The Controls of Porosity on Mineral Alteration Processes in the Shallow Oceanic Lithosphere at Slow-Spreading Ridges

Abstract

Porosity exerts strong controls on mineral replacement and alteration processes mediated by the action of fluids. These processes are ubiquitous in the Earth’s crust, but even more so within the oceanic lithosphere, an environment saturated by infiltrating fluids that can range in composition from hot hydrothermal fluids to low-temperature seawater. This thesis presents examples of fluid-rock interactions that affect mafic and ultramafic lithologies recovered via drilling from the subseafloor of the slow-spreading Mid-Atlantic Ridge. These reactions are enabled by the presence of pores that provide flow pathways for fluids to reach the reaction front of altering minerals. Chapter 2 explores the genesis of petrographic textures observed in samples recovered from the Trans-Atlantic Geotraverse, a hydrothermal sulfide mound that shows black smoker activity. The textures form due to the replacement of anhydrite by pyrite under the action of hot hydrothermal fluids generated via heating of seawater entrained within the oceanic crust, and involved growth of quartz into open space. Intensive investigation of the observed textures detailed in this Chapter provides a mechanistic explanation for the attainment of the sulfur isotopic signatures found in sulfide precipitate within the hydrothermal mound. In Chapter 3, tomographic analysis of serpentinized peridotite at nanometer and micrometer scales reveals the presence of dissolution pores located at the grain boundaries of olivine, which show significant connectivity, suggesting that soluble Mg and Si sourced from the dissolution of olivine are exported to seawater, affecting its chemical budget, alkalinity, and consequently the coupled carbonate-silicate cycle. Chapter 4 tests the hypothesis that the porosity and Fe3+ contents of serpentinized peridotite increase with increasing serpentinization degree, suggested by correlations between density and magnetic susceptibility of serpentinites established in earlier work. The hypothesis was tested by applying statistical analysis to geochemical and porosity data collected on serpentinite samples recovered from 10 different drilling sites, revealing complex alteration patterns that involve seafloor weathering processes and thus extend beyond the transformation imparted by serpentinization reactions. Together, the scientific investigations conducted in this thesis demonstrate the importance and complexities of porosity generation and evolution during alteration of the shallow oceanic lithosphere.