Experimental Study of the Influence of Hydrous Minerals on the Melting Behaviour of Rocks at High Temperatures and Pressures

Abstract The experimental study on the melting of potassic basalt and eclogite with about 2% water at 800—1300°C and 1.0—3.5 GPa shows that the solidi of both rocks are significantly lower than those obtained from the previous experiments of the same type of rocks under dry conditions, and the former which is enriched in potassium has a lower melting point than the latter. It is consistent with the previous study. The melting temperature of eclogite increases with pressure, whereas potassic basalt has similar properties only at 1.5—2.5 GPa and >3.0 GPa, and at 2.5— 3.0 GPa the melting temperature decreases with pressure. This can be explained as follows: (1) eclogite only has one hydrous mineral amphibole and the dehydous temperature is lower than the wet solidus of the rock. (2) Amphibole exists in potassic basalt at the pressures lower than 2.5 GPa and phlogopite exists at pressures higher than 2.5 GPa, and the special compositions of both minerals determine that amphibole has a dehydration temperature higher than or close to that of the wet solidus of the rocks, while phlogopite has a dehydration temperature lower than that of the wet solidus. On the other hand the features of the continuous solidus in the experiment of hydrous eclogite were produced by the fact that the dehydration temperature of its amphibole lower than or close to the melting temperature of the hydrous conditions. So the melting temperature lowers at higher pressures. Therefore, the composition of the rocks in the lithosphère and the types of hydrous minerals and their stable P‐T conditions are the important factors controlling the solidi of rocks. It can quite well explain the partial melting of rocks and the origin of the low velocity zone in the deep lithosphere.