Concurrent Mixing and Cooling of Melts under Iceland

Abstract

The compositions of 75 melt inclusions, their host olivines and 49 whole-rock samples of their carrier lavas have been determined. These compositions were added to a compilation of the trace element composition of 243 melt inclusions from 10 eruptions in the neovolcanic zones of Iceland and used to investigate melt mixing processes. Whereas the compositional variability of whole-rock samples from single eruptions is limited, there are significant compositional differences between eruptions. The compositions of inclusions are more variable than those of whole-rock samples of their carrier lava. On a flow-by-flow basis, the average composition of trace element ratios such as La/Yb in the inclusions is similar to that of their carrier lava. These observations indicate that, for each lava flow, melts with compositions similar to those of the inclusions crystallized and mixed to produce the magma that transported the host olivines to the surface. Although many of the olivines are not in Mg–Fe equilibrium with their host melt, they are not accidental xenocrysts because they crystallized from melts similar to those that mixed to form the carrier magma. The trace element variability of melt inclusions drops with decreasing forsterite content of the host olivine. This relationship is observed both within single flows and in the compilation of data from 10 flows. Concurrent mixing and crystallization dominate the compositional evolution of basaltic melts in lower crustal magma chambers. This coupled mixing and cooling is likely to result from convective motions in magma chambers. The rate of change of the mixing parameter, M, with temperature of the melt is dM/dT = 0·0094 ± 0·0036 per °C. This relative rate may be used to constrain the fluid dynamics of basaltic magma chambers.