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
The presence of hydrogen and carbon in forms of ОН¯ ions, molecules of crystal hydrate water (Н2Оcryst,) 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 Н2Оcryst in mineral structure, which is supported by higher values of wavenumbers of νOH¯ as compared to νН2Оcryst. Water component (ОН¯+Н2Оcryst) 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 – 1450oC), pressure (1.5 – 5.5 GPa) and oxygen fugacity DlogfO2FMQ (-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.