MECHANISMS OF CRUSTAL ACCRETION IN ICELAND

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 lat ter but have not been reported from the Tertiary succession. This obse rvation leads to a useful test of the Palmason model of crustal accret ion in which axial lava flows ultimately form the lower levels of the lava pile, and only flows emplaced off-axis remain accessible in the u pper part of the crust. Axial flows large enough to spill out Of the r ift axis, and flows erupted on the rift flanks, should therefore have a uniform composition similar to that of the Tertiary flows. Here we t est this prediction and thereby the validity of the Palmason model of crustal accretion in oceanic spreading centers, Our data show that dep leted basalt is confined to small-volume lava flows within the rift ax is, Furthermore, large flows that originated within the axial zone and spread far enough to become preserved in the accessible part of the f uture off-axis lava pile are very homogeneous in composition and compa rable to those forming the Tertiary lava successions.