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，并被淹没在海洋之下。洋壳是由洋中脊的岩浆形成的，是板块构造扩张的结果。当它们冷却时，海洋地壳的火成岩与海水和其他流体发生化学反应而发生变化。在伸展脊附近的高温下，这些相互作用导致了海底潜水器观察到的壮观的热液“黑烟”排气系统。远离伸展脊的低温流体-岩石反应可能会持续更长时间，并可能持续数千万年。海洋地壳的变化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，在新鲜的熔岩中是罕见的，但通过蚀变在地壳中变得丰富。这将它们从海洋中移除，并随着时间的推移将它们绑定到海洋地壳中。大多数海洋地壳的最终命运是被俯冲到其他构造板块之下，沉入地幔，在很长一段时间内将地球表面的挥发性元素带走。上述过程对许多挥发性元素的自然循环具有重要意义。由于碳在大气中的表现，它尤为重要。将其困在海洋地壳中并使其俯冲可能是过去全球气候周期的重要组成部分。同样，在地球的历史上，以这种方式去除海水中的氯可能是降低海洋盐度的原因，有助于使海洋适合生命生存。为了了解过去海洋地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（大洋板块分开的速度）和覆盖地壳的沉积物数量都可能影响蚀变，并且可以通过板块构造重建来估计或预测。然而，计算出扩散速率或沉积物厚度如何影响蚀变地壳中挥发物的捕获是复杂的。由于它被海水淹没，我们唯一的海洋地壳直接样本来自钻探或疏浚海底。迄今为止钻探的岩心大多来自非常古老或非常年轻的地壳，并且倾向于更快的扩展速率和更厚的沉积物覆盖。为了解决这些缺点，一项新的钻探探险正在进行，沿着南大西洋一个约1000公里的海洋地壳段钻6个孔的样带，从而可以对一组记录约6000万年变化的独特样本进行询问。新的钻探还将填补现有岩心收集中的关键空白，包括以扩张速度缓慢和薄沉积覆盖序列为特征的海洋地壳。从2022年4月到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，它们将在船上进行研究和整理，以产生一个永久的、公开的回收材料记录。回到岸上，这些岩心将被取样，用于一系列科学研究项目，这些项目将在世界各地的实验室进行。在曼彻斯特大学，我们打算在选定的岩心样本中测量碳、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，记录它们是如何随时间融入岩石的，并重新解释早期研究的结果，从而更全面地了解时间对海洋地壳蚀变的重要性。最终，这也将使我们能够更好地预测海洋地壳中碳等重要挥发物的捕获在过去可能是如何变化的。

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.