Basalt from the Tertiary lava pile in Iceland is much more homogeneous in composition than that erupted in the axial rift zone. Lavas highly depleted in incompatible elements are occasionally erupted in the latter but have not been reported from the Tertiary succession. This observation leads to a useful test of the Pálmason model of crustal accretion in which axial lava flows ultimately form the lower levels of the lava pile, and only flows emplaced off-axis remain accessible in the upper part of the crust. Axial flows large enough to spill out of the rift axis, and flows erupted on the rift flanks, should therefore have a uniform composition similar to that of the Tertiary flows. Here we test this prediction and thereby the validity of the Pálmason model of crustal accretion in oceanic spreading centers. Our data show that depleted basalt is confined to small-volume lava flows within the rift axis. Furthermore, large flows that originated within the axial zone and spread far enough to become preserved in the accessible part of the future off-axis lava pile are very homogeneous in composition and comparable to those forming the Tertiary lava successions.