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

六千万年的海底蚀变：沿 1000 公里洋壳横断面对挥发物合并的时空控制

• 批准号: NE/X002012/1
• 负责人: Brian O'Driscoll
• 金额: $ 6.01万
• 依托单位: University of Manchester
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2022
• 资助国家: 英国
• 起止时间: 2022 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FX002012%2F1
• 关键词: Sixty; million; years; seafloor; alteration

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.

大约60％的地球表面被海洋壳覆盖，并在海洋下淹没。由于板块构造扩散，海洋外壳是由岩浆在中山脊的岩浆形成的。在冷却时，海洋地壳的火成岩会因海水和其他液体的化学反应而改变。在扩散山脊附近的高温下，这些相互作用导致海底下的潜水物观察到了壮观的水热“黑人吸烟者”排气系统。低温流体岩石反应远离扩散山脊的寿命可能更长，并且可能持续数千万年。海洋壳的改变导致地壳与海洋之间的化学交换，从而改变了两者的组成。诸如碳，氯和水等挥发性化学物质在新鲜的熔岩中很少见，但通过改变而在外壳中富集。随着时间的流逝，这将它们从海洋中移出，并将它们粘合到海洋地壳中。大多数海洋地壳的最终命运是在其他构造板下面俯冲，然后沉入地球地幔中，消除了从地球表面的挥发性元素，以实现潜在的长时间尺度。碳在大气中的行为尤其重要。将其捕获在海洋地壳中，并俯冲，认为外壳可能是过去全球气候周期的重要组成部分。同样，以这种方式去除海水氯可能是为了降低海洋的咸味，并帮助使它们在地球历史上可居住。为了了解过去，海洋壳的改变可能会影响过去的海水和大气化学反应，我们需要了解在这种情况下控制挥发性地球化学行为的是什么。地壳的年龄，扩散率（海洋板移动的速率）以及覆盖地壳的沉积物量都可能影响改变，并且可以通过板块构造重建估计或预测。但是，锻炼扩散速率或沉积物厚度如何影响挥发物在改变地壳中的捕获是复杂的。因为它被海水淹没，所以我们唯一的海洋外壳样品来自钻孔或挖出海底。迄今为止，钻探的核心主要来自非常古老或非常年轻的外壳，并且偏向于更快的扩散速率和更厚的沉积物盖。为了解决这些缺点，正在进行一项新的钻探探险，以在南大西洋的一个约1000公里的海洋地壳沿单个约1000公里的海洋壳钻出6个孔，从而允许询问一个独特的样本集，记录了约6000万年的更改。新的钻井还将填补现有核心集合的关键空白，包括以缓慢的扩散率和较薄的沉积覆盖序列为特征的海洋外壳。从4月到2022年8月，一支国际科学家团队将在大西洋上两次在大西洋上航行到科学钻探船上的大西洋。随着核心的回收，他们将在船上进行研究和策划，以产生永久性且公开可用的记录。回到岸上，将对核心进行采样，以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，我们打算在选择核心样品中测量碳，水，卤素和贵重气体的量。这将使我们能够追踪变化流体的来源，记录如何随着时间的推移将它们掺入岩石中，并重新诠释早期研究的结果，以获取时间对海洋壳蚀变的重要性的更完整的了解。最终，这也将使我们能够更好地预测重要的挥发物（例如碳中的碳中的碳中的诱捕）过去可能有所不同。

项目成果

Bimodal Alteration of the Oceanic Crust Revealed by Halogen and Noble Gas Systematics in the Oman Ophiolite

阿曼蛇绿岩中卤素和稀有气体系统学揭示的洋壳双峰蚀变

• DOI: 10.1029/2021jb022669
• 发表时间: 2022
• 期刊: Solid Earth
• 影响因子: 3.4
• 作者: Carter E