Modelling the Lost City hydrothermal field: influence of topography and permeability structure
Modelling the Lost City hydrothermal field: influence of topography and permeability structure
复制标题
失落之城热液场建模：地形和渗透性结构的影响
作者：
Sofya S. Titarenko;A. McCaig
失落之城热液田 (LCHF) 位于亚特兰蒂斯地块顶部的蛇纹岩中，亚特兰蒂斯地块是靠近大西洋中脊的海洋核心复合体。它以其长寿命和排放富含氢气和甲烷的低温（40–91°C）碱性流体而著称。 IODP 孔 U1309D 位于 LCHF 以北 5 公里处，穿透了 1415 m 的辉长岩，包含接近 100°C/km 的近传导热梯度。如此接近活跃的热液场，这是非常了不起的。我们使用穿过喷口场和 IODP 站点 U1309 的地形剖面进行热液建模。在 10e-14 至 10e-15 m2 的中等渗透率和 0.22 W m2 的基础热流下，可以维持与 LCHF 类似的喷口温度的长寿命循环。海底地形是一个重要的控制因素，喷口往往会形成并保留在较高的地形中。整个地块具有均匀渗透率的模型无法同时维持 LCHF 处的循环和钻孔中的近传导梯度，而钻孔中的渗透率需要 <10e-16 m2。 LCHF 和钻孔之间需要陡倾渗透性不连续性，以通过创建横向导电边界层来稳定地块顶部的通风。间断面需要靠近喷口位置，这支持了之前的推论，即高渗透率很可能是由与转换断层相关的断层产生的。模拟流体温度随喷口下方深度的快速增加与之前基于地球化学模拟的反应温度估计一致。
The Lost City hydrothermal field (LCHF) is hosted in serpentinite at the crest of the Atlantis Massif, an oceanic core complex close to the mid-Atlantic Ridge. It is remarkable for its longevity and for venting low-temperature (40–91°C) alkaline fluids rich in hydrogen and methane. IODP Hole U1309D, 5 km north of the LCHF, penetrated 1415 m of gabbroic rocks and contains a near-conductive thermal gradient close to 100°C/km. This is remarkable so close to an active hydrothermal field. We present hydrothermal modelling using a topographic profile through the vent field and IODP site U1309. Long-lived circulation with vent temperatures similar to the LCHF can be sustained at moderate permeabilities of 10e-14 to 10e-15 m2 with a basal heatflow of 0.22 W m2. Seafloor topography is an important control, with vents tending to form and remain in higher topography. Models with a uniform permeability throughout the Massif cannot simultaneously maintain circulation at the LCHF and the near-conductive gradient in the borehole, where permeabilities <10e-16 m2 are required. A steeply dipping permeability discontinuity between the LCHF and the drill hole is required to stabilize venting at the summit of the massif by creating a lateral conductive boundary layer. The discontinuity needs to be close to the vent site, supporting previous inferences that high permeability is most likely produced by faulting related to the transform fault. Rapid increases in modelled fluid temperatures with depth beneath the vent agree with previous estimates of reaction temperature based on geochemical modelling.