Application of immobile element (Al-Ti) methods to the Galápagos mounds in the eastern Pacific Ocean allows determination of the amount of each mobile element added to or removed from precursor calcareous ooze during formation of nontronitic clay, the main hydrothermal phase in the deposits. As calcareous ooze dissolves, Si-Fe-Mg-K mass is added from low-temperature hydrothermal fluids. The calculated net mass change for granular nontronite zones in mound interiors is positive and very large relative to precursor ooze, whereas smaller mass loss occurs in intervening transitional sediment zones. The resulting volume changes produce positive sea-floor relief in the central parts of mounds and negative relief in peripheral parts, forming moats. Mass changes in mobile elements also account for large apparent variations in the concentrations of rare earth elements, which were immobile. In ancient sedimentary terranes, immobile element methods can be used to assess exploration potential by monitoring mass changes and hydrothermal alteration around mineralizing systems.