Lavas from the northern East Pacific Rise with both robust (9¿30'N) and nonrobust (10¿30'N) ridge morphologies have compositionally diverse populations of plagioclase and/or olivine due to magma mixing. We interpret zoning in olivines to be primarily due to diffusion after mixing events, which allows us to calculate the residence times of individual crystals in the magma. For the four studied samples, olivine populations exhibit exponential distributions of calculated diffusion times, with short times exponentially more abundant than longer times. We model this distribution as resulting from mixing in an open-system axial magma chamber. Magma residence times in axial chambers at East Pacific Rise 9¿30'N and 10¿30'N for the four studied samples are of the order of months. Surprisingly, we find that the differences in mineralogy and magma residence times between robust and nonrobust East Pacific Rise segments are not significant. Taken together, our observations and measurements suggest that the seismically imaged melt lens of the axial magma chamber does not play a significant role in controlling the crystal content and characters of erupted lavas.