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 800-1300CC 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 hai 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 tempera. ture lowers at higher pressures. Therefore, the composition of the rocks in the lithosphere and the types of hydrous minerals and their stable PT 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.

Study on the Electrical Conductivity of 0.025 mol NaCl Solution at 0.25–3.75 GPa and 20–370°C

“…These phenomena probably imply that the properties of waterm are obviously different from those of waterN and the difference is probably an important cause for the fact that the melting point at higher pressures (above 2.58 GPa) is lower than that at lower pressures (under 2.11 GPa). This explanation is consistent with the results of melting experiment of rocks containing about 2% water at high temperature and high pressure (Zheng et al, 1995).…”

The Role of Fluids in Melting the Continental Crust and Generating Granitoids: An Overview

“…For example, the dehydration temperature of pure muscovite is usually around 700 • C at a pressure of 0.5 GPa [75], which is lower than the dry solidus temperatures of pelitic rocks (Figure 1). The water produced by the dehydration of hydrous minerals will greatly reduce the solidus temperatures of rocks and then promote the decomposition and melting of other minerals such as feldspar and quartz, resulting in the generation of more magma [76].…”