Quantifying Ridge Flank Hydrothermal Alteration on the Juan de Fuca Ridge: IODP Expedition 327

海洋地壳的形成是板块构造循环的一个组成部分，不断地重新铺设海底（今天最古老的海底是180 Myr）。海洋地壳是由火成岩组成的，火成岩沿着海底火山链形成，称为海洋中脊。洋中脊位于板块边界的边缘，在那里两个构造板块相互远离，使得地幔在洋中脊下方上涌并部分融化。新的海洋地壳形成并远离洋脊，最终将被俯冲回地幔。海洋地壳也是地球上最大的含水层，海水通过裂缝渗透到火山地壳中，在一个被称为热液循环的过程中被加热并发生化学反应。在洋中脊附近，结晶岩浆产生的热量驱动了这种循环，并改变了从黑色烟囱中喷出地壳的液体的化学成分。在山脊两侧，热液循环是由地壳随着年龄增长而冷却所驱动的。脊侧环流时热液温度较低（10 s oC），化学变化较小，但流体环流的体积大得多。因此，脊侧环流在热液环流进入海洋的化学通量中所占比例较大，量化这些通量对了解全球地球化学循环具有重要意义。脊侧热液流体的直接采样是困难的，因为它们通常是分散的，而且它们存在的地理区域太大，无法广泛绘制。通过研究流体与之反应和循环的地壳岩石，可以对脊侧热液系统进行间接取样。洋壳热液蚀变主要表现为原生火成岩矿物的替代和次生矿物充填开放孔隙（裂隙、囊泡）。岩石中次生热液矿物的分布、组成和相对时间是了解海水循环过程中地下作用过程的唯一直接途径。在这个项目中，我们将使用综合海洋钻探计划（远征327）回收的样本，研究胡安德富卡山脊侧面的热液环流。胡安德富卡板块位于东太平洋，形成于胡安德富卡脊，扩张速度为30 mm/年。该地区海底的特点是熔岩流被沉积物覆盖，深海丘陵地形，高角度断裂和基底露头。由于来自北美大陆边缘的年轻冰川沉积物供应充足，胡安德富卡山脊侧翼经历了异常高的沉积速率，形成了水文和热隔离的年轻（1 Ma）海洋地壳。Juan de Fuca Ridge侧翼一直是ODP/IODP钻井十年计划的重点，在0.6至3.6 Ma的范围内，沿山脊垂直和山脊平行的样带有12个ODP/IODP钻井点。能够绘制出地壳中低温变化的模式，将使我们能够全面描述流体流经海洋地壳的影响。将对一系列具有代表性的蚀变类型、流动类型和其他特征进行采样，以便考虑到地质特征的最大可能组合。对整个岩石样品的化学分析将确定玄武岩在热液蚀变作用下的变化情况。对氧化铁（Fe3+）含量的测量将表明过去的变化情况。同位素分析记录了热液流体传递给脊侧的特征，从而确定了流体的来源。

The formation of ocean crust is an integral component of the plate tectonic cycle, repaving the ocean floor continuously (the oldest ocean floor today is 180 Myr). The ocean crust is composed of igneous rocks that form along a chain of underwater volcanoes called the mid ocean ridges. Mid ocean ridges are located at the edges of plate boundaries where two tectonic plates move away from each other, allowing the mantle to upwell beneath the ridge and partially melt. New ocean crust forms and moves away from the ridge and eventually will be subducted back into the mantle. The ocean crust is also the largest aquifer on Earth, with seawater percolating down into the volcanic crust through fractures and becoming heated and chemically reacted in a process called hydrothermal circulation. Near the mid ocean ridges, heat from the crystallising magma drives this circulation and modifies the chemistry of fluids that exit the crust at black smoker chimneys. On the ridge flanks, hydrothermal circulation is driven by the cooling of the crust as it ages. Hydrothermal fluid temperatures are considerably lower during ridge flank circulation (10's oC) and the chemical changes smaller, but the volume of fluid circulation is far greater. Consequently, ridge flank circulation represents a larger proportion of the chemical fluxes from hydrothermal circulation into the oceans and quantifying these fluxes is important for understanding global geochemical cycles.Direct sampling of ridge flank hydrothermal fluids is difficult because they are generally diffuse and the geographical area where they are present is too large to map extensively. Indirect sampling of the ridge flank hydrothermal system is possible by studying the crustal rocks the fluids have reacted with and circulated through. Hydrothermal alteration of the ocean crust manifests as replacement of primary igneous minerals and as secondary minerals filling open porosity (fractures, vesicles). The distribution, composition and relative timing of secondary hydrothermal minerals within the rock provide the only direct insight into the processes operating in the subsurface during the circulation of seawater.In this project we will investigate ridge flank hydrothermal circulation through the Juan de Fuca Ridge Flank using samples recovered by the Integrated Ocean Drilling Program (Expedition 327). The Juan de Fuca plate is located in the NE Pacific, and formed at Juan de Fuca Ridge at a spreading rate of 30 mm/yr. The seafloor in this area is characterised by lava flows overlain by sediments, abyssal hill topography, high angle faulting and basement outcrops. The Juan de Fuca Ridge Flank experiences unusually high sedimentation rates because of the abundant supply of young glacial sediments from the North American continental margin, creating hydrologically and thermally isolated young (1 Ma) ocean crust. The Juan de Fuca Ridge Flank has been the focus of a decadal program of ODP/IODP drilling, with 12 ODP/IODP drill sites located along both ridge perpendicular and ridge parallel transects, ranging from 0.6 to 3.6 Ma.Being able to map out the patterns of low temperature alteration in the sections of crust that are available to us will allow an overall characterization of the effects of fluid flow through the oceanic crust. A representative range of alteration types, flow types, and other features will be sampled so that the maximum possible combinations of geologic features are accounted for. Chemical analyses of powered whole rock samples will determine how changed the basalt is as a result of hydrothermal alteration. Measurements of the levels of oxidised iron (Fe3+) will give an indication of the past conditions of alteration. Isotopic analyses document the signature that the hydrothermal fluids have imparted to the ridge flank and allow determination of the fluid origin.