Experimental Constraints on the Mechanics of Dyke Emplacement in Partially Molten Olivines

Dykes emplacement is a key mechanism of magma transport in the lithosphere. However, a complete dynamic model is not yet available, because the details of coupling between the viscous stresses, the country rock deformation, and the role of fractures are not fully understood. We performed experiments to determine melt concentration and strain distributions around basalt dykes in a San Carlos olivine matrix with 10% wt% MORB matrix, by using a high-pressure, high-temperature Paterson apparatus. Undrained triaxial compression experiments have been conducted after hot-pressing San Carlos olivine with 10% MORB, fully encapsulated by nickel shells. Creep and constant displacement rate experiments were performed at 1473 K and confining pressure of 300MPa, at constant stresses (80-160MPa) and constant strain rates ranging from 3x10-4 to 5x10-5s-1. Microstructural observation and chemical analyses of the melt distribution show an increase of MORB matrix from 10% in proximity of the dyke (1-2mm) to 4-5% at 3-4mm away. Melt migration appears to be controlled by the loading conditions and by the dyke geometry. As expected, the highest melt concentrations, and presumably, the highest stress concentrations, are found at the tip of the dyke. The deformation of matrix appears to be controlled by granular flow, but dilatancy occurs near the tip of the dyke, indicating coupled MORB transport and granular flow.