Abstract

We wished to advance the knowledge of speciation among volatiles during melting and crystallization in the Earth's interior; therefore, we explored the nature of carbon-, nitrogen-, and hydrogen-bearing species as determined in COHN fluids and dissolved in coexisting aluminosilicate melts. Micro-Raman characterization of fluids and melts were conducted in situ while samples were at a temperature up to 825 °C and pressure up to ∼1400 MPa under redox conditions controlled with the Ti-TiO2-H2O hydrogen fugacity buffer. The fluid species are H2O, H2, NH3, and CH4. In contrast, under oxidizing conditions, the species are H2O, N2, and CO2.

The equilibria among silicate structures (Q-species) and reduced carbon and nitrogen species are, 2NH3 + 4Qn ⇋ 2Qn–1(NH2) + 2Qn–1(OH), and 2CH4 + 4Qn ⇋ 2Qn–1(CH3) + 2Qn–1(OH). The Qn and Qn–1 denote silicate species with, respectively, n and n–1 bridging O atoms. The formulation in parentheses, (NH2), (CH3), and (OH), is meant to indicate that those functional groups replace one or more oxygen in the silicate tetrahedra. There is no evidence for O-NH2 or O-CH3 bonding. Therefore, a solution of reduced C- and N-species species in the COHN system results in depolymerization of silicate melts. The ΔH values derived from the XNH2/XNH3 and XCH3/XCH4 evolution with temperature, respectively, were 8.1 ± 2.3 kJ/mol and between –4.9 ± 1.0 and –6.2 ± 2.2 kJ/mol.

The fluid/melt partition coefficients, Kfluid/melt, of the reduced species, H2O, H2, NH3, and CH4, remain above unity at all temperatures. For example, for carbon it is in the 6–15 range with a ΔH = –13.4 ± 2.4 KJ/mol. These values compare with a 0.8–3 range with ΔH = –19 ± 2.4 kJ/mol in N-free silicate-COH systems. The Kfluid/melt values for reduced nitrogen and molecular hydrogen are in the 6–10 and 6–12 range with ΔH values of –5.9 ± 0.9 and = 8 ± 6 kJ/mol, respectively.

A change in redox conditions during melting and crystallization in the Earth sufficient to alter oxidized to reduced carbon- and nitrogen-bearing species will affect all melt properties that depend on melt polymerization. This suggestion implies that changing redox conditions during melting of a COHN-bearing mantle can have a profound effect on physical and chemical properties of melts and on melting and melt aggregation processes.

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