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

洋壳的形成是板块构造循环的一个组成部分，不断地重新铺设洋底（今天最古老的洋底是1.8亿年）。海洋地壳由火成岩组成，这些火成岩沿着一系列被称为大洋中脊的水下火山形成。洋中脊位于板块边界的边缘，两个构造板块在此相互远离，使地幔在脊下上升并部分融化。新的洋壳形成并远离洋脊，最终将俯冲回地幔。海洋地壳也是地球上最大的含水层，海水通过裂缝进入火山地壳，并在一个称为热液循环的过程中被加热和化学反应。在大洋中脊附近，来自结晶岩浆的热量驱动着这种循环，并改变了从黑烟囱中排出地壳的流体的化学性质。在海脊两侧，热液循环是由地壳随着年龄的增长而冷却所驱动的。海脊侧翼环流期间热液流体温度相当低（10 ℃），化学变化较小，但流体循环量要大得多。因此，脊侧翼环流代表了从热液环流进入海洋的化学通量的较大比例，量化这些通量对于了解全球地球化学循环非常重要，脊侧翼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，无法广泛绘制地图。通过研究流体与之发生反应并在其中循环的地壳岩石，可以对海脊侧翼热液系统进行间接取样。海洋地壳的热液蚀变表现为原生火成岩矿物的替换和次生矿物填充开放孔隙（裂缝、小泡）。岩石中次生热液矿物的分布、成分和相对时间提供了对海水循环过程中地下运行过程的唯一直接了解。在本项目中，我们将使用综合大洋钻探计划（327号考察队）回收的样品，通过胡安·德富卡海脊侧翼调查海脊侧翼热液循环。胡安·德富卡板块位于东北太平洋，形成于胡安·德富卡海脊，扩张速率为30 mm/年。该区域海底的特征是沉积物覆盖的熔岩流、深海丘陵地形、高角度断层和基底露头。胡安·德富卡海脊侧翼的沉积速率异常高，这是因为北美大陆边缘提供了大量年轻的冰川沉积物，形成了水文和热隔离的年轻（1 Ma）洋壳。Juan de Fuca海脊侧翼一直是ODP/IODP钻探十年计划的重点，12个ODP/IODP钻探点位于沿着海脊垂直和平行断面，如果能够绘制出我们所掌握的地壳各部分的低温蚀变模式，就可以全面描述通过海洋的流体流动的影响，地壳。将对蚀变类型、流动类型和其他特征的代表性范围进行采样，以便考虑地质特征的最大可能组合。对粉状全岩样品的化学分析将确定玄武岩是如何由于热液蚀变而发生变化的。氧化铁（Fe 3+）水平的测量将显示过去的蚀变情况。同位素分析记录了热液流体赋予山脊侧面的特征，并可以确定流体的来源。

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.