Silicate solution, cation properties, and mass transfer by aqueous fluid in the Earth\u2019s interior

Bjorn Mysen

doi:10.1186/s40645-018-0198-1

Abstract

Aqueous fluids in the Earth’s interior are multicomponent systems with silicate solubility and solution mechanisms strongly dependent on other dissolved components. Here, solution mechanisms that describe the interaction between dissolved silicate and other solutes were determined experimentally to 825 °C and above 1 GPa with in situ vibrational spectroscopy of aqueous fluid while these were at high temperature and pressure. The silicate content in Na-bearing, silicate-saturated aqueous fluid exceeds that in pure SiO2 at high temperature and pressure. Silicate species were of Q0 (isolated SiO4 tetrahedra) and Q1 (dimers, Si2O7) type. The temperature dependence of its equilibrium constant, K = XQ1/(XQo)2, yields enthalpies of 22 ± 12 and 51 ± 17 kJ/mol for the SiO2–H2O and Na-bearing fluids. In contrast, in Ca-bearing fluids, the solubility is more than an order of magnitude lower, and only Q0 species are present. The present data together with other published experimental information lead to the conclusion that the silicate solubility in aqueous fluids in equilibrium with mafic rocks such as amphibolite and peridotite is an order of magnitude lower than the solubility in fluids in equilibrium with felsic rocks such as andesite and rhyolite compositions (felsic gneiss) under similar temperature and pressure conditions. The silicate speciation also is more polymerized in the felsic systems. This difference is also why second critical end-points in the Earth are at lower temperature and pressure in felsic compared with mafic systems. Alkali-rich fluids formed by dehydration of felsic rocks also show enhanced high field strength element (HFSE) solubility because alkalis in such solution form oxy complexes with the HFSE cations. Fluids formed by dehydration of felsic rocks in the Earth’s interior are, therefore, more efficient transport agents of silicate materials than fluids formed by dehydration of mafic and ultramafic rocks, whether for major, minor, or trace elements.

Highlights

The solubility and solution mechanism(s) of silicate components in aqueous fluids in the Earth’s interior are central to our understanding of the roles of aqueous fluids in material transport within the Earth (Fockenberg et al 2006; Newton and Manning 2009; Burchard et al 2011)
The Raman spectra indicate that the silicate concentration and speciation in the fluid at given temperature and pressure is dependent on the nature of the crystalline materials in the equilibrium with the fluid
Solubility Silicate solubility in the aqueous fluids might be estimated by combining silica solubility in the fluid in the system SiO2–H2O (Manning 2004) with the intensity of relevant Raman bands in order to generate a calibration curve from the intensity of Raman bands in spectra of

Summary

Introduction

The solubility and solution mechanism(s) of silicate components in aqueous fluids in the Earth’s interior are central to our understanding of the roles of aqueous fluids in material transport within the Earth (Fockenberg et al 2006; Newton and Manning 2009; Burchard et al 2011) This is so in and near subduction zones where water-rich fluids released from dehydrating slab materials can transport chemical components into source regions of melting and, at the same time, deplete dehydrating slab materials of fluid-soluble components (Mibe et al 2008; Kawamoto et al 2012). The abundance of components known to affect silicate solubility in aqueous fluid differs significantly among those rock types in the Earth’s interior It is necessary, to determine how individual compositional variables affect the silicate solubility and structure of dissolved silicate in aqueous fluids at relevant high-temperature and high-pressure conditions.

Results

Discussion

Conclusion