A linear, axis-parallel, array of glacial and postglacial basalt samples from the elevated mid-ocean ridge in southwest Iceland, the Reykjanes Peninsula, shows three MgO lows and two MgO highs at ∼40 km intervals. Similar patterns are observed with other fractionation indices, e.g., Cr. These along-axis variations in elements affected by fractional crystallization are interpreted as evidence for segment-scale variation in crustal residence times arising from the focusing of magmatic activity at regular intervals along this elevated mid-ocean ridge. In the majority of the samples, Nb/Zr, generally considered to be unaffected by crystal fractionation, does not show a systematic variation with MgO. Lavas with unusually low Nb/Zr, erupted at the end of the last glaciation, are the only exception. These low-Nb/Zr lavas are generally restricted to the MgO highs, resulting in a wider range of lava Nb/Zr in these areas than in the MgO lows. It is proposed that low-Nb/Zr melts are available along the entire ridge section at all times, but are modified before eruption by mixing with melts that are more enriched in incompatible elements. Crustal processes at this ridge axis are governing the distribution of chemistry associated with the mantle.