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

新的海洋地壳是由火山爆发形成的，这些火山沿着一系列被称为大洋中脊的海底火山。这一系列火山代表了两个大洋板块相互分离的构造边界。这一过程是地幔与地壳进行热交换的主要机制。海洋地壳覆盖了地球表面的60%，并且都是在过去的1.8亿年中形成的。新的洋壳形成后，海水渗入洋壳，与岩石发生反应。在海脊轴附近，来自形成地壳的岩浆房的热量驱动海水的循环，并导致海水的加热和化学改性，然后从黑烟囱中排出海洋地壳，成为集中的高温热液流体（~400摄氏度的富含金属的流体）。海水渗入洋壳的过程持续了几千万年，越过海脊轴线进入海脊侧翼区域。在海脊侧翼区域，海水的循环发生在低得多的温度（几十摄氏度），并由海洋板块随着年龄的增长而冷却驱动。在海脊侧翼区域循环的海水量远远大于轴向区域，对海洋某些元素的化学性质影响更大，因此对了解全球地球化学循环非常重要，由于无法进入海脊侧翼区域，难以对离开洋壳的热液进行定位和取样，因此对这一重要水文地质储层的直接观测有限。直接对流体取样的另一种方法是对热液脉取样;这些是海洋地壳中充满次生矿物的裂缝，这些矿物随着流体的迁移和海洋地壳的反应而沉淀下来。这些热液脉的分布、组成和相对时间提供了对海水循环期间地下过程的唯一直接洞察。本项目将调查位于东北太平洋的胡安德富卡板块，那里的洋壳形成速度为30毫米/年。该区域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。这一地区的沉积速率很高，因为北美大陆边缘有大量年轻的冰川沉积物，埋藏着相对年轻的洋壳。这创造了水文和热隔离的年轻（1马）洋壳，具有强大的横向压力和温度梯度，非常适合研究脊侧翼热液过程。Juan de Fuca山脊侧翼一直是ODP/IODP钻探的十年期长期计划的重点，最近完成的IODP Expedition 327使ODP/IODP钻探地点总数达到12个，这些钻探地点位于沿着山脊垂直和山脊平行断面，范围从0.6到3.6 Ma。在Exp 327期间，从该地区较深的钻孔之一（海底以下496米）U1362 A孔中回收的火山岩含有1200多个热液脉。这些矿脉中存在一系列矿物成分，主要由粘土矿物和氧化铁矿物组成。也存在碳酸钙脉。在该地区首次发现了具有较高温度和流体混合特征的热液矿物。通过记录不同矿脉类型之间的分布和关系，可以了解流体流动的途径。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变，这反过来又反映了流体与海洋地壳之间的反应程度。不同次生矿物的年代测定将对热液脊侧翼流体的时间提供直接的限制。

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.