QUANTIFICATION OF MELT DISTRIBUTION, MELT CONNECTIVITY, AND ANISOTROPIC PERMEABILITY OF DEFORMED PARTIALLY MOLTEN ROCKS USING X-RAY MICROTOMOGRAPHY

Abstract

Volcanic activity plays a dominant role in shaping the surface of Earth and other planets. For example, Earth’s ocean floor is created by volcanic activity at mid-ocean ridges. There, magma is sourced from a ~60 km deep, ~100 km wide region of the mantle, from which partial rock ascends and erupts along narrow ridges that run along the middle of Earth’s oceans. Volcanic activity at mid-ocean ridges, the strength of Earth’s mantle, and the geochemical composition of volcanic rocks and ocean water are all influenced by how the melt is distributed in partially molten rocks, and how easily it can flow through the partially molten mantle beneath these ridges. In particular, it is often assumed that most of the melt ascends through isolated channels that direct it towards the mid-ocean ridges, making melt transport localized and anisotropic. A possible origin of these channels is the differential stress induced by upwelling mantle material beneath the ridge, which has been shown in laboratory experiments to localize melt into planar regions named “melt-rich bands.” To date, the development and characteristics of melt-rich bands have been studied principally using theoretical models and two-dimensional (2D) images of sheared partially molten rocks. There has been little experimental research using 3D techniques until now. This thesis uses 3D images of sheared partially molten rocks created in the laboratory, obtained using high-resolution x-ray microtomography (X-ray µCT), to investigate how the distribution of melt, its orientation, its connectivity, and its ability to flow through the rocks changes when stress is applied. This study shows how melt connectivity and, therefore, rock permeability changes as melt changes from being dispersed through a partially molten rock to being localized on well-developed melt-rich bands. This work shows that melt forms melt volumes that are preferentially elongated within the plane of melt-rich bands even before these bands form. This discovery emphasizes the importance of permeability anisotropy at all stages of melt-rich band development. We also measured permeability and melt connectivity at all scales, both inside and outside melt-rich bands. Our results show that melt can hardly flow perpendicular to melt-rich bands over distances larger than a few grains. Additionally, the permeability along the melt-rich bands is also reduced by half compared to that in a partially molten rock that is not subjected to differential stress. This research quantifies the uneven distribution of permeability in a sheared, partially molten rock. It also proposes a scheme to average local permeability estimates, helping us understand and quantify how melt travels along melt-rich bands at various scales. These findings provide valuable insights into how magma flows in the Earth’s mantle, especially at active plate boundaries like mid-ocean ridges. Overall, this research provides the first experimental constraints, based on 3D microtomography images, on the melt network and how melt flows in the presence of stress in partially molten rocks.