Flow mechanism and viscosity in basaltic magma chambers

Magmatic flow in the dense suspension of crystallizing gabbros below the free surface of basaltic magma chambers is considered from the point of view of flow mechanisms and rheology. Hyperdense suspensions (∼20% melt fraction) may arise if flat plagioclase crystals develop a strong preferred orientation induced by magmatic flow. With the help of Nomarski differential interference contrast and back scattered electron figures, we show that suspension flow is possible even for smaller melt fractions if impingements between moving crystals are reduced by chemical dissolution at their contact points. This dissolution process is rate controlling. With strain rates near 10−9 s−1 and viscosities near 1014–16 Pa.s, such crystalline mushes should be closer to plastically deforming solids than to the overlying basaltic suspension. If we characterize magma chambers by suspension flow, no matter how small the melt fraction, magma chambers below oceanic fast spreading centers should not be restricted to a perched melt lens, but should extend to the Moho and comprise the entire volume of observed strong seismic attenuation.