Prof. Bjorn Mysen [Carnegie Institution of Washington, USA]

Title: Mass Transport by Aqueous Fluids at High Temperature and Pressure - Role of Fluids in MaterialsTransport in the Earth’s Interior

Language:  English presentation

Time: 9:30 - 11:00 AM, May 24, 2024

Place:  Onsite：Conference room A417, HPSTAR (Beijing)       Meeting ID: 713-398-526

Host:  Prof. Yanhao Lin & Prof. Ho-kwang Mao

Abstract:  文摘:

H2O, often together with halogens, in particular chlorides, dominate the fluid budgets and affect physical and chemical properties of the high-pressure/-temperature environments terrestrial and planetary interiors. For example, the physical and chemical properties of fluids, themselves, as well as those of magmatic liquids and crystalline material with which fluids interact can change profoundly with temperature, pressure and chemical composition(s). This behavior results from changes in the 3-dimensional H2O structure which change with high temperature, high pressure, and the presence of other solutes.

Aqueous fluids mediate terrestrial and planetary mass and transport in their interiors. The fluid migration necessary for mass transport is accomplished via percolation through channels along grain boundaries. Percolation velocity is controlled by wetting angles, q, at the interface between melt and fluid and those of mineral surfaces of surrounding rocks. Wetting angle is governed by the interfacial energy, which, in turn, reflects the structural similarity between fluid and adjoining solid materials and, therefore, the composition of fluid and solids in contact at the wetting angles.

The solubility in aqueous fluids of important major element components in the Earth and planets such as, for example, SiO2 and Al2O3, also depends on the presence of additional solutes, as well as temperature and pressure, in significant ways. Silicate solubility, for example, tends to increase through complexing with alkali metals, whereas alkaline earths tend to have lesser effects. The solubility of Al2O3, which is practically insolubility in simple Al2O3-H2O systems, increases by more than an order of magnitude when K or Na is added. The solubility of minor and trace elements such as Ti, Zr, and Hf together with other HFSE can also be affected in significant ways by the presence of alkali metals forming complexes with those minor and trace solutes. The solubility of these complexes can be from one to five or six orders of magnitude greater in aqueous solutions than the solubility of those elements when simple dissolved in pure H2O without additional solutes. It is likely that the solubility of such complexes increases the more electropositive the metal cation. Moreover, additional solutes such as halogens under certain circumstance also affect the solubility behavior.

This solubility behavior is in response to solvation, ion pairing and clustering in the aqueous fluid structure. As these structural variables vary with fluid composition, temperature, and pressure, the solubility of other compositions also will vary. Temperature and pressure cause major disordering of the tetrahedrally interconnected H2O structure, which, in turn, affects on the physical and chemical properties of the fluid. Pressure is particularly important because the structural response of aqueous fluids changes quickly with increasing pressure so that at a few GPa aqueous fluid structure  and, therefore, physical and chemical properties to behave as a molecular fluid.