Porosity-dependent physical property changes of the oceanic crust at the South Atlantic Transect (IODP X390-393)

The physical properties of oceanic crust evolve significantly with age as the lithosphere cools, densifies, and subsides. At the crustal scale, the oceanic crust undergoes a progressive reduction in porosity and permeability (due to pore space and fracture infill), leading to an overall increase in seismic velocity. In particular, alteration of basaltic crust by low-temperature hydrothermal fluids produces the largest modification to the upper oceanic crust. This means that understanding the impact of porosity changes is critical for quantifying crustal physical property evolution through time.

Here, we present a new dataset of physical property measurements from the upper oceanic crust recovered during the South Atlantic Transect (IODP Expeditions X390–393), spanning basalt ages of approximately 6 to 61 Ma. The dataset includes P-wave velocity (Vp), pycnometry measurements, and X-ray micro-CT image analyses. The new dataset, integrated with existing shipboard data, provides a comprehensive view of low-temperature alteration processes.

Micro-CT analyses reveal that basalt samples exhibit a highly heterogeneous porosity structure. Primary porosity is dominated by vesicles that are variably filled with secondary minerals; many vesicles remain partially unfilled or display clay coatings, indicating incomplete calcite precipitation. Secondary porosity occurs as micro-porosity (< 10 micron) associated with volcanic glass, olivine and plagioclase alteration, as well as fracture networks. Two generations of cross-cutting fractures are identified, filled by clay and calcite, respectively, reflecting multiple stages of fluid circulation and mineral precipitation.

Variations in porosity are closely linked to volcanic emplacement style and microstructural characteristics, including groundmass grain size, phenocryst abundance, vesicle distribution, and are positively proportional to the degree of alteration.

Our findings provide new constraints on the mechanisms governing physical property evolution in ageing oceanic crust and have important implications for upscaling models of CO₂ sequestration in basaltic formations, where porosity, permeability, and fracture connectivity are critical parameters.