Granite melt convecting in an experimental micro-magma chamber at 1050 degrees C, 15 kbar

Contamination of hydrous granite melt by reaction with ultramafic rocks, as might occur near the crust-mantle boundary, was investigated. The experiments showed that the rate of advance of the melt front decreases from >10 (super -4) cm/s in the first minutes to <10 (super -5) cm/s after 1 hour, suggesting that H (sub 2) O diffusion to the melting front controls the melting rate. Experiments of longer duration show evidence of convective flow of the granitic melt, and the flow features defined by graphite distributed in glass develop continuously with time. Glass analyses reveal initial steep compositional gradients adjacent to the peridotite but with increasing run duration the gradients become less steep and compositional changes are spread throughout the main body of the melt. After 11 hours the granite is completely melted, the graphite has disappeared and the granite is compositionally similar to the starting material. A small horizontal T gradient and/or small variations in melt water content seem likely explanations for the driving forces of the observed flow.