Experimental conditions are shown in Table 1. In the temperature range from 475 to 750 °C, the samples comprise 30–40 μm diameter melt spheres in a matrix of fluid (Fig. 1). At 800 °C, there is a single fluid phase only. The melt spheres and the fluid matrix could be probed quantitatively with the confocal micro-Raman system because with the ~ 1 μm diameter laser beam the sampling depth is within the range of the sphere diameter or less. In the FTIR spectrometer, however, the minimum aperture is > 30 μm, which means that with the 30–40 μm diameter melt spheres in these experiments, quantitative infrared absorption spectra of melts were not possible because a significant portion of the infrared beam will pass through neighboring fluid. Only infrared absorption spectra of fluid spectra are, therefore, reported.
Infrared absorption spectra
Infrared absorption spectra of fluid in the frequency range of the combination and overtone bands of OH and H2O (4000–7500 cm−1) can be used to infer changes in total H2O content as well as proportion of molecular H2O and OH groups that form bonding with Si4+ and Al3+ in silicate (e.g., Scholze 1959; Malfait and Xue 2014). The presence of molecular H2O in fluids (and melts) characteristically results in absorption centered near 5200 cm−1, which has been assigned to a combination band of the H–O–H bending vibration near 1600 cm−1 and the OH fundamentals near 3600 cm−1 (Fig. 2) (Scholze 1959). The 7200 cm−1 band is the first overtone of the 3600 cm−1 OH stretch vibrations and has been used to obtain total water content of the sample (Scholze 1959; Dixon et al. 1995; Behrens and Nowak 2003). The 5200 and 7200 cm−1 bands broaden significantly with increasing temperature with the broadening of the 7200 cm−1 including development of an asymmetry toward higher frequency at the highest temperatures.
There is also minor intensity centered near 4500 cm−1 at the higher temperatures in the infrared spectra of fluids (see insert in Fig. 2). The 4500 cm−1 band is a combination band of the OH fundamentals at 3600 cm−1 and vibrations in OH groups attached to Si and Al to form (Si,Al)–OH bond with its main vibrational band between 900 and 950 cm−1 (Malfait and Xue 2014). Its integrated intensity increases approximately linearly with increasing temperature to a maximum relative intensity of few percent relative to the 5200 cm−1 intensity (Fig. 3). Conversion of this information to concentration requires molar absorption coefficients, which are not available for these fluids. However, if we assume that the 10% difference in these coefficients between the 4500 and 5200 cm−1 absorption from Na2O–SiO2 glasses (Yamashita et al. 2008), from the relationship;
$$ {C}_{\mathrm{OH}}/{C}_{\mathrm{H}2\mathrm{O}}={A}_{4500}/{\upvarepsilon}_{4500}/\left({A}_{5200}/{\upvarepsilon}_{5200}\right), $$
(1)
where C is concentration, A is integrated area, and ε is molar absorption coefficient. The ε5200 is about 10% larger than ε4500, which means that the area ratio in Fig. 3 at most underestimates the proportion of OH by about 10%.
Infrared spectroscopy also was employed to probe for species that comprise carbon or nitrogen (CO2, CO, N2, N2O, etc.) or both (e.g., carboxyl groups or CN groups). Among these species, molecular N2 is infrared inactive, but can be identified, if present, with Raman spectroscopy (see below).
In the spectra of fluids, the only absorption observed is that near 2350 cm−1 (Fig. 4), which is assigned as the v3 antisymmetric stretching in the CO2 molecule (see Nowak et al. 2003, and references therein). Its integrated intensity is essentially independent of temperature and pressure (see insert A in Fig. 4). Notably, though, the width of this absorption broadens with increasing temperature. It also is asymmetric, wherein a second band could be fitted near 2320 cm−1 (see insert B, Fig. 4). This second absorption might exist because there may be variations in the geometry of the CO2 molecule when dissolved in silicate-saturated C–O–H–N fluid.
Raman spectra
Raman spectra provide information on a range of potentially volatile species as well as silicate species in fluids and melts.
H2O component
The 3000–4000 cm−1 frequency region of the Raman spectra comprises that where signals from the fundamental OH vibrations will occur when water is present in a sample (Walrafen 1967; Holtz et al. 1996). This Raman intensity includes contributions from both molecular H2O and structurally bound OH groups.
Sharp, nearly symmetric bands with intensity maxima slightly below 3600 cm−1 were recorded in the spectra of all fluids and all melts (Fig. 5a, b). By normalizing the integrated intensities to that in the spectrum of single-phase fluid at 800 °C (diamond in Fig. 5c), we find that this intensity passes through at maximum near 625 °C before decreasing with further temperature increase. The intensities in the spectra of fluid always exceed those of coexisting melt.
The width of the 3600 cm−1 Raman bands decreases with increasing temperature (Fig. 5d), which is qualitatively similar to that reported for pure H2O as well as that of water in silicate-rich hydrous fluids and melts in silicate–H2O systems (Frantz et al. 1993; Walrafen et al. 1999; Foustoukos and Mysen 2015). This decrease is associated with diminishing asymmetry on the low-frequency-side of the spectrum, a feature that has been ascribed to less influence of hydrogen bonding among water species with increasing temperature (Walrafen et al. 1996, 1999; Foustoukos and Mysen 2012). At all temperatures and pressures, the width of this band in spectra of hydrous melts is greater than in the spectra of silicate-bearing aqueous fluids. This difference reflects either different hydrogen bonding environment in fluids and melts or a broader range of different forms of OH groups might exist in hydrous melts, or both, in hydrous melts as compared with the silicate-bearing C–O–H–N fluids. It is likely that both features contribute. It has been shown, for example, that the enthalpy of hydrogen bond formation of water in silicate melts is less than 50% of the value in aqueous fluids (Mysen 2014). Given the much more silica-rich nature of hydrous melts compared with silicate-saturated C–O–H–N fluids in the temperature and pressure range under a study, it is not surprising that a broader range of types of OH formation would be found in water-bearing silicate melts.
Nitrogen component
The most common N-bearing species in the C–O–H–N system could be N2, N2O, (N..H)-species, nitride and perhaps CN complexes. Their individual stability depends on bulk composition, temperature, pressure, and redox conditions. There is no evidence in the present spectra for (N..H) complexes (in the 3000–3400 cm−1 range; Yeo and Ford, 1994), nitride substitution in SiO4 tetrahedra (> 1200 cm−1; Sato et al. 1990), and CN-complexes (2000–2200 cm−1 frequency range; see, for example, Nakamoto 2009). The lack of Raman signals in the frequency ranges of these possible compounds is consistent with the fact that nitrogen was added as N2 (from AgN3 decomposition) and no reducing agent was added to the sample.
Absent a reducing agent, nitrogen in the C–O–H–N system may exist as N2O, azide (N3−) or as N2, which are all detectable with Raman spectroscopy. For example, in an experimental study of N-bearing silicate glasses in which Raman and 15N MAS NMR spectroscopies were the structural tools, Roskosz Mysen and Cody (2006) reported that the principal Raman signals from nitrosyl (N2O) and azide (N≡N) are near 2100 and 2200 cm1. No Raman signals were detected in this frequency region thus ruling out detectable amounts of such species in either fluid or melt. Whether or not NO3 groups are formed will be addressed below in the presentation of the low-frequency range comprising vibrations of the silicate network because N–O vibrations occur in the same frequency range (between 1100 and 1200 cm−1; Fung et al. 1994). There is no Raman signal in this frequency range in spectra of fluids. The signal intensity in spectra of melts can be satisfied without resorting to the presence of Raman bands assignable to N–O vibrations in NO3 groups. This leaves molecular N2 as the only likely nitrogen species in fluids and melts in the silicate–C–O–H–N system under study here.
The N=N stretch vibrations from molecular N2 dissolved in silicate melts at high pressure result in Raman bands slightly above 2300 cm−1 (Roskosz Mysen and Cody, 2006). The spectra of fluids and melts in the silicate–C–O–H–N system exhibit a single sharp line near 2320 cm−1 after the background from second-order Raman peaks in this region from the diamonds in the diamond cell, itself, was subtracted (Fig. 6a, b). The peaks are Gaussian in shape, which is similar to that reported by Roskosz Mysen and Cody (2006) in Raman spectra of N2-saturated silicate glasses quenched from melts at pressures ≥ 1 GPa. In contrast, gaseous N2 spectra have sharp Lorentzian peaks. This difference likely reflects some interaction between the N2 molecules and the hydrous silicate melt matrix in which they are dissolved.
The integrated Raman intensities of these bands, normalized to the same acquisition time and retaining the same focal plane for all measurements show distinctly different abundance behavior with temperature and pressure relative to the value in the spectra of single phase fluid (Fig. 6c). Whereas the normalized abundance in fluid passes through a maximum before decreasing rapidly with increasing temperature and pressure, the normalized abundance of N2 in the coexisting melt is insensitive to temperature (and pressure).
Carbon component
Carbon speciation in fluids and melts is redox sensitive. Under reducing conditions such as those near the magnetite–wüstite buffer and more reducing, the most common species in silicate–COH system is CH4 with the dominant Raman signals near 2900–3000 cm−1 (Kadik et al. 2004; Mysen et al. 2011; Armstrong et al. 2015). In the absence of hydrogen and under reducing conditions, oxycarbide complexes have been proposed (Libourel et al. 2003; Renlund et al. 1991; Soraru et al. 1995) with the two dominant Raman bands near 1350 and slightly above 1500 cm−1, respectively (Soraru et al. 1995). There is no evidence for any of these species in the Raman spectra of fluids and melts, which is, of course, understandable given the conditions of no reducing agent in samples were carbon was added to the starting materials as CO2.
Molecular CO2, CO32-, and HCO3- groups are the three main C-bearing species under oxidizing conditions. Molecular CO2 was detected in the infrared absorption spectra of fluid as discussed above (Fig. 4). However, the Fermi diad of molecular CO2, which at ambient conditions occurs at 1280 and 1380 cm−1, respectively (Rosso and Bodnar 1995; Lamadric et al. 2017), cannot be identified in Raman spectra of the samples contained in diamond cells at high temperatures because the one-phonon Raman shift from the natural diamonds used as anvils in the cell results in a Raman band with intensity that covers those frequency regions. As noted above, infrared spectra could not be obtained from the melt spheres because of their small size. It could not be established, therefore, whether molecular CO2 also exists in the hydrous silicate melts at high temperature and pressure.
The Raman spectra of fluids and melts may, contain information relevant to possibility of CO32- and HCO3- groups because of their dominant C–O stretch vibrations near 1070 and 1020 cm−1 at ambient temperatures and pressures. The C–O stretch vibration in HCO3- is particularly sensitive to temperature and is located near 950 cm–1-near 500 °C, for example (Frantz 1998). In simple COH fluids equilibrated at 700 °C and above (without nitrogen and without silicate components), there is evidence for CO32- groups from Raman intensity near 1070 cm−1 (Fig. 7), which is assigned to C–O stretch vibrations in CO32- groups (White 1974; Frantz 1998). A peak that occurs slightly below 1100 cm−1 in the spectra of silicate–C–O–H–N melt from 550 °C to the highest temperatures (Fig. 8). The same peak can be detected in spectra of fluids from 550 °C to the highest temperatures. It is assigned to C–O vibrations in CO32- groups. Within the scatter of the data, there are no other peaks in this frequency region in the spectra of fluid (Fig. 8a). In the spectra of melts, a second band appears near 1000 cm−1 (Fig. 8b), which is assigned to C–OH vibrations in HCO3- groups (Frantz 1998). We cannot detect HCO3- groups in spectra of fluids. Neither is there any signal above 1100 cm−1, which could be due to N–O vibrations in NO3 groups in the fluids (Fung et al. 1994).
Absolute concentration of CO32- and HCO3- groups cannot be obtained from the spectra without assuming that the Raman cross-sections for the two vibrations are similar. It is clear, nevertheless, that in both fluid and coexisting melt, the CO32- abundance increase with temperature and pressure until the univariant line that separates melt + fluid from a single fluid is crossed (near 800 °C) (Fig. 9). In this regard, the evolution of the CO32- abundance with temperature and pressure resembles those of N2 and H2O (Figs. 5c and 6c). There is, however, a difference in that for N2 and H2O, these components are favored by the fluid, whereas for the CO32- groups, these are favored by melt.
Silicate components
Silicate components and their species exist in coexisting silicate melts and C–O–H–N fluid. There are, however, significant differences between the silicate-saturated C–O–H–N fluid and the C–O–H–N-saturated silicate melts as seen, for example, in their significantly different Raman spectra (Fig. 10). In spectra of fluid, the dominant peak is near 780 cm−1, and is assigned to Si–O vibrations in Q0 species (Zotov and Keppler 2002; Mibe et al. 2008). The spectra of fluid characteristically also comprise a broad band centered near 600 cm−1, with two weaker bands near 810 and 900 cm−1, respectively. The latter two Raman bands are assigned Si–O vibrations in Q1 and Q2 species, respectively, whereas that centered near 600 cm−1 is an Si–O–Si bending vibration, which requires, therefore, bridging oxygen such as, for example, in Q1 and Q2 species (Zotov and Keppler 2002). At frequencies higher than 1000 cm−1 in spectra of fluids, there is a weak band near 1070 cm−1, assigned to C–O stretching in CO3 groups, as discussed under C-bearing species above (see also Fig. 8).
The spectra of coexisting melts differ significantly from those of fluids. They do comprise the 780, 810, and 900 cm−1 bands also seen in spectra of fluids, but in addition, there is a band near 1100 cm−1 not seen in the fluid spectra. There is also much more intensity at frequencies below 700 cm−1 (Fig. 10). The 1100 cm−1 is assigned to Si–O vibrations in Q3 species, whereas the extensive intensity below 700 cm−1 is due to bending and rocking motions in polymerized silicate species (McMillan 1984; Neuville et al. 2006; Mibe et al. 2008). The intensity near 500 cm−1, coupled with the shoulder near 550 cm−1, in the spectra of melts suggests that Q4 species may exist in the C–O–H–N-saturated silicate melts. The 1070 and 1000 cm−1 bands, assigned to C–O stretching in CO32- and HCO3- species, were discussed above (Fig. 8).
The abundance of Q-species and degree of polymerization, NBO/T, of silicate in fluids and melts were derived from the Raman spectra. This was accomplished by assuming that the Raman cross sections of Si–O stretch vibrations in Q0, Q1, Q2, and Q3 species were the same. The NBO/T can then be calculated from the simple relationship;
$$ \mathrm{NBO}/\mathrm{T}=\sum \limits_{n=1}^{n=4}{\left(\mathrm{NBO}/\mathrm{T}\right)}_{Q^n}\bullet {X}_{Q^n}, $$
(2)
where (NBO/T)Qn is the degree of polymerization of a specific Q-species and XQn its mol fraction.
The C–O–H–N-saturated melts become increasingly depolymerized (NBO/T increases) with increasing temperature and pressure, whereas that of silicate in the C–O–H–N fluid follows the opposite trend (Fig. 11). These trends qualitatively are the same as those observed in the simpler silicate–H2O systems (Mysen 2009) and reflect how the H2O component in silicate–C–O–H–N melts in breaks up the silicate network. The effect becomes more pronounced with increasing temperature and pressure because such increases also most likely are associated with increasing water concentration in the melt. In the C–O–H–N fluids, on the other hand, the silicate solubility increases with increasing temperature and pressure. Increasing silicate concentration in water-bearing fluids results in increasing silicate polymerization (Mysen et al. 2013).
The NBO/T evolution with temperature and pressure reflects the abundance evolution of the Q-species (Fig. 11). For melts, the Q0 abundance increases with temperature and pressure, that of Q1 is insensitive, whereas the abundance of more polymerized species, Q2 and Q3, decreases with increasing temperature and pressure and, therefore, increasing water content. In other words, the C–O–H–N-saturated melts behave as simpler H2O-saturated melts, which become increasingly depolymerized with increasing water content (Zotov and Keppler 1998). This behavior contrasts with the silicate dissolved in C–O–H–N fluid, where the abundance of Q0 species decreases and that of Q1 increases with increasing temperature and pressure and, therefore, likely total water content (Fig. 11).