Hydrogen, fluorine and lithium investigation in hydrogarnets by Secondary Ion Mass Spectrometry: Comparison with X-ray and spectroscopic techniques

The incorporation of OH- in trace but measurable amounts in the structure of nominally anhydrous minerals is to date well documented. OH- occurring in this manner may constitute the dominant reservoir of hydrogen in the Earth's interior and is believed to play an important role in the physical properties of the mantle. It may also affect the evolution of the hydrosphere through its influence on mantle melting and isotopic fractionation [1]. In Ti-garnets, hydrogen may be incorporated via the hydrogarnet substitution, where a SiO4 unit may locally be replaced by a H4O4-tetrahedron. However, more complex mechanisms have also been proposed [2]. In addition, the uptake of fluorine into the structure is more complex than the simple exchange reaction F- OH- [3]. In this study, Secondary Ion Mass Spectrometry (SIMS) has been used to analyse hydrogen -quantified conventionally as H2O (wt%)-, fluorine and lithium in a suite of Ti garnets of different origin and geological provenance. From Electron Probe Micro-Analysis (EPMA) and structure refinements from single crystal X-ray diffraction (SCXRD) data, such garnets were expected to have tetrahedral substitutions and a hydrogarnet component. FTIR preliminary spectra in the OH- stretching region evidenced that, comparatively, the samples were characterized by various degrees of hydration. The results of our SIMS analyses confirmed the presence of significant amount of H2O (0.091 - 0.459 wt%), low concentration of F (0.0089 - 0.048 wt%) as well as of Li2O (0.0024 - 0.0139 wt%). The comparison with data obtained by X-ray site scattering refinement and Mössbauer spectroscopy allowed to ascertain that the combination of VITi4+VIM3+-1 IVFe3+IVSi-1 (schorlomite substitution) and VIM2+VITi4+VIFe3+-2 (morimotoite substitution), with M3+ = Fe3+, Al3+, and M2+ = Fe2+, Mg2+, Mn2+, can explain most of the chemical variation of tetrahedral and octahedral sites. However, the accurate evaluation of H and F by SIMS favoured the assessment of minor substitution mechanisms. A good qualitative agreement between SIMS and FTIR data was found.