Geochemistry of sheeted dike complex minerals of ODP Hole 140-504B@en

Diabases were recovered during Legs 137 and 140 at Hole 504B from depths between 1621.5 and 2000.4 meters below seafloor in the lower sheeted dike complex. The samples contain multiple generations of millimetric to centimetric veins. The orientation of the measured veins suggests that two main vein sets exist: one characterized by shallow dipping and the other by random trend. Thermal contraction during rock cooling is considered the main mechanism responsible for fracture formation. Vein infill is related to the circulation of hydrothermal fluids near the spreading axis. Some veins are surrounded by millimeter-sized alteration halos due to fluid percolation from the fractures through the host rock. Vein-filling minerals are essentially amphibole, chlorite, and zeolites. Amphibole composition is controlled by the microstructural site of the rock. Actinolite is the main amphibole occurring in the veins and also in the groundmass away from the halos. In the alteration halos, amphibole shows composition of actinolitic hornblende and Mg-hornblende. Late-stage tension gashes and interstitial spaces in some amphibole-bearing veins are filled with zeolites, suggesting that the veins likely suffered multiple opening stages that record the cooling history of the circulating fluids. Evidence of deformation recorded by the recovered samples seems to be restricted to veins that clearly represent elements of weakness of the rock. On the basis of vein geometry and microstructure we infer structural interpretations for the formation mechanism and for deformation of veins.在大洋钻探第137航次与第140航次期间，从504B钻孔海底以下1621.5至2000.4米深度的下部席状岩墙群中采获了辉绿岩（diabase）样品。该批样品发育多期次毫米级至厘米级脉体。所测脉体的产状显示存在两组主要脉系：一组以缓倾斜为特征，另一组走向任意。岩石冷却过程中的热收缩作用被认为是裂缝形成的主导机制。脉体充填物与扩张轴附近的热液流体循环密切相关。部分脉体因流体沿裂缝渗透进入围岩而被毫米级蚀变晕环绕。脉体充填矿物主要为角闪石、绿泥石与沸石。角闪石的成分受其赋存岩石的显微构造部位控制。阳起石是脉体及蚀变晕外围基质中产出的主要角闪石种属；在蚀变晕内部，角闪石的成分为阳起质角闪石与镁角闪石。部分含角闪石脉体中的晚期张性裂隙及粒间空隙均被沸石充填，表明脉体经历了多期张开过程，该过程记录了循环热液流体的冷却历史。所获样品记录的变形证据仅局限于明显属于岩石薄弱带的脉体。基于脉体的几何形态与显微构造特征，我们对脉体的形成机制及变形过程提出了构造学解释。