Mineralogy and chemistry of basalts and secondary minerals from DSDP Site 417, Atlantic Ocean

Basalts from DSDP Site 417 (109 Ma) exhibit the effects of several stages of alteration reflecting the evolution of seawater-derived solution compositions and control by the structure and permeability of the crust. Characteristic secondary mineral assemblages occur in often superimposed alteration zones within individual basalt fragments. By combining bulk rock and single phase chemical analyses with detailed mineralogic and petrographic studies, chemical changes have been determined for most of the alteration stages identified in the basalts.
1) Minor amounts of saponite, chlorite, and pyrite formed locally in coarse grained portions of massive units, possibly at high temperatures during initial cooling of the basalts. No chemical changes could be determined for this stage.
2) Possible mixing of cooled hydrothermal fluids with seawater resulted in the formation of celadonite-nontronite and Fe-hydroxide-rich black halos around cracks and pillow rims. Gains of K, Rb, H20, increase of Fe 3 +/FeT and possibly some losses of Ca and Mg occurred during this stage.
3a) Extensive circulation of oxygenated seawater resulted in the formation of various smectites, K-feldspar, and Fe-hydroxides in brown and light grey alteration zones around formerly exposed surfaces. K, Rb, H20, and occasionally P were added to the rocks, Fe3+/FeT increased, and Ca, Mg, Si and occasionally Al and Na were lost.
3 b) Anoxic alteration occurred during reaction of basalt with seawater at low water-rock ratios, or with seawater that had previously reacted with basalt. Saponite-rich dark grey alteration zones formed which exhibit very little chemical change: generally only slight increases in Fe 3 +/FeT and H20 occurred.
4) Zeolites and calcite formed from seawater-derived fluids modified by previous reactions with basalt. Chemical changes involved increases of Ca, Na, H20 , and CO2 in the rocks.
5) A late stage of anoxic conditions resulted in the formation of minor amounts of Mn-calcites and secondary sulfides in previously oxidized rocks. No chemical changes were determined for this stage.
Recognition of such alteration sequences is important in understanding the evolution of submarine hydrothermal systems and in interpreting chemical exchange due to seawater-basalt reactions.

深海钻探计划（Deep Sea Drilling Project, DSDP）417站位（距今1.09亿年）产出的玄武岩记录了多期蚀变作用的痕迹，这些蚀变既反映了海水源流体成分的演化过程，也受地壳结构与渗透性的调控。单个玄武岩岩屑内部常发育叠加式蚀变带，其中产出特征性的次生矿物组合。通过将全岩与单矿物化学分析结果，与精细的矿物学及岩相学研究相结合，研究人员已明确了该玄武岩中识别出的多数蚀变阶段的化学变化特征。
1) 少量皂石、绿泥石与黄铁矿局部赋存于块状玄武岩单元的粗粒区域，其形成可能对应玄武岩初始冷却阶段的高温环境，该阶段未检测到显著化学变化。
2) 冷却后的热液流体与海水发生混合，在裂缝及枕状熔岩边缘形成了绿纤石-绿脱石组合与富氢氧化铁的黑色晕圈。此阶段岩石发生了钾（K）、铷（Rb）、水（H₂O）的富集，三价铁与总铁的比值（Fe³⁺/Fe_T）升高，同时可能伴随钙（Ca）与镁（Mg）的部分亏损。
3a) 含氧海水的大规模循环作用，在原暴露表面周围的棕色与浅灰色蚀变带中形成了多种蒙脱石族矿物、钾长石及氢氧化铁。该阶段岩石获得了钾、铷、水，部分样品还富集了磷（P）；三价铁与总铁的比值升高，同时钙、镁、硅（Si）发生亏损，部分样品还伴随铝（Al）与钠（Na）的流失。
3b) 当玄武岩与低水岩比的海水发生反应，或与先前已与玄武岩作用过的海水接触时，发生缺氧蚀变。此时形成富皂石的深灰色蚀变带，该带几乎未发生化学变化，仅普遍出现三价铁与总铁的比值及水含量的轻微升高。
4) 沸石与方解石由经前期玄武岩反应改造的海水源流体沉淀形成，该阶段岩石的钙、钠、水及二氧化碳（CO₂）含量均出现升高。
5) 晚期缺氧环境阶段在先前已发生氧化的岩石中形成少量锰方解石与次生硫化物，该阶段未检测到化学变化。
识别此类蚀变序列，对于理解海底热液系统的演化过程，以及解释海水-玄武岩反应引发的化学交换过程均具有重要意义。