Sixty million years of seafloor alteration: spatial-temporal controls on volatile incorporation along a 1000 km transect of oceanic crust

大约60%的地球表面被海洋地壳覆盖并淹没在海洋之下。洋壳是板块构造扩张的结果，由洋中脊的岩浆形成。当它们冷却时，海洋地壳的火成岩通过与海水和其他流体的化学反应而改变。在扩张脊附近的高温下，这些相互作用导致海底潜水器观察到壮观的热液“黑烟”喷口系统。远离扩张海脊的低温流体-岩石反应可能会持续更长时间，并可能持续数千万年。海洋地壳的蚀变导致地壳和海洋之间的化学交换，改变了两者的组成。挥发性化学物质，如碳、氯和水在新鲜熔岩中很少见，但通过蚀变在地壳中富集。随着时间的推移，这将它们从海洋中移除并将它们结合到海洋地壳中。大多数海洋地壳的最终命运是俯冲到其他构造板块之下，沉入地幔，在可能很长的时间内从地球表面去除挥发性元素，上述过程对许多挥发性元素的自然循环具有重要意义。碳由于其在大气中的行为而特别重要。将其捕获在海洋地壳中并俯冲该地壳可能是过去全球气候循环的重要组成部分。同样，在地球历史上，以这种方式去除海水中的氯可能是降低海洋盐度并帮助使其适合生命居住的原因。为了了解过去洋壳的变化如何影响海水和大气化学，我们需要了解在这种情况下是什么控制着挥发性地球化学行为。地壳的年龄、扩张速度（大洋板块分离的速度）和覆盖地壳的沉积物数量都可能影响蚀变，并可以从板块构造重建中估计或预测。然而，计算扩展速率或沉积厚度如何影响蚀变地壳中挥发物的捕获是复杂的。因为它被海水淹没，我们唯一的直接海洋地壳样本来自钻探或疏浚海底。迄今为止所钻取的岩心大多来自非常古老或非常年轻的地壳，并偏向于更快的扩展速度和更厚的沉积物覆盖层。为了解决这些不足之处，正在进行一次新的钻探考察，沿南大西洋约1 000公里的一段洋壳钻探一个沿着6个孔的断面，以便能够对记录约6 000万年蚀变的独特样本集进行调查。新的钻探还将填补现有岩心收集的关键空白，包括以缓慢扩张速率和薄沉积覆盖序列为特征的海洋地壳。从2022年4月至8月，一个国际科学家团队将乘坐科学钻探船JOIDES Resolution两次穿越大西洋。随着岩心的回收，将在船上进行研究和策划，以制作回收材料的永久和公开记录。回到海岸，岩芯将被取样，用于在世界各地的实验室进行的一系列科学研究项目。在曼彻斯特大学，我们打算测量一些岩心样本中的碳、水、卤素和惰性气体的含量。这将使我们能够追踪蚀变流体的来源，记录它们如何随着时间的推移被纳入岩石中，并重新解释早期研究的结果，以获得时间对海洋地壳蚀变的重要性的更完整的图片。最终，这也将使我们能够更好地预测过去海洋地壳中碳等重要挥发物的捕获方式可能会发生变化。

Roughly 60% of the Earth's surface is covered by oceanic crust and submerged beneath the oceans. Oceanic crust is formed from magma at mid-ocean ridges as a result of plate tectonic spreading. As they cool, the igneous rocks of the oceanic crust are altered by chemical reactions with seawater and other fluids. At high temperatures near the spreading ridge these interactions result in the spectacular hydrothermal 'black smoker' vent systems observed by submersibles on the seafloor. Low temperature fluid-rock reactions away from the spreading ridges are likely to be longer-lived and may continue for tens of millions of years. The alteration of oceanic crust results in chemical exchange between the crust and the oceans, altering the composition of both. Volatile chemicals such as carbon, chlorine and water are rare in fresh lavas but become enriched in the crust through alteration. This removes them from the oceans and binds them into the oceanic crust over time. The eventual fate of most oceanic crust is to be subducted beneath other tectonic plates and sink into Earth's mantle, removing volatile elements from the Earth's surface for potentially long timescales.The processes described above have significant implications for the natural cycles of many volatile elements. Carbon is particularly important due to its behaviour in the atmosphere. Trapping it in the oceanic crust and subducting that crust might be an important part of past global climate cycles. Similarly, removal of seawater chlorine in this way may have been responsible for reducing the saltiness of the oceans and helping to make them habitable for life, over Earth's history. In order to understand how alteration of the oceanic crust may have affected seawater and atmospheric chemistry in the past we need to understand what controls volatile geochemical behaviour in this setting. The age of crust, the spreading rate (the rate at which the oceanic plates are moving apart) and the amount of sediment covering the crust are all likely to affect alteration and can be estimated or predicted from plate tectonic reconstructions. However, working out spreading rate or how sediment thickness affects the trapping of volatiles in altered crust is complex. Because it is submerged by seawater, our only direct samples of the oceanic crust come from drilling or dredging the seafloor. The cores drilled to date are mostly from very old or very young crust and are biased to faster spreading rates and thicker sediment cover. To address these shortcomings, a new drilling expedition is being undertaken to drill a transect of 6 holes along a single ~1000 km segment of oceanic crust in the South Atlantic, allowing a unique sample set recording ~60 million years of alteration to be interrogated. The new drilling will also fill in key gaps in the existing collection of cores including oceanic crust characterised by slow spreading rates and thin sedimentary cover sequences.From April to August 2022, an international team of scientists is due to sail twice across the Atlantic Ocean onboard the scientific drilling vessel JOIDES Resolution. As cores are recovered, they will be studied and curated onboard to produce a permanent and publicly available record of the material recovered. Back on shore, the cores will be sampled for a range of scientific research projects to be carried out at laboratories across the world. At the University of Manchester, we intend to measure the amounts of carbon, water, halogens and noble gases in a selection of the core samples. This will allow us to trace the sources of the alteration fluids, document how they are incorporated into the rocks over time and reinterpret the results of earlier studies to obtain a more complete picture of the importance of time to oceanic crustal alteration. Ultimately this will also enable us to make better predictions of how trapping of important volatiles such as carbon in the oceanic crust may have varied in the past.