Abstract
Understanding what governs the speciation in the C–O–H–N system aids our knowledge of how volatiles affect mass transfer processes in the Earth’s interior. Experiments with aluminosilicate melt + C–O–H–N volatiles were, therefore, carried out with Raman and infrared spectroscopy to 800 °C and near 700 MPa in situ in hydrothermal diamond anvil cells. The measurements were conducted in situ with the samples at the desired temperatures and pressures in order to avoid possible structural and compositional changes resulting from quenching to ambient conditions prior to analysis. Experiments were conducted without any reducing agent and with volatiles added as H2O, CO2, and N2 because both carbon and nitrogen can occur in different oxidation states.Volatiles dissolved in melt comprise H2O, CO32-, HCO3-, and molecular N2, whereas in the coexisting fluid, the species are H2O, CO2, CO32-, and N2. The HCO3-/CO32- equilibrium in melts shift toward CO32- groups with increasing temperature with ∆H = 114 ± 22 kJ/mol. In fluids, the CO2 abundance is essentially invariant with temperature and pressure. For fluid/melt partitioning, those of H2O and N2 are greater than 1 with temperature-dependence that yields ∆H values of − 6.5 ± 1.5 and − 19.6 ± 3.7 kJ/mol, respectively. Carbonate groups, CO32- are favored by melt over fluid.Where redox conditions in the Earth’s interior exceed that near the QFM oxygen buffer (between NNO and MW buffers), N2 is the stable nitrogen species and as such acts as a diluent of both fluids and melts. For fluids, this lower silicate solubility, in turn, enhances alkalinity. This means that in such environments, the transport of components such as high field strength cations, will be enhanced. Effects of dissolved N2 on melt structure are considerably less than on fluid structure.
Highlights
Characterization of the processes that govern the budget and recycling of volatile components in the Earth’s interior is central to our understanding of the formation and evolution of the solid Earth, its oceans, and atmosphere (e.g., Kasting et al 1993; Jambon 1994; Tolstikhin and Marty 1998; Deines 2002)
The objective of the present study is to address this latter type of system in order to provide experimental data with which to further our understanding the structure of fluids and melts of aluminosilicate–C–O–H–N systems relevant in the temperature and pressure range of the Earth’s crust and upper mantle under oxidizing conditions
By normalizing the integrated intensities to that in the spectrum of singlephase fluid at 800 °C, we find that this intensity passes through at maximum near 625 °C before decreasing with further temperature increase
Summary
Characterization of the processes that govern the budget and recycling of volatile components in the Earth’s interior is central to our understanding of the formation and evolution of the solid Earth, its oceans, and atmosphere (e.g., Kasting et al 1993; Jambon 1994; Tolstikhin and Marty 1998; Deines 2002). The nitrogen abundance likely is heterogeneously distributed with considerably greater nitrogen contents in subduction zone environments where abundances above 0.1% N2 have been reported (Fisher et al 2002). This latter environment likely is where most of the exchange of volatiles (including nitrogen) occurs between the solid Earth and the atmosphere (Schmidt and Poli, 2003; Bebout, 2007).
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