<p>Suites of mineral-hosted melt inclusions commonly display variability in their H₂O and CO₂ concentrations that are challenging to explain by a single process. Recent work has shown that H₂O can diffuse rapidly through olivine, allowing olivine-hosted melt inclusions to diffusively equilibrate with an external melt that has a different H₂O concentration. This process can cause the concentrations of H₂O, and in many cases CO₂, in melt inclusions to deviate from isobars and degassing paths. Diffusion of H⁺ through the olivine lattice can fractionate deuterium (²H or D) from hydrogen, and here we use a discretized diffusive equilibration model to investigate the D⁺/H⁺ fractionation associated with passive degassing, CO₂ flushing, and magma ascent – three processes that affect the concentration of H₂O in the external melt and thereby induce diffusive H₂O exchange – on H₂O and D⁺/H⁺ in olivine-hosted melt inclusions. Our model results show that these three processes lead to distinctly different trends in the H₂O concentration and D⁺/H⁺ signature of a melt inclusion. However, the timescale of a given process will impact whether the hallmarks of that process are preserved in natural inclusions. While our models focus on olivine-hosted melt inclusions and basanitic melt compositions, the conceptual framework and effects of these processes should apply to any melt inclusion-host crystal system that can be affected by diffusive exchange of hydrogen. Capturing these signatures in datasets is possible, particularly in the case of magma ascent, but it requires high-precision H₂O and D⁺/H⁺ analyses in a melt inclusion suite that has a wide range of inclusion sizes. </p>