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

洋中脊在海洋中形成了一系列山脉，像棒球上的接缝一样环绕着地球。这些脊是建设性的板块边界，在这里，板块构造扩张和地幔岩浆喷发形成了新的海洋地壳。这是地球释放内部热量的主要过程，大约60%的地球表面是在过去1.8亿年里形成的。由于岩浆在约1200摄氏度的温度下喷出并侵入海洋地壳，渗透到地壳中的海水被强烈加热，通常导致被称为黑烟的海底间歇泉排出约350摄氏度的富含硫化物的液体。岩浆和热液“热水”循环之间有非常密切的关系。由于洋脊大多在水下2000米以下，许多与它们形成有关的过程仍然知之甚少。科学家们使用潜水器只能观察到最近喷发的熔岩。地球物理学家可以用地震速度（波穿过岩石的速度）来测量海洋地壳，但他们的结果必须与实际的岩石进行校准才能理解。海洋地壳的地下采样需要进行深度钻探，而基岩岩芯的回收是自20世纪60年代以来科学海洋钻探的主要目标。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，通常呈枕状；过于垂直的侵入物，被称为片状岩脉；反过来，过于粗粒的辉长岩，是结晶的岩浆室。直到最近，只有一个钻孔将熔岩穿透到岩脉中，而且没有一个钻孔对岩脉-辉长岩边界进行采样，尽管就岩浆和海水之间的热交换而言，这个区域是最关键的，因为这两个阶段只被一层薄薄的热接触层隔开。IODP考察队309-312最近回到了1256D洞，这是东太平洋的一个深钻孔，并将这个洞加深到1500米，完全穿过熔岩、岩脉，进入辉长岩。1256D孔钻入的地壳扩展速度非常快（约200毫米/年），因为预计那里的辉长岩最浅。虽然只有约20%的全球脊轴正以快速扩张的速度（约80毫米/年）扩张，但目前60%的海洋盆地和约30%的地球表面是由快速扩张形成的。因为在快速扩张速率下形成的地壳应该是相对均匀的，所以在一个地点钻探可以合理地推断出地壳的很大一部分。1256D孔的岩心是了解海洋地壳火成岩构造和岩浆与海水相互作用的独特资源。通过与化学分析相结合的详细描述，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通过地壳。利用1256D孔的新信息，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算）、其他不太完整的海洋地壳部分和蛇绿岩的热液通量估算结果进行比较。冷却海洋地壳的海水的数量，以及它被加热到的温度，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，我们将利用这些约束条件进行岩浆侵入和结晶与热液循环耦合的计算机模拟。这些计算机模型将使我们能够确定岩浆房的大小和排列以及热液流体循环在物理上是合理的。

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.