UK-IODP Moratorium Co-Chief Support for Expedition 393: Quantifying the geochemical impacts of ocean crustal ageing.

大洋中脊是一系列围绕地球旋转的海底海脊，就像棒球上的接缝一样。这就是新的洋壳形成的地方，岩浆是由上地幔部分熔化形成的，然后向上涌，侵入并喷发到海底。每年形成的洋壳面积超过3000平方公里。平均而言，大洋地壳的厚度约为6.5+/-1千米，因此，每年有超过20立方千米的岩浆从地幔移动，形成新的地壳，使这一过程成为从地球内部向地球表面传递热量和质量的主要机制。地球物理测量表明，洋壳在扩张海脊一公里左右的范围内完全固化。这些岩浆的冷却和结晶得益于海水产生的“黑烟”热液流体(高达400摄氏度)的循环，这些流体在壮观的黑烟烟囱的海底喷发，形成火山托管的大量硫化物贱金属矿床(铜、锌)，并为各种外来化学合成生物体和微生物群落提供能源和环境。随着洋壳从海脊轴线向外扩散，在覆盖地球表面60%以上的巨大海脊两侧，与海水产生的流体发生了进一步的反应。这种循环是由新的板块--岩石圈--在远离山脊的地方成熟时的冷却驱动的。海底测量表明，与平均模型相比，向外流出地壳的可察觉的传导性热流赤字约为6500万年，表明热通过热液循环平流。然而，与脊轴上的高温系统相比，脊侧的水热环流发生在通常远低于100度的低温下，通常只有几十度或更低。这导致火山岩的蚀变和次生矿物的沉淀，如粘土、铁氧化物、碳酸盐、硫化物和沸石。由于规模巨大，这些脊侧反应涉及大量来自海水的热液，是U、K、CO_2、H_2O、S、镁、钙和Fe~(3+)/Fettal(氧化)等重要元素和化合物的全球地球化学收支的重要组成部分。这些反应大多发生在地壳最上层的火山熔岩流中。综合海洋发现计划(IODP)探险390/393是第一次系统地量化大洋中脊两侧海水-玄武岩热液交换的演化方式和程度的实验。重要的是，它将钻探具有正常厚度沉积物覆盖的地点，而不是早期的海洋钻探，后者倾向于采样具有异常厚沉积覆盖的地点。它将钻探沉积覆盖层，并在大西洋中低扩张速率海脊西侧6.6、15.2、30.6、49.2和6120万年的洋壳中采集200至300米的熔岩序列，穿过南纬约30度的东西走向横断面。在最古老的6120万年前的地壳，IODP考察队390/393将钻探两个地点--一个仅有较薄的沉积物覆盖层(约300米)，以与已被更厚的沉积覆盖层(约600米)埋藏的同时代洋壳进行对比和比较。仔细的定量岩芯描述、矿物鉴定和地球化学分析，以及来自熔岩流、角砾岩和矿脉的较强蚀变岩石和矿物的代表性样品，将使我们能够了解热液-岩石反应的演化方式，并量化海水和洋壳之间化学交换的进展程度。这些分析还将揭示过去6000万年来海底水域成分变化的信息。

The mid-ocean ridges are a range of submarine ridge that spiral around Earth like the seams on a baseball. This is where new ocean crust is constructed from magma formed from the partial melting of the upper mantle that then upwells to be intruded into and erupted onto the ocean floor. More that 3 square kilometres of ocean crust are formed each year. On average the ocean crust is approximately 6.5 +/- 1 km thick, and consequently more than 20 cubic kilometres of magma moves from the mantle to make new crust every year, making this process the dominant mechanism for heat and mass transfer from the interior of the Earth to the surface of our planet. Geophysical surveys demonstrate that the ocean crust is completely solidified within a kilometre or so of the spreading ridge. The cooling and crystallisation of these magmas is facilitated by the circulation of seawater-derived "black-smoker" hydrothermal fluids (upto 400 degrees C) that vent on the seafloor at spectacular black smoker chimneys, form volcanic-hosted massive sulphide base metal deposits (Cu, Zn), and provide energy and environments for diverse communities of exotic chemosynthetic organisms and microbes. As the ocean crust spreads away from the ridge axis further reactions with seawater-derived fluids occur on the vast ridge flanks that cover more than 60 percent of the surface of our planet. This circulation is driven by the cooling of the new plate-the lithosphere-as it matures away from the ridge. Seafloor measurements indicate that compared to models on average there is a discernible conductive heat flow deficit out to crust of ~65 million years, indicating the advection of heat by the circulation of hydrothermal fluids. However, in contrast to the high temperature systems at the ridge axis, hydrothermal circulation on the ridge flanks occurs at low temperatures generally much less than 100 degrees and generally at only a few tens of degrees or lower. This leads to the alteration of volcanic rocks and the precipitation of secondary minerals such as clays, iron oxides, carbonates, sulphides and zeolites. Because of their vast scale, these ridge flank reactions involve huge volumes of seawater-derived hydrothermal fluids and are important components in the global geochemical budgets of important elements and compounds such as U, K, CO2, H2O, S, Mg, Ca and Fe3+/Fetotal (oxidation). Most of these reactions occur in the volcanic lava flows of the uppermost crust. Integrated Ocean Discovery Program (IODP) Expedition 390/393 are the first experiment to systematically quantify the evolving styles and extents of seawater-basalt hydrothermal exchange across the flank of a mid-ocean ridge. Importantly it will drill sites with normal thickness sediment cover in contrast to earlier ocean drilling that has tended to sample sites with anomalously thick sediment cover. It will drill through the sedimentary overburden and sample 200 to 300 metres of the lava sequences in ocean crust of 6.6, 15.2, 30.6, 49.2 and 61.2 million years old on the western flank of the slow to medium spreading rate Mid-Atlantic Ridge across an east-west transect at approximately 30 deg South. At the oldest 61.2 million-year-old crust IODP Expedition 390/393 will drill two sites - one with only a thin sediment cover (<300 m) to contrast and compare with ocean crust of the same age that has been buried by a much thicker sediment cover (~600 m).Careful quantitative core description, mineral identification and geochemical analyses of fresh, little altered ocean basalt and representative samples of more strongly altered rocks and minerals from lava flows, breccias and veins will enable us to understand the evolving styles of hydrothermal fluid-rock reactions and quantify the progressive extents of chemical exchange between seawater and the ocean crust. These analyses will also reveal information about changes in the composition of ocean bottom waters over the past 60 million years.