Seafloor hydrothermal clay alteration at Jade in the back-arc Okinawa trough: Mineralogy, geochemistry and isotope characteristics
Seafloor hydrothermal clay alteration at Jade in the back-arc Okinawa trough: Mineralogy, geochemistry and isotope characteristics
复制标题
DOI：
10.1016/s0016-7037(99)00158-1
复制
发表时间：
1999-09
影响因子：
5
通讯作者：
K. Marumo;K. Hattori
中科院分区：
文献类型：
--
作者：
K. Marumo;K. Hattori
玉石的海底热液活动导致了原生碎屑沉积物和浮质凝灰岩的广泛蚀变，形成了云母、高岭石（高岭石和高岭石）、富镁绿泥石、滑石、蒙脱石以及一种双八面体绿泥石和蒙脱石的混合层矿物（Chl/Mont）。粘土矿物组合呈现垂直变化，反映了地下沉积物和凝灰岩中热液流体中冷海水的数量变化。但单纯的混合作用并不能解释玉石中高岭土矿物丰富的成因。高岭土矿物的形成需要比热液和冷海水简单混合所需的酸性流体多得多的酸性流体。低pH值可能是由于热液流体中溶解的H2S或在减压过程中从流体中释放出来的H2S氧化而获得的。到达海底的流体被排放到冷海水中，导致靠近喷口的硫化物和喷口边缘的天然硫和重晶石的沉淀。高岭石、重晶石和硬石膏的Sr同位素组成与海洋Sr相似，表明海洋Sr直接来源于海水或钙质纳米浮游生物的溶解作用。高岭石（δ18O = +7.4‰，δD =−23‰）、Chl/Mont （δ18O = +7.0‰，δD =−32‰）和云母（δ18O = +5.4 ~ +9.9‰，δD =−30 ~−26‰）的同位素组成表明其为加热海水流体。O同位素数据显示，高岭石的地层温度为170℃，高岭石为61 ~ 110℃，云母为145 ~ 238℃。重晶石δ34S值（+21.0 ~ +22.5‰）与海相硫酸盐值非常接近，证实了重晶石的形成是含钡热液与富硫酸盐海水混合作用的结果。原生硫的δ34S变化较大，可能是由于海底流体呼出过程中S的快速不平衡沉淀所致。热液流体中的硫通常被消耗形成金属硫化物。因此，玉石中丰富的天然硫表明热液流体具有较高的硫化氢/金属比。玉石热液矿物的蚀变组合和同位素数据与中中新世黑子型重晶石矿床非常相似，形成于低于200℃的高S/金属比流体中。在Jade，只有一个黑色的烟囱在活跃地排放高温（~ 320°C）流体，但在该地区有许多化石硫化物烟囱和土丘。这些沉淀物的矿物学特征和高Au、Cu含量表明过去存在高金属热液活动。这些活动可能导致热液羽流的排放以及海底硫化物和硫酸盐的沉降。这些放射性沉降物被纳入远离火山口的沉积物中。它们现在被记录为沉积物中金属含量高，没有岩石学和矿物学证据表明原位热液活动。有的铜含量高达8100 ppm，锌为12500 ppm，砷为1000 ppm，银为100 ppm，铅为21000 ppm。这种沉积物中的蒙脱石碎屑颗粒在沉淀到海底之前，在海水中悬浮期间被铁氧氢氧化物包裹。沉积物中这种金属异常的深度表明，在1800至300 ybp之间存在高水平的含金属热液活动。
Seafloor hydrothermal activity at Jade has resulted in extensive alteration of the host epiclastic sediments and pumiceous tuffs, forming mica, kaolins (kaolinite and halloysite), Mg-rich chlorite, talc, montmorillonite, and a mixed-layer mineral of dioctahedral chlorite and montmorillonite (Chl/Mont). Clay mineral assemblages show a vertical variation, which reflects variable amounts of cold seawater incorporated into hot hydrothermal fluids in subsurface sediments and tuff. However, mixing alone cannot explain the occurrence of abundant kaolin minerals at Jade. The formation of kaolin minerals requires much more acidic fluid than expected from simple mixing of hydrothermal fluids and cold seawater. Low pH values are likely attained by oxidation of H2S either dissolved in the hydrothermal fluid or released from the fluid during decompression. The fluid reaching the seafloor is discharged into cold seawater, which caused precipitation of sulfides close to vents and native sulfur and barite at the margins of the vent areas. Halloysite, barite and anhydrite show Sr isotope compositions similar to marine Sr, indicating the derivation of marine Sr directly from seawater or by the dissolution of calcareous nannoplanktons. The isotopic compositions of kaolinite (δ18O = +7.4‰, δD = −23‰), Chl/Mont (δ18O = +7.0‰, δD = −32‰), and mica (δ18O = +5.4 to +9.9‰, δD = −30 to −26‰) suggest fluids of a heated seawater origin. The O isotopic data yielded formation temperatures of 170°C for kaolinite, 61 to 110°C for halloysite, and 145 to 238°C for mica. Barite δ34S values (+21.0 to +22.5‰) are very similar to the marine sulfate value, confirming that the barite formation took place due to mixing of Ba-bearing hydrothermal fluids and sulfate-rich seawater. Native sulfur shows a large variation in δ34S in one hand specimen probably because of rapid disequilibrium precipitation of S during fluid exhalation on the seafloor. Sulfur in hydrothermal fluids is usually consumed to form metal sulfides. Therefore, abundant native sulfur at Jade suggests high H2S/metals ratios of the hydrothermal fluids. The alteration assemblages and isotopic data of hydrothermal minerals from Jade are very similar to those of Kuroko-type barite deposits of middle Miocene age, which formed from fluids of high S/metals ratios at less than 200°C. At Jade, there is only one black smoker actively discharging high temperature (∼320°C) fluid, but there are many fossil sulfide chimneys and mounds in the area. The mineralogy and high Au and Cu in these precipitates suggest highly metalliferous hydrothermal activity in the past. These activities likely resulted in discharge of hydrothermal plumes and fall-outs of sulfides and sulfates on the seafloor. These fall-outs were incorporated in sediments far from the vent areas. They are now recorded as high metal contents in sediments with no petrographic and mineralogical evidence of in-situ hydrothermal activity. Some are high as 8,100 ppm for Cu, 12,500 ppm for Zn, 1,000 ppm for As, 100 ppm for Ag and 21,000 ppm for Pb. Detrital grains of montmorillonite in such sediments are coated with Fe-oxyhydroxides during the suspension in seawater before settling on the seafloor. The depths of such metal anomalies in sediments suggest high levels of metalliferous hydrothermal activities from 1,800 to 300 ybp.