DIFFUSING BARIUM OXIDE IN A PARTIALLY MOLTEN ROCK TO TRACE MELT MIGRATION WITHIN SHEAR ZONES

In ophiolite rocks abducted onto continental crust, chemical disequilibrium is evident in the tabular dunites, which are undersaturated in pyroxene. This observation suggests that when the melt; which was produced deep within Earth’s mantle; segregated and penetrated the overlying country rock, the melt had to flow through channels fairly quickly so that there was no time for the melt to chemically equilibrate with the rocks through which it traveled. Our investigation of melt migration commences with the creation and testing a homogenized sample of olivine, 10% chromite, and 4% mid-ocean ridge basalt (MORB). Before the rock was mechanically deformed under torsion, a notch was created on a side of the cylindrical sample and filled in with barium oxide. The barium oxide would be used to trace the flow of the melt in the sample. With a Paterson high-temperature, high-pressure testing apparatus, the sample experienced a strain of 2 (γ=2) at a confining pressure of 300MPa and a temperature of 1200°C. Previous studies recognized that melt-rich bands (>20%) will form, leaving regions depleted of melt (<1%); and these bands will align themselves ~20° to the shear plane. Using the techniques of optical microscopic imaging and the electron microprobe analysis, the movement of the melt and the barium oxide tracer were mapped. Observations made with the microscope, demonstrated that the barium did not venture too far from its initial spot, where the notch was created. Based on results from the electron microprobe, we quantitatively proved that diffusion was higher in the melt-rich band discontinuities than in the depleted melt-rich zones. One reason why the melt may have traveled quicker in the melt-rich bands is because in the bands there is an interconnected network of grains that increased the bands porosity. In this case, the distance the melt had to travel in between the grains was significantly lower than it was for the depleted melt-rich regions. These results have important implications in explaining a method for how melt migrates out of the interior of a terrestrial planet to its surface. The data would especially be interesting for understanding planets like Venus, which has no plate tectonics, as an explanation for the rapid transfer of its melt from depth to surface.