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

大洋中脊侧翼热液循环期间矿物形成的年代测定：来自胡安德富卡海脊的证据，IODP 327 号探险队

• 批准号: NE/L000032/1
• 负责人: Damon Teagle
• 金额: $ 4.56万
• 依托单位: University of Southampton
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2013
• 资助国家: 英国
• 起止时间: 2013 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FL000032%2F1
• 关键词: Dating; mineral; formation; during; mid; Dating; mineral; formation; during; mid

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.

新的海壳是由沿着海洋中的一系列火山爆发形成的，称为中海山脊。这条火山链代表了构造边界，其中两个海洋板彼此不同。该过程代表了从地幔到地壳交换热量的主要机制。海壳占地约60％，在过去的1.8亿年中成立。在新海皮形成后，海水将其渗入海角，并与岩石反应。在山脊轴附近，形成地壳的岩浆腔中的热量驱动了海水的循环，并导致海水的加热和化学修饰，然后将海壳从黑色吸烟烟囱中退出，因为聚焦的高温水热流体（〜400摄氏度C金属富含液体）。海水进入海角的渗透持续到山脊轴上，直到山脊侧面区域持续了数千万年。在山脊侧面区域，海水的循环发生在低得多的温度（几个10摄氏度），并由海洋板的冷却随着年龄的增长而驱动。在山脊侧面循环区域循环的海水量远大于轴向区域，并且对某些元素的化学影响更大，因此对于理解全球地球化学周期的不可接受和难以在山脊中引起的水平范围内的水平范围内，对山coll的位置的局限度的影响很重要，因此很重要。液体直接采样的一种替代方法是采样水热静脉。这些是海壳中的裂缝，这些裂缝充满了二级矿物质，随着流体迁移并与海皮反应，这些矿物会沉淀。这些水热静脉的分布，组成和相对时机为海水循环期间在地下中运行的过程提供了唯一的直接见解。该项目将研究位于Ne Pacific的Juan de Fuca板，在那里，海壳在30 mm/yr的情况下形成。该区域的特征是熔岩流覆盖着沉积物，深渊山地形，高角度断层和地下室露头。由于北美大陆边缘衍生出的年轻冰川沉积物的大量供应，该地区的沉积率很高，该沉积物掩埋了相对年轻的海洋壳。这会在水文和热隔离的幼型（1 MA）海壳中产生强烈的侧向压力和温度梯度，非常适合研究山脊侧面热液过程。胡安·德·富卡山脊的侧面一直是ODP/IODP钻探的十年长期计划的重点，在该计划中，最近完成的IODP Expedition 327将总数达到了Ridge Perpenticular Perpenticular和Ridge Parallel Tractects的12个ODP/IODP钻机​​，范围为0.6 MA。从U1362a孔中回收的火山岩是该区域中较深的钻孔之一（海底496米）的孔中回收的，含有1200多种水热静脉。这些静脉中存在着一系列矿物质成分，这些矿物由粘土矿物和氧化铁矿物支配。碳酸钙静脉也存在。还恢复了该区域的水热矿物质的特征和流体混合的特征。通过记录不同静脉类型之间的分布和关系，可以洞悉流体流动的途径。对静脉矿物质本身的化学分析将记录其形成的液体的化学成分和演变，这反过来反映了液体和海角之间的反应程度。不同二级矿物的日期将在水热山脊侧流体的时机上提供直接限制。