Oceanic detachment faults focus very large volumes of black smoker fluids
Oceanic detachment faults focus very large volumes of black smoker fluids
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DOI：
10.1130/g23657a.1
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发表时间：
2007-10-01
期刊：
影响因子：
5.8
通讯作者：
MacLeod, Christopher J.
中科院分区：
文献类型：
--
作者：
McCaig, Andrew M.;Cliff, Robert A.;MacLeod, Christopher J.
一般认为，流入海底黑烟囱喷口场的海水衍生液体是从大约 1-3 公里的深度垂直排出的。我们提供的新的氧和锶同位素数据表明，黑烟温度为 300-400 摄氏度的流体沿着大西洋中脊附近北纬 15 度 45' 处的低角度脱离断层聚集。同位素变化是迄今为止报道的在相似温度下变化的海洋岩石中最极端的变化，表明热液系统的补给和排放部分中的流体流动强烈集中。断层岩中的稀土元素迁移率表明，同位素变化是由演化的热液而非传导加热的海水引起的。断层带原岩主要是超镁铁质，但也包括镁铁质岩石，交代蚀变为滑石-透闪石-绿泥石片岩，主要是由于流体流动过程中这些岩性之间的化学交换所致。与该组合处于平衡状态的流体类似于含有超镁铁质的黑色烟熏液。我们提出了一种新模型，其中滑脱断层周围的热液循环从玄武岩托管（TAG型）演变为下盘超镁铁质托管（彩虹型），再到低温超镁铁质托管（失落之城型）。我们模型的主要特征是辉长岩体直接侵入滑脱层下方，为循环提供热源，并将流体集中流入滑脱断层，以允许从新火山轴排出。
It is generally assumed that the seawater-derived fluids that feed black smoker vent fields on the seafloor are discharged vertically from depths of similar to 1-3 km. We present new oxygen and strontium isotope data that show that fluids at black smoker temperatures of 300-400 degrees C were focused along a low-angle detachment fault at 15 degrees 45'N near the Mid-Atlantic Ridge. Isotopic alteration is the most extreme ever reported from oceanic rocks altered at similar temperatures, indicating intensely focused fluid flow both in recharge and discharge parts of the hydrothermal system. Rare earth element mobility in the fault rocks demonstrates isotopic alteration by evolved hydrothermal fluids, not conductively heated seawater. The fault zone protolith was predominantly ultramafic, but also included mafic rocks, with metasomatic alteration to talc-tremolite-chlorite schists resulting mainly from chemical exchange between these lithologies during fluid flow. Fluids in equilibrium with this assemblage would be similar to ultramafic-hosted black smoker fluids. We present a new model in which hydrothermal circulation around detachment faults evolves from basalt hosted (TAG type), to footwall ultramafic hosted (Rainbow type), to low-temperature ultramafic hosted (Lost City type). Key features of our model are the intrusion of gabbro bodies immediately below the detachment to provide a heat source for circulation, and focusing of fluid flow into the detachment fault to allow venting away from the neovolcanic axis.