Silicate solution, cation properties, and mass transfer by aqueous fluid in the Earth’s interior

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 SiO 2 at high temperature and pressure. Silicate species were of Q 0 (isolated SiO 4 tetrahedra) and Q 1 (dimers, Si 2 O 7 ) type. The temperature dependence of its equilibrium constant, K = X Q1 /(X Qo ) 2 , yields enthalpies of 22 ± 12 and 51 ± 17 kJ/mol for the SiO 2 -H 2 O and Na-bearing fluids. In contrast, in Ca-bearing fluids, the solubility is more than an order of magnitude lower, and only Q 0 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.

In-situ Raman spectroscopic analysis of dissolved silica structures in Na2CO3 and NaOH solutions at high pressure and temperature

“…The observation of this band in the NaOH solution (Fig. 4b) was consistent with that reported by Mysen (2018). The band was detected above ~ 300-400 °C in the Na 2 CO 3 solution, ~ 500 °C in the NaOH solution, and ~ 600 °C in pure H 2 O.…”

“…The Raman spectra revealed the presence of neutral monomers Si(OH) 4 and deprotonated monomers Si(OH) 3 O − in the solutions, but lacked evidence of silica oligomers in the spectral region of interest (> ~ 700 cm −1 for the observation of carbon species). On the other hand, a Raman spectroscopy study with SiO 2 -NaOH-H 2 O systems by Mysen (2018) showed the Q 1 -species band under high P-T conditions, but the presence of anionic silica species in the solution is still uncertain. Thus, the information available on the Raman observations is insufficient to understand silica solubility and speciation in Na 2 CO 3 and NaOH solutions under high P-T conditions.…”

Redox control on nitrogen isotope fractionation during planetary core formation

“…In comparison, S dissolves in metal alloys as Fe–S with vibration modes at 310 and 360 cm −1 , and in silicate melts as Fe–, Mg–, and Ca–S with vibration modes between 285 and 350 cm −1 (66). Although the vibration modes of complexes involving N or S have been determined for quenched silicate and metal, their equivalents in silicate melts and liquid metals at HP–HT are expected to vary only by a few cm −1 , as shown by Mysen (71) for the N 2 vibration mode in silicate melts and aqueous fluids. The observed vibration modes are directly related to the reduced masses µ N and µ S of the bonds formed by N and S, respectively, in the metal and silicate phases.…”

Origin of the Paleoproterozoic “Giant Quartz Reef” System in the Bundelkhand Craton, India: Constraints from Fluid Inclusion Microthermometry, Raman Spectroscopy, and Geochemical Modelling

“…The high solubility obtained by using the SIT database is too high compared to values reported at 300 °C taking crystalline silica only as the equilibrium phase [63]. Mysen [64] observed a reduced solubility of silica in deeper crustal fluid in presence of Ca-rich basic rocks corresponding to deeper crust. In that context, high solubility obtained in the present case at 300 °C in the presence of anorthite needs to be confirmed.…”