Exploring Multi-Phase Boundary Layer Phenomena during Magma Mixing

Abstract:

Marsh and his colleagues have been pioneers in linking of fluid mechanical templates with the chemistry and petrologic constraints in magmatic systems. Such interplay becomes particularly important during the assembly and growth of magma bodies of intermediate composition where thermal and chemical gradients are frequently reset due to recharge and mixing. The resulting boundary layers and their dynamics remain poorly understood given the complex interplay of interface kinetics, phase equilibria and heat and mass transfer in those environments. The multitude of parameters makes this problem commonly intractable. Therefore, we have devised diffusion couple experiments to investigate specifically the kinetic petrologic phenomena associated with magma recharge and mafic enclave formation.

Magma-magma diffusion couples were run by using natural andesite and dacite to synthesize, at 150 MPa and fO2 ~FMQ+3, two initial glass compositions: 1) andesite with ~1 wt. % H2O, ~700 ppm S, ~500 ppm Cl at 1030°C; and 2) dacite with ~3 wt. % H2O, ~100 ppm S, ~1500 ppm Cl at 900°C. The diffusion-couples were run at 950°C and 1000°C, 150 MPa, and FMQ+3, for 0.1 to 100 h. At 950°C and 1000°C the andesite crystallized significantly at the interface between the two magmas, and became volatile saturated within the solidification front. EPMA traverses across the interface and x-ray maps indicate the mass transfer by diffusion in the melt, however, delayed as a consequence of transient crystallization and subsequent dissolution as the magmas work their way towards equilibrium. Mineral dissolution fronts can be tracked for clinopyroxenes, olivines and plagioclase with decreasing extent into the andesite. These results provide a temporal and spatial framework for one important aspect of magma mixing, the formation mafic enclave as well as their slow destruction.