In situ study at high pressure and temperature of the environment of water in hydrous Na and Ca aluminosilicate melts and coexisting aqueous fluids

Abstract

The bonding and speciation of water dissolved in Na silicate and Na and Ca aluminosilicate melts were inferred from in situ Raman spectroscopy of the samples, in hydrothermal diamond anvil cells, while at crustal temperature and pressure conditions. Raman data were also acquired on Na silicate and Na and Ca aluminosilicate glasses, quenched from hydrous melts equilibrated at high temperature and pressure in a piston cylinder apparatus. In the hydrous melts, temperature strongly influences O‐H stretching ν(O‐H) signals, reflecting its control on the bonding of protons between different molecular complexes. Pressure and melt composition effects are much smaller and difficult to discriminate with the present data. However, the chemical composition of the melt + fluid system influences the differences between the ν(O‐H) signals from the melts and the fluids and, hence, between their hydrogen partition functions. Quenching modifies the O‐H stretching signals: strong hydrogen bonds form in the glasses below the glass transition temperature Tg, and this phenomenon depends on glass composition. Therefore, glasses do not necessarily record the O‐H stretching signal shape in melts near Tg. The melt hydrogen partition function thus cannot be assessed with certainty using O‐H stretching vibration data from glasses. From the present results, the ratio of the hydrogen partition functions of hydrous silicate melts and aqueous fluids mostly depends on temperature and the bulk melt + fluid system chemical composition. This implies that the fractionation of hydrogen isotopes between magmas and aqueous fluids in water‐saturated magmatic systems with differences in temperature and bulk chemical composition will be different.

A comprehensive molecular dynamics simulation study of hydrous magmatic liquids

“…16 shows that the solvation structure around water depends strongly on the protonated species under investigation, and only marginally on the melt composition The observation of MOH groups, in addition to TOH ones, is quite in accordance with NMR spectroscopic studies of depolymerized silicate glasses (Xue and Kanzaki, 2004, 2006, 2007, 2008Cody et al, 2005;Mysen and Cody, 2005). Nevertheless we have to be aware that the environment of hydroxyl groups in quenched glasses is not necessarily representative of that in the liquid phase, so the comparison with spectroscopic studies dealing with quenched glasses must be made with some caution (Le Losq et al, 2017). With regard to the environment of molecular water, our results are new and point out that H 2 O is preferentially solvated in the depolymerized region of the melt (the same rationale holds for H 3 O +…”

“…Hence, the structural information obtained from spectroscopic studies (NMR, IR, and Raman) on hydrous silicate glasses and suggesting that water acts as a structure modifier agent tending to depolymerize silicate melts (Bartholomew et al, 1980;Mysen et al, 1980;Stolper, 1982;Mysen and Virgo, 1986;McMillan and Remmelé, 1986;Kohn et al, 1989;Zotov and Keppler, 1998;Zeng et al, 2000;Xue and Kanzaki, 2004, 2006, 2008Malfait, 2014;Malfait and Xue, 2014;Le Losq et al, 2015a), has to be reevaluated for describing hydrous melts at upper mantle conditions. More relevant clues on the way OH groups and molecular water incorporate into the melt structure can be inferred from in situ Raman studies (Mysen, 2010; Le Losq et al, 2013Losq et al, , 2017. However, a clear interpretation of the evolution of the water band shape with melt composition and temperature in terms of water (OH, H 2 O)silicate interactions is difficult due to a lack of a robust theoretical basis relating O-H vibration band data and melt structure.…”

In Situ Raman Study of Liquid Water at High Pressure

“…Centroid was also used by Le Losq et al. 10 for description of the H 2 O Raman band in hydrous aluminosilicate melts. An even more complex behavior in the same pressure range of 0.2–0.4 GPa was reported by Sun et al.…”

“…The measured position of the centroid reaches its maximum at 0.2 GPa, where it changes the slope to negative. Centroid was also used by Le Losq et al 10 for description of the H 2 O Raman band in hydrous aluminosilicate melts. An even more complex behavior in the same pressure range of 0.2-0.4 GPa was reported by Sun et al 11 for the maximum point of n 1 Raman band calculated using fitting software; however, no details of the fitting procedure were reported.…”

The HXD95: a modified Bassett-type hydrothermal diamond-anvil cell for in situ XRD experiments up to 5 GPa and 1300 K

“…The HXD95 RH-DAC is based on a modified Bassett-type design (HDAC-III; Bassett, 2003), optimized for in situ synchrotron XRD measurements of liquids and glasses within the specific moderately high P-T domain relevant to industrial and crustal magmatic processes. The Bassett-type resistive heater design, in which heat from the resistive elements is transmitted to the sample through both diamond anvils, is suited for routine operational usage up to 1100 K, with higher-T heating reported up to 1300 K (Audé tat & Keppler, 2005;Le Losq et al, 2017a;Louvel et al, 2013;Mayanovic et al, 2007). Advantages of the Basset-type design include: (i) a simple 'in-house' mounting procedure, where all parts and connections, including the heating elements, can be prepared by the user from affordable components; (ii) easy diamond alignment facilitated by direct access to both diamond seats when the cell is closed; and (iii) the ability to perform high-T operations directly in air by flushing an inert Ar-H 2 gas (98%-2%) inside the cell to prevent oxidation of heaters, WC seats and diamond anvils.…”