Hydrogen and carbon speciation in nominaly anhydrous minerals of lithospheric mantle rocks: a comprehensive FTIR and STA + QMS study

Babushkina M. S., Nikitina L. P., Goncharov A. G., Ugolkov V. L.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2020. V. 149. N 1. P. 23-46

https://doi.org/10.31857/S0869605520010050

Language: English

Abstract

The presence of hydrogen and carbon in forms of ОН¯ ions, molecules of crystal hydrate water (Н₂О_cryst,) and specific groups СO₂, СН, CH₂, CH₃ in structure of minerals from mantle peridotites and pyroxenites has been confirmed by the Fourie-transform infrared microspectroscopy (FTIR) and simultaneous thermal analysis in combination with quadrupole mass spectrometry of thermal decomposition products (STA+QMS) methods. Temperature of the m17 (OH¯) release is higher (by STA+QMS data) in comparison with m18 (Н₂О), that testify to weaker bonds of Н₂О_cryst in mineral structure, which is supported by higher values of wavenumbers of ν_OH¯ as compared to ν_Н2Оcryst. Water component (ОН¯+Н₂О_cryst) is predominant among volatiles. It is equal to 40 – 400 ppm in diopsides, 20 – 220 in enstatites, 10 – 230 in forsterites and 30 – 340 in pyropes. Bulk water content in xenoliths varies from 20 to 200 ppm. These volatiles are preserved in the NAMs structure in the wide range of conditions: at temperature (750 – 1450^oC), pressure (1.5 – 5.5 GPa) and oxygen fugacity DlogfO₂^FMQ (-4.5... – +0.3). The main mechanism of their release from minerals is, probably, the partial melting of rock: the total content of water components decreases with increase of its degree.

Keywords: hydrogen, carbon, FTIR, STA+QMS, lithospheric mantle xenoliths, peridotites, pyroxenites, NAMs.