Hydrothermal systems, thermal boundary layers and detachment faults in slow-spread ocean crust

在过去的50年里，我们已经知道欧洲和美洲一直在以每年约2厘米的速度分开，这是由于海底扩张的过程，在大西洋中脊海底产生了新的海洋地壳。这是板块构造的基本过程之一，塑造了我们赖以生存的星球。然而，由于我们不能使用使用电磁辐射的标准遥感技术来研究海底，在许多方面，我们对火星表面的了解比对大西洋海底的了解还要多！在过去的12年里，改进了的大西洋中脊声纳调查揭示了一种新的海底扩张模式，在这种模式下，板块分裂的很大一部分是由长期存在的凸向上分离断层的滑动所占据的，而不是主要由岩浆侵入所占据的。多达一半的大西洋海底可能是这样形成的。这些剥离断层与大型热液系统有关，这些热液系统在海底产生黑色烟囱，排出400℃的流体。在快速（10-15厘米/年）扩张的山脊上，如东太平洋隆起，黑烟系统很小，寿命很短，位于活火山活动带，由浅（1-2公里）的岩浆房提供热量，这些岩浆房或多或少一直存在。这些系统已经被广泛地模拟，其中一个关键因素是在熔融岩浆和热液之间存在一个薄的导电边界层。在大西洋中脊，黑烟系统分布更广、更大、寿命更长，通常位于离活火山活动区几公里远的地方。在某些情况下，这些系统可能是由分离断层上的流体流动控制的，由海底以下7公里处的幕式岩浆房提供热量，而对它们进行的数值模拟工作要少得多。我们在钻取的剥离断层中发现了一个化石热边界层。在本文中，我们计划更深入地研究这一边界层，以及慢扩张脊上断裂、岩浆活动和热液循环之间的复杂相互关系。我们将通过建立热液数值模型来解决这一问题，以预测滑脱断裂产生的不对称热结构以及与渗透性断裂带和局部岩浆活动相关的热液循环模式。由于水的复杂性质，水的密度和粘度在黑烟系统的温度范围内变化非常快，因此热液模型必须非常复杂。因此，我们将与经验丰富的建模师在巴黎工作，以实现我们的目标。我们将使用来自大西洋暴露的剥离断层下盘的IODP岩心的岩石冷却速率数据来测试这些模型——这些冷却速率是通过比较天然矿物的组成和微量元素扩散速率的实验数据来计算的。我们的模型的目的不是精确地复制自然（有太多的未知因素无法做到这一点），而是测试产生可接受结果的参数值范围。例如，该模型必须根据海底测量的温度和地球化学数据估计的热量输出来产生喷口——允许这种情况发生的断层带厚度和渗透率的最小值是多少？然后可以将这些值与基于裂缝密度和地震活动分布的渗透率物理模型进行比较。由于很难直接观察地下地质或流体流动，建模通常是确定假设是否真实的唯一方法。在这个项目结束时，我们将更好地了解地球系统中最重要但最难以接近的部分之一——在脊顶形成新的岩石圈，以及由于这个过程而发生的海洋和地壳之间复杂的相互作用。

We have known for the last 50 years that Europe and America have been moving apart at about 2cm/yr by processes of seafloor spreading that generate new oceanic crust at the submarine mid-Atlantic Ridge. This is one of the fundamental processes of Plate Tectonics, and has shaped the planet that we live on. Yet because we cannot use standard remote sensing techniques using electromagnetic radiation to study the seafloor, in many ways we know more about the surface of Mars than we do about the floor of the Atlantic! Over the last 12 years improved sonar surveys of the mid Atlantic Ridge have revealed a new mode of seafloor spreading where a significant part of the plate divergence is taken up by slip on long-lived, convex upward detachment faults, rather than mainly by magmatic intrusion. Up to half of the Atlantic seafloor may have formed in this way. These detachment faults are associated with large hydrothermal systems producing black smokers venting 400 C fluids on the seafloor. On fast (10-15 cm/yr) spreading ridges such as the East Pacific Rise, black smoker systems are small, short-lived, and located in zones of active volcanism, and are supplied with heat by shallow (1-2 km) magma chambers that are there more or less all the time. These systems have been modelled extensively, and a key element is the existence of a thin conductive boundary layer between molten magma and the hydrothermal fluid. On the mid-Atlantic ridge, black smoker systems are more widely spaced, larger, and longer lived, and often are located a few km away from the zone of active volcanism. These systems may in some cases be controlled by fluid flow up detachment faults, with heat supplied by episodic magma chambers as deep as 7km below seafloor, and much less numerical modelling work has been done on them. We have identified a fossil thermal boundary layer in a detachment fault sampled by drilling. In this proposal we plan to investigate this boundary layer more thoroughly, as well as the complex interrelationships between faulting, magmatism and hydrothermal circulation at slow spreading ridges. We will address this problem by building thermal and hydrothermal numerical models to predict both the asymmetric thermal structure produced by detachment faulting and the hydrothermal circulation patterns associated with permeable fault zones and localised magmatism. The hydrothermal models have to be very sophisticated because of the complicated properties of water, which changes density and viscosity very rapidly in the temperature range of black smoker systems. Hence we will work with experienced modellers in Paris to achieve our aims. We will test these models using data on cooling rates of rocks from IODP core in the footwall of an exposed detachment fault in the Atlantic - these cooling rates are calculated by comparing the compositions of natural minerals with experimental data on diffusion rates of trace elements. The aim of our models is not to replicate nature precisely (there are too many unknowns to do that) - but to test the range of parameter values that generate acceptable results. For example, the model must generate vents with the temperature measured on the seafloor and the heat output estimated from geochemical data - what are the minimum values of fault zone thickness and permeability that allow this to happen? These values can then be compared with physical models of permeability based on fracture densities and seismicity distributions. Because it is hard to observe subsurface geology or fluid flow directly, modelling is often the only way of determining whether hypotheses are realistic. At the end of this project we will have a better understanding of one of the most important but least accessible parts of the Earth System - the formation of new lithosphere at ridge crests, and the complex interactions between the ocean and the crust that occur as a result of this process.