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

洋中脊是一系列围绕地球旋转的海底脊，就像棒球上的接缝一样。在这里，上地幔部分熔融形成的岩浆形成了新的海洋地壳，然后岩浆上涌，侵入并喷发到海底。每年形成的海洋地壳面积超过3平方公里。海洋地壳的平均厚度约为6.5±1公里，因此，每年有超过20立方公里的岩浆从地幔中移动，形成新的地壳，使这一过程成为从地球内部到地球表面的热量和质量传递的主要机制。地球物理调查表明，在伸展脊附近一公里左右的范围内，海洋地壳已经完全凝固。这些岩浆的冷却和结晶是由海水衍生的“黑烟”热液（高达400摄氏度）的循环促进的，这些热液在海底壮观的黑烟烟囱上喷发，形成火山携带的大块硫化物基础金属矿床（Cu, Zn），并为各种奇异的化学合成生物和微生物群落提供能量和环境。当海洋地壳从脊轴向外扩散时，在覆盖了地球表面60%以上的巨大脊翼上发生了与海水衍生流体的进一步反应。这种循环是由新的板块——岩石圈——在远离洋脊成熟的过程中冷却所驱动的。海底测量结果表明，与模型相比，平均存在约6500万年的可识别的传导热流赤字，表明热液流体循环带来的热量平流。然而，与脊轴的高温系统相比，脊侧的热液环流发生在低温下，通常远低于100度，通常只有几十度或更低。这导致了火山岩的蚀变和次生矿物的沉淀，如粘土、氧化铁、碳酸盐、硫化物和沸石。由于其规模巨大，这些脊侧反应涉及大量的海水热液流体，是U、K、CO2、H2O、S、Mg、Ca和Fe3+/Fetotal（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最上层地壳的火山熔岩流中。综合海洋发现计划（IODP）第390/393远征队是第一个系统量化海洋中脊侧面海水-玄武岩热液交换的演变方式和程度的实验。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，倾向于取样具有异常厚沉积物覆盖的地点。它将钻穿沉积覆层，在缓慢至中等扩张速度的大西洋中脊西侧横跨大约南纬30度的东西样带，在660万年、1520万年、3060万年、4920万年和6120万年的海洋地壳中取样200至300米的熔岩序列。在最古老的6120万年前的地壳中，IODP探险队390/393将钻探两个地点——一个只有薄薄的沉积物覆盖层（<300米），与被更厚的沉积物覆盖层（~600米）掩埋的相同年龄的海洋地壳进行对比和比较。对新鲜的、蚀变较小的海洋玄武岩以及来自熔岩流、角砾岩和岩脉的蚀变较强的岩石和矿物的代表性样品进行细致的定量岩心描述、矿物鉴定和地球化学分析，将使我们能够了解热液流体-岩石反应的演化样式，并量化海水与海洋地壳之间化学交换的渐进程度。这些分析还将揭示过去6000万年海底水成分变化的信息。

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.