Composition and age of ores and bottom sediments accumulated within and near the Rainbow hydrothermal field

Results of direct geological and geochemical observations of the modern Rainbow hydrothermal field (Mid-Atlantic Ridge, 36°14'N; 33°54'W) carried out from the deep-sea manned Mir submersibles during Cruises 41 and 42 of the R/V Akademik Mstislav Keldysh in 1998-1999 and data of laboratory studies of collected samples are under consideration in the paper. The field lacks neovolcanic rocks and the axial part of the rift is filled in with a serpentinite protrusion. In this field there occur metalliferous sediments, as well as active and relict sulfide edifices composed of sulfide minerals; pyrrhotite, chalcopyrite, isocubanite, sphalerite, marcasite, pyrite, bornite, chalcosine, digenite, magnetite, anhydrite, rare troilite, wurtzite, millerite, and pentlandite have been determined. Sulfide ores are characterized by concentric-zoned textures. During in situ measurements during 35 minutes temperature of hydrothermal fluids was varying within a range from 250 to 350°C. Calculated chemical and isotopic composition of hydrothermal fluid shows elevated concentrations of Cl, Ni, Co, CH4, and H2. Values of d34S of H2S range from +2.4 to +3.1 per mil, of d13C of CH4 from -15.2 to -11.2 per mil, and d13C of CO2 from +1.0 to -4.0 per mil. Fluid inclusions are homogenized at temperatures from 140 to 360°C, whereas salinity of the fluid varies from 4.2 to 8.5 wt %. d34S values of sulfides range from +1.3 to +12.5 per mil. 3He/4He ratio in mineral-forming fluid contained in the fluid inclusions from sulfides of the Rainbow field varies from 0.00000374 to 0.0000101. It is shown that hydrothermal activity in the area continues approximately during 100 ka. It is assumed that the fluid and sulfide edifices contain components from the upper mantle. A hypothesis of phase separation of a supercritical fluid that results in formation of brines is proposed. Hydrothermal activity is related to the tectonic, not volcanic, phase of the Mid-Atlantic Ridge evolution.

本文呈现了1998-1999年由“姆斯季斯拉夫·凯尔迪什院士”号科研考察船（R/V Akademik Mstislav Keldysh）第41、42航次期间，借助载人深海“和平”号潜水器（Mir submersibles）对现代彩虹热液田（Rainbow hydrothermal field，大西洋中脊Mid-Atlantic Ridge，36°14'N；33°54'W）开展的直接地质与地球化学观测结果，以及采集样品的实验室测试数据。该热液田缺失新火山岩，裂谷轴部被蛇纹岩凸起所充填。区内发育含金属沉积物，以及由硫化物矿物构成的活动与残余硫化物构造；已识别出的矿物包括磁黄铁矿、黄铜矿、等轴古巴矿、闪锌矿、白铁矿、黄铁矿、斑铜矿、铜蓝、蓝辉铜矿、磁铁矿、硬石膏、少见的陨硫铁、纤锌矿、针镍矿及镍黄铁矿。硫化物矿石具有同心环带结构特征。35分钟的原位实测结果显示，热液流体温度波动于250~350℃之间。热液流体的计算化学与同位素组成显示，其Cl、Ni、Co、甲烷（CH4）及氢气（H2）浓度显著升高。硫化氢（H2S）的δ34S值介于+2.4‰~+3.1‰之间，甲烷（CH4）的δ13C值介于-15.2‰~-11.2‰之间，二氧化碳（CO2）的δ13C值介于+1.0‰~-4.0‰之间。流体包裹体均一温度介于140~360℃之间，流体盐度为4.2~8.5 wt%。硫化物的δ34S值介于+1.3‰~+12.5‰之间。彩虹热液田硫化物流体包裹体中成矿流体的3He/4He比值介于3.74×10^-6~1.01×10^-5之间。研究表明，该区域的热液活动持续时长约为10万年（ka）。推测热液流体与硫化物构造含有上地幔来源的组分。本文提出超临界流体相分离形成卤水的假说。该热液活动与大西洋中脊演化的构造阶段（而非火山阶段）相关。