Hydrothermal alteration of an intact section of the upper oceanic crust formed at a superfast spreading rate

大洋中脊在海洋中形成一系列山脉，环绕地球，就像棒球上的接缝一样。这些海脊是板块构造扩张和地幔岩浆喷发形成新洋壳的构造性板块边界。这是地球释放内部热量的主要过程，地球表面的60%是在过去的1.8亿年中形成的。由于岩浆在约1200摄氏度的温度下喷发并侵入海洋地壳，因此渗入地壳的海水变得剧烈加热，通常导致被称为黑烟的海底间歇泉喷出>350摄氏度的富含硫化物的流体。岩浆活动与热液“热水”循环关系密切。由于洋脊大多位于水深2 000米以上的地方，因此对洋脊形成的许多过程仍然知之甚少。使用潜水器的科学家只能观察最近喷发的熔岩。地球物理学家可以使用地震速度（波穿过岩石的速度）测量海洋地壳，但他们的结果必须根据实际岩石进行校准才能理解。对海洋地壳的次表层取样需要进行深钻，自1960年代以来，回收基底岩芯一直是科学海洋钻探的一个主要目标。保存在陆地上的古代海洋岩石，被称为蛇绿岩，表明海洋地壳由三个基本层组成：喷发的熔岩，通常具有枕形形状，过度垂直的侵入体被称为席状岩脉，反过来又覆盖着粗粒辉长岩，它们是结晶的岩浆房。直到最近，只有一个钻孔将熔岩穿透到岩脉中，没有钻孔对岩脉-辉长岩边界进行取样，尽管该区域在岩浆和海水之间的热交换方面是最关键的，因为这些相只被一个薄的热接触层分开。IODP Expeditions 309-312最近返回到1256 D孔，这是东太平洋的一个深钻孔，并将该孔加深到>1500米，完全穿过熔岩，岩脉并进入辉长岩。1256 D孔是在地壳中钻成的，地壳的扩展速度非常快（>200毫米/年），因为预测辉长岩在那里最浅。虽然只有约20%的全球海脊轴以快速扩张速度（>80毫米/年）扩张，但目前60%的海洋盆地和约30%的地球表面是通过快速扩张形成的。由于快速扩张形成的地壳应该是相对均匀的，因此可以合理地推断在单个位置进行钻探可以描述地壳的重要部分。1256 D孔的岩芯是一个独特的，迄今为止无法获得的资源，用于了解洋壳的火成岩结构和岩浆与海水之间的相互作用。通过仔细描述并结合化学分析，我们将能够计算出流体-岩石相互作用的物理条件，并估计流体的化学和同位素组成。根据化学交换的程度，我们可以估计有多少海水穿过地壳。利用1256 D孔的这些新信息，我们可以将我们的结果与全球化学收支（如鲜为人知的河流流量估计），其他不太完整的海洋地壳部分和蛇绿岩的热液通量估计进行比较。冷却洋壳的海水量和海水被加热的温度极大地影响了洋中脊岩浆房的位置和大小。一旦我们确定了热液蚀变的条件和通量，我们将在计算机模拟中使用这些约束条件，将岩浆侵入和结晶与热液循环结合起来。这些计算机模型将使我们能够确定岩浆房和热液流体循环的大小和排列在物理上是合理的。

The mid-ocean ridges form a chain of mountains in the oceans that circuit the Earth like seams on a baseball. These ridges are constructive plate boundaries where new ocean crust is formed by plate tectonic spreading and the eruption of magma from the mantle. This is the major process by which the Earth releases its internal heat, and ~60% of the Earth's surface was formed in the past 180 million years. Because magma is erupted onto and intruded into the ocean crust at ~1200 deg C, seawater that percolates into the crust becomes vigorously heated, commonly resulting in submarine geysers known as black smokers that disgorge >350 deg C, sulfide-rich fluids. There is a very close relationship between magma and hydrothermal 'hot water' circulation. Because the ocean ridges are mostly beneath >2000 m of water many of the processes related to their formation remain poorly understood. Scientists using submersibles can observe only the most recently erupted lavas. Geophysicists can make measurements of the ocean crust using seismic velocities (the speed that waves travel through rocks), but their results must be calibrated against actual rocks to be understood. Sampling the sub-surface of the ocean crust requires deep drilling and the recovery of basement rock cores has been a major goal of scientific ocean drilling since the 1960s. Ancient oceanic rocks preserved on land, known as ophiolites, suggest that the ocean crust is made of three basic layers: erupted lavas, commonly with pillowed shapes, that overly vertical intrusions known as sheeted dikes, which in turn overly, coarse-grained gabbroic rocks that are crystallized magma chambers. Until very recently, only one drill hole had penetrated the lavas into the dikes and no hole had sampled the dike-gabbro boundary even though that zone is the most critical in terms of heat exchange between magma and seawater, because those phases are separated by only a thin thermal contact layer. IODP Expeditions 309-312 recently returned to Hole 1256D, a deep drill hole in the eastern Pacific, and deepened that hole to >1500 m, completely through the lavas, dikes, and into the gabbros. Hole 1256D was drilled into crust that formed at a very fast spreading rate (>200 mm/yr), because gabbros were predicted to be at their shallowest there. Although only ~20% of the global ridge axis is spreading at fast spreading rates (>80 mm/yr full rate), 60% of the current ocean basins and ~30% of the Earth's surface was formed by fast spreading. Because crust formed at fast spreading rates should be relatively uniform, drilling at a single location can be reasonably extrapolated to describe a significant portion of the Earth's crust. The cores from Hole 1256D are a unique, hitherto unavailable, resource for understanding the igneous construction of the ocean crust and interactions between magma and seawater. By making careful descriptions linked with chemical analyses we will be able to work out the physical conditions of fluid-rock interaction and estimate the chemical and isotopic composition of the fluids. From the extent of chemical exchange we can make estimates of how much seawater passed through the crust. Using this new information for Hole 1256D we can compare our results with estimates of hydrothermal fluxes from global chemical budgets (such as poorly known estimates of river discharge), other less complete sections of ocean crust, and ophiolites. The amount of seawater that cools the ocean crust, and the temperatures to which it is heated, greatly influences the location and size of mid-ocean ridge magma chambers. Once we have worked out the conditions and fluxes of hydrothermal alteration, we will use these constraints in computer simulations that couple magma intrusion and crystallization with hydrothermal circulation. These computer models will allow us to establish what size and arrangements of magma chambers and hydrothermal fluid circulation are physically plausible.