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

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!