Investigation of reaction porosity and permeability, IODP Expedition 399, Atlantis Massif

Atlantis Massif Oceanic Core Complex（OCC）是海洋地壳中研究得最好的地点之一，迄今为止有四次IODP探险（304，305，340 T和357）。它是非常重要的迷城热液场（LCHF）的所在地，在40-90摄氏度下喷出富含氢和甲烷的碱性流体。LCHF被认为是一个生命可能在30亿年前的早期地球上首次进化的环境。类似的碱性喷口在蛇形岩石中被认为出现在太阳系的冰冷世界中，如土卫二。在迷城的流体中发现了氢、甲烷、烷烃和有机酸，在以前的地块钻探岩心中发现了可能是非生物来源的氨基酸。我们新的IODP远征399号将于2023年4月至6月航行，将寻求了解这些“生命的基石”如何通过海水与地下岩石的相互作用非生物地产生，以及生活在高温和高pH值下的“极端微生物”群落可以建立和繁荣的条件。IODP孔U1309 D位于LCHF以北5公里处，是迄今为止在年轻（<2 Ma）洋壳中钻探的最深的孔（1415 m），该孔将在Ep. 399至海底以下2050米深处，温度约为220摄氏度，这有利于对从未低于已知生命温度极限的岩石进行取样，并研究大洋中脊地壳增生的火成岩过程。然而，这不是我们在这里提出的主要目标。相反，我们将从覆盖亚特兰蒂斯地块的100米厚的拆离断层带收集新样本，已知该断层带包含反应渗透区，其中矿物已被高温反应性流体溶解，然后流体填充孔隙空间，导致在低得多的温度下被次生矿物填充，在生命的极限内。探险队的其他科学家将研究可能被困在这种孔隙中的微生物，以及可能帮助它们生长的孔隙流体化学。我们将通过扫描电子显微镜（SEM）、电子探针显微分析（EPMA）和计算机化X射线断层扫描（XCT）等最先进的技术来验证相同的孔隙率。最后一种技术与医学中使用的CAT扫描相同，但针对的是微观尺度。我们将能够量化孔隙度，并计算出它在被次生矿物部分填充之前的大小。反应孔隙度和渗透率被认为对某些类型的矿床的形成很重要，也可能在地热能源项目中形成，例如在冰岛。孔隙的产生和填充在石油工程和储集岩中的碳封存中以及可能在诸如亚特兰蒂斯地块中的反应性火成岩中也是重要的。因此，我们的工作可能会应用于寻找稀有矿物，可再生能源和地下碳捕获地球科学的所有关键目标，以实现净零。更不用说地球生命的起源了！

The Atlantis Massif Oceanic Core Complex (OCC) is one of the best studied locations in the ocean crust, the site of four IODP expeditions so far (304, 305, 340T and 357). It is the site of the very important Lost City Hydrothermal Field (LCHF), venting alkaline fluids rich in hydrogen and methane at 40-90 centigrade. the LCHF has been suggested as an environment where Life may first have evolved on the early Earth > 3 billion year ago. Similar alkaline vents in serpentinising rocks are thought to occur on icy worlds in the Solar System such as Enceladus. Hydrogen, methane, alkanes and organic acids have all been found in the Lost City fluids, and amino acids of probably abiotic origin in previous drill core from the Massif. Our new IODP Expedition 399, sailing in April to June 2023, will seek to understand how these "building blocks of life" can be produced abiotically by interaction of seawater with rocks in the subsurface, and also the conditions under which "extremophile" microbial communities living at high T and high pH can be established and flourish.IODP Hole U1309D, located 5km north of the LCHF, is the deepest (1415m) hole so far drilled in young (<2 Ma) ocean crust; This hole will be deepened in Exp. 399 to depths of 2050 metres below seafloor, and temperatures of around 220 C, inoodre to sample rocks that have never been below the known temperature limit for life, and to study igneous processes of crustal acretion at mid-ocean ridges. However this is not the main target of our proposal here. Instead we will collect new samples from the 100m thick detachment fault zone which caps the Atlantis Massif, which is known to contain zones of reaction permeability, where minerals have been dissolved away by high temperature reactive fluids, and then the fluid filled pore spaces that result filled in by secondary minerals at much lower temperatures, within the limits of life. Other scientists on the Expedition will study microbes that may be trapped in this porosity, and the pore fluid chemistry that may have helped them to grow. We will characterise the same porosity by state of the art techniques of Scanning Electron microscopy (SEM), Electron Probe Microanalysis (EPMA) and computerised X-ray Tomography (XCT). This last technique is the same as the CAT-scan used in medicine, but targeted at a micro scale. We will be able to quantify the porosity and also work out how large it was before it was partially filled by secondary minerals.Reaction porosity and permeability is thought to be important in the formation of some types of ore deposits, and also may be formed in geothermal energy projects, for example in Iceland. the creation and filling in porosity is also important both in petroleum engineering, and in sequestration of carbon in reservoir rocks and perhaps in reactive igneous rocks such as those in the Atlantis Massif. Hence there may be applications of our work both in the search for rare minerals, in renewable energy, and in carbon capture underground all key objectives of geoscience in the aim for net-zero. Not to mention the origin of Life on Earth!