Mechanisms of fluid penetration into gabbroic crust, IODP Site 1309, mid-Atlantic Ridge.

摘要：洋壳是地壳地质调查的最后一个重要领域。在大陆上，我们可以通过实地绘制露头地图来指导我们的采样工作，因此，即使是南极洲等最偏远的地区也比大多数海底地区更为人所知。在海洋中，由于沉积物覆盖，大多数地壳无法接近，露头测绘仅限于用几艘潜水艇进行勘测，并对海底碎石斜坡进行疏浚取样。这就是为什么大洋钻探计划（ODP，现在的IODP）如此重要，我们可以从沉积物覆盖层下恢复核心，在连续的核心中，我们可以看到地质关系，告诉我们过程。通过大洋钻探，我们现在对海洋地壳的沉积覆盖层有了相当多的了解，但我们对“硬岩”地壳的了解还远远不够。在年轻的断裂玄武岩中开孔是非常困难的，因此我们对大洋地壳中更深的席状岩墙和辉长岩层的了解仅限于两个深孔（东太平洋的504 B和西南印度洋脊的735 B）和少数几个最多几百米的孔。因此，在大西洋中脊的年轻（<200万年）地壳中钻一个新孔（IODP孔U1309 D），穿透1 400米的辉长岩，是海洋科学界非常兴奋的时刻。洋壳是地球系统中最重要的组成部分之一。地壳的形成和扩展，再加上海水热液循环的冷却，是我们星球失去热量的主要方式。热液循环对地壳的改变是对海水成分的主要控制，并在海底产生热液喷口，在地球历史早期，生命可能起源于海底。我们对喷口流体的成分（变化很大）了解很多，我们也知道海水的成分，但由于上文提到的取样困难，我们对洋壳内部发生的情况了解甚少。例如，流体是如何进入最初不可渗透的岩石的，是什么控制了流动路径、循环的深度和温度，以及在流动路径上的什么地方具有喷口流体的独特化学性质？我们对这些过程的大多数模型都是基于蛇绿岩（古老的海洋型地壳被推到大陆上），它可能来自与俯冲有关的盆地，而不是真正的海洋地壳，并且可能不具有代表性。在这个项目中，我们希望通过进一步研究在船上观察到的极好的蚀变关系来回答其中的一些问题。特别是，我们已经确定了几个阶段的蚀变，并可以采样蚀变和未蚀变岩石之间的边界。由于钻孔是在钻孔后用地球物理工具记录的，我们将能够将岩心恢复到其真实的地理方位（通常不可能），并测量矿脉和裂缝的方位，将其与构造情况联系起来。我们在岩芯中发现了蚀变反应纹理，表明体积增加/这可能是增加海底渗透性的一种新方法。我们的计划是使用地球化学分析来测量通过岩石的水量，将其与裂缝的方向和密度联系起来，并开发一些数值模型（通过项目合作伙伴）来联系应力，热历史，压裂和变质反应。此外，我们还将首次获得捕获在洋壳中的热液流体包裹体的分析结果，从而制约热液流体沿着流动路径的化学演化。

Summary: The ocean crust is the last great frontier for geological investigation of the Earth's crust. On the continents, we can guide our sampling by field mapping of outcrops, and hence even the remotest areas such as Antarctica are far better known than most of the ocean floor. In the oceans, most of the crust is inaccessible due to sediment cover, and outcrop mapping is restricted to surveys by a few submersible craft, and to dredge sampling of submarine scree slopes. This is why the Ocean Drilling Program (ODP, now IODP) is so important / we can recover core from beneath the sediment cover, and in continuous core we can see geological relationships that inform us about process. Through ODP we now know quite a lot about the sedimentary cover of the ocean crust, but our knowledge of the 'hard rock' crust is far from adequate. It is very difficult to start holes in young fractured basalts, and hence our knowledge of the deeper sheeted dyke and gabbro layers in the ocean crust is restricted to two deep holes (504B in the east Pacific, and 735B on the SW Indian Ridge) and a handful of other sites with holes up to a couple of hundred metres at most. Hence the drilling of a new hole (IODP Hole U1309D) penetrating 1400 m of gabbro in young (<2 million year) crust on the mid-Atlantic ridge is an occasion for great excitement in the ocean science community. The ocean crust is one of the most important parts of the Earth System. Formation and spreading of the crust, coupled with cooling through hydrothermal circulation of seawater, is the main way in which our planet loses heat. Alteration of the crust by hydrothermal circulation is a major control on the composition of seawater, and produces hydrothermal vents on the seafloor where life may have originated early in Earth history. We know quite a lot about the composition of vent fluids (which is quite variable), and we know the composition of seawater, but because of the sampling difficulties referred to above, we know far less about what happens inside the ocean crust. For example, how does fluid get into the initially impermeable rock, what controls the flow paths, depth and temperature of circulation, and where on the flow path does it the distinctive chemistry of vent fluids? Most of our models for these processes are based on ophiolites (pieces of old ocean-type crust thrust up onto the continents), which probably come from subduction-related basins rather than true ocean crust, and may not be representative. In this project we hope to answer some of these questions by further study of superb alteration relationships observed on board ship. In particular, we have identified several phases of alteration and can sample the boundaries between altered and unaltered rock. Because the hole was logged with geophysical tools after it was drilled, we will be able to restore the core to its real geographical orientation (not normally possible) and measure the orientations of veins and fractures, relating these to the tectonic situation. We identified alteration reaction textures in the core indicating volume increase / this could be a new way of increasing permeability in the ocean floor. Our plan is to measure the amount of water which has passed through the rock using geochemical analyses, relate this to the orientation and density of fractures, and develop some numerical models (through project partners) to relate stress, thermal history, fracturing and metamorphic reactions. In addition we will obtain the first ever analyses of hydrothermal fluid trapped in the ocean crust in fluid inclusions, and thus constrain the chemical evolution of hydrothermal fluid along the flow path.