Mid-ocean ridges are our best window into the upper mantle because of their relatively well-understood dynamics and their broad and continuous sampling through extraction of partial melts. They therefore provide the context to address a persistent challenge: understanding the mechanism of sampling, as well as the nature, origin and consequences of chemical heterogeneity in the mantle. These processes have consequences for the structure of the oceanic lithosphere, from mantle-dominated to layered, and for hydrothermal systems and processes including serpentinization and carbonation. Both magmatic and hydrothermal processes contribute to the mass budgets of key chemical compounds (e.g. C, S, Fe, Cl). Questions that are especially relevant to this session include: what is the "initial condition" of the MOR mantle source? How do heterogeneities melt? How is magma extracted and mixed? How is the chemical signature of the mantle source transposed to the erupted basalts? To what extent do these magmatic processes influence ridge hydrothermalism? This interdisciplinary session welcomes all contributions to answering these and related questions.