Silica‐Rich Vein Formation in an Evolving Stress Field, Atlantis Bank Oceanic Core Complex

Drilling 809‐m Hole U1473A in the gabbro batholith at the Atlantis Bank Oceanic Core Complex (OCC) found two felsic vein generations: late magmatic fractionates, rich in deuteric water, hosted by oxide gabbros, and anatectic veins associated with dike intrusion and introduction of seawater‐derived volatiles. Microtextures show a change from compressional to tensional stress during vein formation. Temperatures and oxidation state were obtained from amphibole‐plagioclase and oxide pairs in the adjacent gabbros. Type I veins generally have reverse shear‐sense, with restricted ΔFMQ, high Mt/Ilm ratios, and low‐amphibole Cl/F indicating deuteric fluids. They formed during percolation and fractionation of Fe‐Ti‐rich melts into the primary olivine gabbro. Type II veins are usually hosted by olivine gabbro, occur at dike contacts and the margins of normal‐sense shear zones. They are undeformed or weakly deformed, with highly variable ΔFMQ, low Mt/Ilm ratios, and high‐amphibole Cl/F, indicating seawater‐derived fluids. The detachment fault on which the gabbro massif was emplaced rooted near the base of the dike‐gabbro transition beneath the rift valley. The ingress of seawater volatiles began at >800°C and penetrated at least ~590 m into the lower crust during extensional faulting in the rift valley and adjacent rift mountains. The sequence of the felsic vein formation likely reflects asymmetric diapiric flow, with a reversal of the stress regime, and a transition from juvenile to seawater‐derived volatiles. This, in turn, is consistent with fault capture leading to the large asymmetries in spreading rates during OCC formations and heat flow beneath the rift mountains.

在亚特兰蒂斯浅滩大洋核杂岩（OCC）的辉长岩岩基中钻探809米的U1473A孔时，发现了两代长英质岩脉：由氧化物辉长岩所含的富含热液水的晚期岩浆分异物，以及与岩墙侵入和海水来源挥发物引入相关的深熔岩脉。微观结构显示在岩脉形成过程中应力从压缩变为拉伸。温度和氧化态是从相邻辉长岩中的角闪石 - 斜长石和氧化物对获得的。I型岩脉通常具有逆剪切方向，有限的ΔFMQ（氧逸度相对值）、高的磁铁矿/钛铁矿比率以及低的角闪石Cl/F值，表明为热液流体。它们是在富铁 - 钛熔体渗透和分异进入原生橄榄石辉长岩过程中形成的。II型岩脉通常位于橄榄石辉长岩中，出现在岩墙接触处和正剪切带的边缘。它们未变形或轻微变形，具有高度变化的ΔFMQ、低的磁铁矿/钛铁矿比率以及高的角闪石Cl/F值，表明为海水来源的流体。辉长岩体所就位的拆离断层扎根在裂谷下方岩墙 - 辉长岩过渡带的底部附近。海水挥发物的进入始于>800°C，并在裂谷和相邻裂谷山脉的伸展断层作用期间至少渗透到下地壳约590米深处。长英质岩脉形成的顺序可能反映了不对称的底辟流、应力状态的反转以及从原生挥发物到海水来源挥发物的转变。这反过来又与断层捕获相一致，导致了大洋核杂岩形成过程中扩张速率以及裂谷山脉下方热流的巨大不对称性。