Axial magma reservoirs located by variation in lava chemistry along Iceland's mid-ocean ridge

A linear, axis-parallel, array of glacial and postglacial basalt samples fr om the elevated midocean ridge in southwest Iceland, the Reykjanes Peninsul a, shows three MgO lows and two MgO highs at similar to 40 km intervals. Si milar patterns are observed with other fractionation indices, e,g,, Cr. The se along-axis variations in elements affected by fractional crystallization are interpreted as evidence for segment-scale variation in crustal residen ce times arising from the focusing of magmatic activity at regular interval s along this elevated mid-ocean ridge. In the majority of the samples, Nb/Z r, generally considered to be unaffected by crystal fractionation, does not show a systematic variation with MgO. Lavas with unusually low Nb/Zr, erup ted 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 r ange 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,vith melts that are more enriche d in incompatible elements. Crustal processes at this ridge axis are govern ing the distribution of chemistry associated with the mantle.