Submersible-based geologic observations and geochemical, magnetic paleointensity, and (210Pb/226Ra) radioactive disequilibria data indicate that at least five distinct lava sequences (three normal mid-ocean ridge basalt (N-MORB) and two transitional mid-ocean ridge basalt (T-MORB)) have been erupted within the last several hundred years along a 27-km-long portion of the fast spreading East Pacific Rise near 17°30′S. Isotopic and geochemical variations, both within and between eruptive units, indicate mixing of different primary magmas concurrently with differentiation in shallow-level subaxial magma reservoirs. Differentiation trends are linked to geographical variations in axial magma chamber (AMC) characteristics, with the lowest MgO samples erupted above the shallowest portion of the AMC, suggesting that pre-eruptive magma temperature is in part controlled by the depth-dependent efficacy of hydrothermal cooling. A third-order axial discontinuity at ∼17°29′S coincides with a narrowing of the subaxial melt lens and an increase in lava MgO to the south; we interpret the latter to reflect a sharp increase in the mixing proportion of recharge to low-MgO magma residing in the melt lens. Magmatic evolution of this area over the last few hundred years reflects continually evolving conditions in the subsurface and mantle melting processes that vary rapidly at rates that are at least as great as the eruption rate.