­Characterization of Reduced Magmatic C-O-H-N Volatiles By Isotopic Labeling

Abstract:

Characterization of COHN species in silicate melts [1-10] is required to understand the role of reduced volatiles in planetary and early Earth processes, including partitioning between planetary cores, mantles, and atmospheres during early differentiation. Vibrational spectroscopy has been used to examine volatile speciation, but for a number of absorptions there is uncertainty as to whether they relate to species containing N, C, or both [1,3]. In particular, an IR band at 3370 cm^-1 is commonly attributed to N-H stretching [1,4,5,7], but associated Raman bands near 3280 cm^-1 have also been attributed to alkyne (C-H) bonds [8-10]. The 3370 cm^-1 IR band appears even in nominally N-free experiments owing to trapped air and is accompanied by a feature at 1615 cm^-1 which could be caused by C=O or N-H bonds [1,3,8]. We sought to determine whether N and C were responsible for various IR bands by dissolving different isotopes of N and C in basaltic melts at high pressure and temperature and observing the shift in position of the resulting absorptions. Experiments were conducted at 1.2 GPa and 1400 ^oC and volatiles were added to a basaltic oxide mix in the form of unlabeled, ¹³C labeled, and ¹⁵N labeled urea [(NH₂)₂CO]. The resulting glasses were analyzed using FTIR and the theoretical band shifts were predicted based on a classical approximation of a diatomic molecule. Relative to isotopically normal glasses, bands at both 3370 cm^-1 and 1615 cm^-1 decrease by 4-8 wavenumbers for ¹⁵N and not at all for ¹³C, consistent with origination by N-H bonds in amines or metal-ammine complexes. [1] Stanley et al. (2014) GCA 129, 54-76. [2] Wetzel et al. (2013) PNAS 110, 8010-8013. [3] Armstrong et al. (in prep). [4] Kadik et al. (2011) Geochem. Int. 49, 429-438. [5] Kadik et al. (2013) PEPI 214, 14-24. [6]Mysen (2013) Chem. Geo. 346, 113-124. [7] Mysen et al. (2008) Am. Min. 93, 1760-1770. [8] Mysen et al. (2009) GCA 73, 1696-1710. [9] Dasgupta et al. (2013) GCA 102, 191-212. [10] Chi et al. (2014) 139, 447-471. [11] Socrates (2001).