Chlorite formation temperatures are constant downhole and do not exceed 300°C. The presence of marcasite and 4C-type pyrrhotite indicates a formation temperature of <= 250°C. At a later stage, illite was formed at the top of the volcanic series at temperatures well below 200°C.

在水深4810米的劳盆地（Lau Basin）弧前区域，钻取了一套长英质火山岩系（felsic volcanic series，605~825米海底以下（meters below seafloor，mbsf）），其上覆有始新世晚期浅海沉积物（500~605米海底以下，mbsf）以及侵入于全新世至中新世沉积物（0~500米海底以下，mbsf）中的玄武安山岩床（basaltic andesitic sills）。841站位（Site 841）的火山岩序列包含蚀变且矿化的钙碱性流纹岩、英安岩，英安质凝灰岩、火山砾凝灰岩、流动角砾岩以及熔结凝灰岩。这些岩石形成于陆上或极浅水环境，自始新世以来该区域经历了超过5000米的沉降。对蚀变过程中主要组分的增减量计算结果显示，火山岩序列最上部的70米发生了最为显著的变化。该层段中铝（Al）、铁（Fe）、镁（Mg）与钾（K）发生富集，而硅（Si）与钠（Na）则发生强烈亏损。该钻孔段以伊利石（illite）、蛭石（vermiculite）、绿泥石（chlorite）以及赤铁矿（hematite）为主。整个岩心剖面均可见石英（quartz）、斜长石（plagioclase）、高岭石（kaolinite）以及方解石（calcite）。体积分数可达10%的硫化物矿化（sulfide mineralization）广泛发育，其主要由浸染状黄铁矿（pyrite，伴少量磁黄铁矿（pyrrhotite）包裹体）、白铁矿（marcasite）以及少量黄铜矿（chalcopyrite）组成。局部可见少量含硫化物的石英-碳酸盐脉，以及钛角闪石（Ti-amphibole）先后被金红石（rutile）、黄铁矿交代的现象。同一深度段内硫化物的砷（As）含量变化幅度极大（0~0.69重量百分比，wt%），且黄铁矿中局部富集钴（Co）、镍（Ni）与铜（Cu），上述现象被认为是流体成分波动所致。氧逸度（oxygen fugacities）与硫逸度（sulfur fugacities）的计算结果显示，岩序列顶部的fO₂与fS₂较高，而底部则较低。对两个样品中分选得到的黄铁矿颗粒进行硫同位素测定，获得的δ³⁴S值分别为+6.4‰与+8.4‰，该数值与日本琉球岛弧（Ryukyu Island Arc）的黑矿型（Kuroko）矿床、冲绳海槽（Okinawa Trough）块状硫化物矿床以及钙碱性火山岩的已报道数据相近。岩序列顶部的绿泥石（chlorite）形成温度计算值为265~290℃，与次生石英中流体包裹体的最低形成温度一致，限定了温度区间为270~297℃。绿泥石形成温度沿钻孔深度保持稳定，且未超过300℃。白铁矿与4C型磁黄铁矿（4C-type pyrrhotite）的存在指示其形成温度不高于250℃。在后期阶段，火山岩系顶部形成了伊利石，其形成温度远低于200℃。