Fault geometry and permeability contrast control vent temperatures at the Logatchev 1 hydrothermal field, Mid-Atlantic Ridge
Fault geometry and permeability contrast control vent temperatures at the Logatchev 1 hydrothermal field, Mid-Atlantic Ridge
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DOI：
10.1130/g36113.1
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发表时间：
2015-01-01
期刊：
影响因子：
5.8
通讯作者：
Petersen, Sven
中科院分区：
文献类型：
--
作者：
Andersen, Christine;Ruepke, Lars;Petersen, Sven
沿沿着缓慢扩张的大西洋中脊发现的高温（> 300摄氏度）离轴热液系统显然一直位于出露断层带。虽然热流体沿着高渗透性断裂岩石的优先流动似乎是直观的，但这种有效的流动不可避免地导致冷环境海水的夹带。这应该导致的温度下降很难与观察到的高温黑烟活动和相关块状硫化物矿床的形成相一致。在这里，我们联合收割机新获得的地震数据从高温，离轴Logatchev 1热液场（LHF 1）与热液流的数值模拟，以解决这一明显的矛盾。数据显示，强烈的离轴地震与震源机制建议从LHF 1向山脊轴倾斜的断层带。我们的模拟预测高温通风LHF 1只有一个有限的范围内的故障宽度和渗透率的对比，表示为故障的相对传输率（两个参数的产品）。相对传输率必须足以“捕获”上升的热液羽流并将其重新定向到LHF 1，但又足够低以防止与环境冷流体的广泛混合。此外，与非均匀地壳中任何高渗透区有关的温度下降，可以解释为什么大部分热液沿着缓慢扩张的海脊排出时温度较低。
High-temperature (> 300 degrees C) off-axis hydrothermal systems found along the slow-spreading Mid-Atlantic Ridge are apparently consistently located at outcropping fault zones. While preferential flow of hot fluids along highly permeable, fractured rocks seems intuitive, such efficient flow inevitably leads to the entrainment of cold ambient seawater. The temperature drop this should cause is difficult to reconcile with the observed high-temperature black smoker activity and formation of associated massive sulfide ore deposits. Here we combine newly acquired seismological data from the high-temperature, off-axis Logatchev 1 hydrothermal field (LHF1) with numerical modeling of hydrothermal flow to solve this apparent contradiction. The data show intense off-axis seismicity with focal mechanisms suggesting a fault zone dipping from LHF1 toward the ridge axis. Our simulations predict high-temperature venting at LHF1 only for a limited range of fault widths and permeability contrasts, expressed as the fault's relative transmissibility (the product of the two parameters). The relative transmissibility must be sufficient to "capture" a rising hydrothermal plume and redirect it toward LHF1 but low enough to prevent extensive mixing with ambient cold fluids. Furthermore, the temperature drop associated with any high permeability zone in heterogeneous crust may explain why a significant part of hydrothermal discharge along slow-spreading ridges occurs at low temperatures.