Melt Generation and Movement beneath Theistareykir, NE Iceland

A detailed study of the volume and composition of all the lavas from the Theistareykir segment of the Northern Volcanic Zone of Iceland was designed to study basaltic melt generation and movement beneath a spreading ridge. The trace element compositions of the lavas are variable, and those of melt inclusions in olivine, clinopyroxene and plagioclase phenocrysts even more so. We show that this variability can be produced by mixing instantaneous melts produced by isentropic decompression of mantle whose initial potential temperature is 1480[degrees]C, and that the calculated volume and composition of the average melt is consistent with geophysical and petrological observations. Pressure and temperature estimates suggest that the phenocrysts form in the upper mantle, at depths of 30-40 km, and trap melts formed at greater 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 area of the Mid-Atlantic Ridge, and from the Gorda and Juan de Fuca Ridges.