Melt generation and movement beneath Theistareykir, NE Iceland

A detailed study of the volume and composition of all the lavas from the Th eistareykir segment of the Northern Volcanic Zone of Iceland was designed t o study basaltic melt generation and movement beneath a spreading ridge. Th e trace element compositions of the lavas are variable, and those of melt i nclusions in olivine, clinopyroxene and plagioclase phenocrysts even more s o. We show, that this variability can be produced bg mixing instantaneous m elts produced by isentropic decompression of mantle whose initial potential temperature is 1480 degreesC, and that the calculated volume and compositi on of the average melt is consistent with geophysical and petrological obse rvations. Pressure and temperature estimates suggest that the phenocrysts f orm in the upper mantle, at depths of 30-40 km, and trap melts formed at gr eater depths. Some mixing of the instantaneous melts occurs before the melt is trapped and more mixing occurs before the lavas are erupted. A similar model can account for the composition of melt Inclusions from the FAMOUS ar ea of the Mid-Atlantic Ridge, and from the Gorda and Juan de Fuca Ridges.