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

大约60%的地球表面被洋壳覆盖，并淹没在海洋之下。洋壳由大洋中脊的岩浆形成，是板块构造扩张的结果。当它们冷却时，洋壳的火成岩通过与海水和其他流体的化学反应而改变。在扩张海脊附近的高温下，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”喷口系统。远离扩张海脊的低温流体-岩石反应可能寿命更长，可能会持续数千万年。洋壳的变化导致了洋壳和海洋之间的化学交换，改变了两者的组成。碳、氯和水等挥发性化学物质在新鲜熔岩中很少见，但通过蚀变在地壳中变得丰富。随着时间的推移，这会将它们从海洋中移走，并将它们绑定到洋壳中。大多数洋壳的最终命运是被其他构造板块俯冲到其他板块之下，并沉入地幔，在潜在的很长一段时间内从地球表面移除挥发性元素。上述过程对许多挥发性元素的自然循环具有重要影响。由于碳在大气中的行为，它尤其重要。将其困在洋壳中，并将其俯冲到洋壳，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含盐性并帮助使它们适合生命生存的原因。为了了解过去洋壳的变化可能如何影响海水和大气化学，我们需要了解是什么控制了这种环境中的挥发性地球化学行为。地壳的年龄、扩张速率(大洋板块分开的速率)和覆盖地壳的沉积量都可能影响到蚀变，可以通过板块构造重建来估计或预测。然而，计算扩散速率或沉积物厚度如何影响蚀变地壳中挥发分的捕获是复杂的。由于洋壳被海水淹没，我们对洋壳的唯一直接样本来自于海底的钻探或疏浚。到目前为止，钻探的岩心大多来自非常古老或非常年轻的地壳，偏向于更快的扩张速度和更厚的沉积物覆盖。为了解决这些缺点，正在进行一次新的钻探探险，沿着南大西洋一段约1000公里的洋壳钻6个孔，以便对记录约6000万年变化的独特样本集进行讯问。新的钻探还将填补现有岩心的关键空白，包括洋壳，其特点是扩散速度慢，沉积盖层薄。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 Resolve两次横渡大西洋。在回收岩芯时，将在船上对它们进行研究和整理，以产生回收材料的永久和公开可用记录。回到陆地上，这些岩芯将被取样，用于在世界各地的实验室进行一系列科学研究项目。在曼彻斯特大学，我们打算测量选定的岩心样品中碳、水、卤素和惰性气体的数量。这将使我们能够追踪蚀变流体的来源，记录它们是如何随着时间的推移并入岩石中的，并重新解释早期研究的结果，以更全面地了解时间对海洋地壳变化的重要性。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发生了怎样的变化。

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.