Typical Raman spectra are shown in fig. S4. The narrow band at ~2335 cm−1, which appears across the entire experimental range, belongs to the main vibron of the N2 molecule (51, 52). The peak near 3290 cm−1 has been assigned to N-H stretching in N+H–bearing complexes, such as Embedded Image (51, 52). As all our experiments were performed in a limited fO2 range, we did not see an interchange of N speciation from relatively oxidized N2 molecules to reduced N-H–bearing species as experimental conditions become more reduced (51, 53). The broad peak at ~3570 cm−1 has been attributed to the total water content, either as OH species bonded as Si-OH or as molecular H2O (3, 4, 52, 54, 55). A weak band at ~1600 cm−1 has also been assigned to vibrations of the OH group (52, 55). A small peak at ~2900 cm−1 has been associated with dissolved CH4 in silicate glasses (3, 4, 54).

In previous studies with C-bearing and N-free systems, the peak at ~2110 cm−1 has been assigned to either vibrations of isolated CO groups (56) or CO vibrations in Fe-carbonyl complexes (54, 57). Graphite-saturated C solubility in N-bearing experiments is systematically higher than previous studies under similar conditions for N-free systems. Figure S3 shows that the C solubility in the silicate glasses increases with an increase in the N content, which would point toward the possible formation of C-N species. As CN and CO are isoelectronic species, as well as strong-field, π-bonding ligands, vibrational similarity is expected between them (26). The vibrational spectra of CN in various inorganic molecules were placed at a Raman peak of ~2090 to 2130 cm−1 (26, 27, 58, 59). Taking into account our elevated C solubility data and the vibrational similarity of CN and CO, we assigned the peak at ~2110 cm−1 to CN. The observation of a similar peak in N-free studies might have been due to contamination related to atmospheric N during preparation and packing of the experimental capsules. However, the presence of both CN and CO peaks in a similar vibrational frequency range cannot be ruled out.

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