Calcite vein microstructures along the South Atlantic Transect: Implications for intraplate stress variations

The South Atlantic Transect (SAT) is a multidisciplinary scientific ocean drilling experiment designed to investigate the evolution of the ocean crust and overlying sediments across the western flank of the Mid-Atlantic Ridge (Coggon et al., 2024). The SAT comprises International Ocean Discovery Program (IODP) Expeditions 390 and 393, built on engineering preparations during Expeditions 390C and 395E. It targeted six sites on 7, 15, 31, 49, and 61 Ma ocean crust to sample intact in situ crust regarding crustal age, spreading rate, and sediment thickness and to investigate the hydrothermal interactions within the aging ocean crust.

An integrated petrological, geochemical and microstructural study unravels the conditions of host rock alteration and the formation conditions of mineralization within hydrothermally formed veins and voids.

This contribution focuses on the internal microstructure of hydrothermal veins in drill cores sampled during IODP Expeditions 390 and 393. Microstructures, preferably in calcite, were analyzed using Electron Backscatter Diffraction (EBSD). Here, data on the density and misorientation of calcite sub-grains potentially allow the assessment of intraplate stresses and stress variations with depth and distance from the Mid-Atlantic Ridge, related to the increasing density of crust with cooling and age.

Microstructures in vein calcite are characterized by the formation of sub-grains, indicating that calcite deformation is mainly characterized by dislocation glide. Mechanical twinning is very subordinate and does not substantially contribute to internal deformation. The evaluated misorientation axes between the calcite sub-grains indicate that basal and prism planes are the main intracrystalline gliding planes. The activation of these slip planes requires relatively high differential stresses, which are far above the critical stresses for twinning. Analysis of average calcite sub-grain sizes shows a general trend characterized by a continuous decrease in sub-grain size with decreasing distance from the mid-ocean ridge.

Oxygen stable isotope data from vein calcite indicate low precipitation temperatures in the range of 2° to 10 °C, without a correlation between precipitation temperature and the age of the oceanic host rock, and with a very minor influence of magmatic fluids. Therefore, we assume that vein calcite precipitated from seawater.

The microstructural and stable isotope data imply that several calcite veins formed in situ at the drilled sites. The microstructures, particularly the calcite sub-grain sizes, seem to indicate that the related differential stresses decrease with increasing distance from the Mid-Atlantic Ridge. This can be related to the higher cooling rates of the oceanic host rocks situated closer to the Mid-Atlantic Ridge; higher cooling rates presumably generate higher internal stresses due to higher rates of density increase and volume loss with cooling. Alternatively, it may also be related to the fact that the ridge-push forces, and therefore the related intraplate stresses, decrease with increasing distance to the Mid-Atlantic Ridge.

References:

Coggon, R.M. et al., 2024. South Atlantic Transect. Proceedings of the International Ocean Discovery Program, 390/393: College Station, TX (International Ocean Discovery Program). https://doi.org/10.14379/iodp.proc.390393.101.2024