Dating mineral formation during mid-ocean ridge flank hydrothermal circulation: Evidence from the Juan de Fuca Ridge, IODP Expedition 327

新的洋壳是由沿大洋下方一系列称为大洋中脊的火山链喷发而成的。这一系列火山链代表了两个大洋板块彼此分离的构造边界。这一过程代表了从地幔到地壳的热量交换的主要机制。洋壳覆盖了地球表面的60%，全部形成于过去1.8亿年。新的洋壳形成后，海水向下渗入洋壳并与岩石发生反应。在海脊轴线附近，来自形成地壳的岩浆室的热量驱动海水的循环，导致海水加热和化学修饰，然后以集中的高温热液(~400℃富金属流体)的形式从黑烟烟囱中流出洋壳。海水渗入大洋地壳的过程持续了数千万年，从海脊轴线进入海脊侧翼区域。在海脊两侧，海水的环流发生在低得多的温度(10‘S℃)，并由大洋板块随着年龄的冷却而驱动。海脊两侧区域的海水循环体积远远大于轴线区域，对某些元素的海洋化学有更大的影响，因此对了解全球地球化学循环具有重要意义。由于无法在海脊两侧区域的洋壳中找到和采样热液流体，因此对这一重要的水文地质储集层的直接观测受到限制。直接对流体采样的另一种方法是对热液矿脉进行采样；这些矿脉是洋壳中的裂缝，填充了次生矿物，随着流体的迁移和与洋壳的反应而沉淀下来。这些热液脉体的分布、组成和相对时间提供了唯一直接的洞察在海水循环期间在地下运行的过程。这个项目将调查位于东北太平洋的Juan de Fuca板块，那里的洋壳形成速度为30 mm/年。该地区的特点是沉积覆盖的熔岩流、深海丘陵地形、高角度断裂和基底露头。该地区经历了较高的沉积速率，因为北美大陆边缘的年轻冰川沉积物供应充足，埋藏了相对年轻的洋壳。这创造了水文和热隔离的年轻(1 Ma)洋壳，具有强大的侧向压力和温度梯度，是研究脊侧热液过程的理想选择。胡安德富卡海脊侧翼一直是十年长的ODPIODP钻探计划的重点，最近完成的IODP 327探险使IODP钻探地点总数达到12个，分布在海脊垂直和海脊平行断面上，范围从0.6 Ma到3.6 Ma。在Exp 327期间从U1362a孔(该地区较深的钻孔之一)(海底以下496米)发现的火山岩包含1200多个热液矿脉。这些矿脉中存在一系列矿物成分，以粘土矿物和氧化铁矿物为主。还存在碳酸钙矿脉。该地区还首次发现了具有高温和流体混合特征的热液矿物。通过记录不同脉型之间的分布和关系，可以洞察流体流动的路径。对矿脉矿物本身的化学分析将记录它们形成的流体的化学成分和演化，这反过来又反映了流体与洋壳之间的反应程度。不同次生矿物的定年将直接制约热液脊侧向流体的形成时间。

New ocean crust is formed from volcanic eruptions along a chain of volcanoes under the oceans called mid ocean ridges. This chain of volcanoes represents the tectonic boundary where two oceanic plates diverge away from each other. This process represents the major mechanism for the exchange of heat from the mantle to the crust. The ocean crust covers ~60% of the Earth's surface and all formed during the past 180 million years. After new ocean crust has formed, seawater percolates down into ocean crust and reacts with the rocks. Near the ridge axis the heat from the magma chamber that formed the crust drives the circulation of the seawater, and results in the heating and chemical modification of seawater that then exit the ocean crust from black smoker chimneys as focused high temperature hydrothermal fluids (~400 deg C metal rich fluids). The percolation of seawater into the ocean crust continues beyond the ridge axis onto the ridge flank region for tens of millions of years. In the ridge flank region, the circulation of seawater occurs at much lower temperatures (a few 10's deg C) and is driven by the cooling of the oceanic plate as it ages. The volume of seawater that circulates in the ridge flank region is far greater than in the axial region and has more impact on the chemistry of the oceans for some elements and is thus important for understanding global geochemical cycles.The inaccessibility and difficulty in locating and sampling hydrothermal fluids as they exit the ocean crust in the ridge flank region has resulted in limited direct observations of this important hydrogeologic reservoir. An alternative to direct sampling of the fluids is to sample hydrothermal veins; these are fractures in the ocean crust that have been filled with secondary minerals that are precipitated as the fluid migrates and reacts with the ocean crust. The distribution, composition and relative timing of these hydrothermal veins provide the only direct insight into the processes operating in the subsurface during the circulation of seawater.This project will investigate the Juan de Fuca plate located in the NE Pacific, where ocean crust formed at 30 mm/yr. This area is characterised by lava flows overlain by sediments, abyssal hill topography, high angle faulting and basement outcrops. This regions experiences high sedimentation rates because of the abundant supply of young glacial sediments derived from the North American continental margin that bury relatively young oceanic crust. This creates hydrologically and thermally isolated young (1 Ma) ocean crust with strong lateral pressure and temperature gradients, ideal for the study of ridge flank hydrothermal processes. The Juan de Fuca Ridge Flank has been the focus of a decadal long program of ODP/IODP drilling, where the recent completion of IODP Expedition 327 took the total to 12 ODP/IODP drill sites located along both ridge perpendicular and ridge parallel transects, ranging from 0.6 to 3.6 Ma. Volcanic rocks recovered during Exp 327 from Hole U1362A, one of the deeper boreholes in this region (496 metres below seafloor), contains over 1200 hydrothermal veins. A range of mineral compositions is present within these veins that are dominated by clay minerals and oxidised iron minerals. Calcium carbonate veins are also present. For the first time in this region hydrothermal minerals characteristic of higher temperatures and mixing of fluids were also recovered.By documenting the distribution and relationships between different vein types, insight into the pathways of fluid flow can be achieved. Chemical analyses of the vein minerals themselves will document the chemical composition and evolution of the fluids they formed from, that in turn reflect the extent of reaction between the fluid and the ocean crust. Dating of the different secondary minerals will provide direct constraints on the timing of hydrothermal ridge flank fluids.