A mapping approach for the investigation of Ti–OH relationships in metamorphic garnet

Garnet is a nominally anhydrous mineral that can incorporate several hundreds of ppm H2O in the form of OH groups, where H+ substitutes for cations in the garnet structure. To understand the effect of such small amounts of H2O on the physical and chemical properties of garnet, it is essential to determine where and how the OH groups are incorporated into the mineral structure. We investigated correlations between major and minor element maps acquired with the electron probe micro-analyser and H2O maps measured with Fourier transform infrared spectroscopy coupled to a focal plane array detector at the microscale to determine possible coupled substitutions. A set of algorithms was developed to match the maps pixel by pixel. They allow the computation of the garnet structural formula taking the H2O content into account and the calculation of correlations between H2O and other elements on the basis of 10,000 s of points. This new approach was applied to two hydrous garnet samples both showing H2O and chemical zoning. The first sample consists of a grossular-rich garnet from a high-pressure metarodingite ranging from 200 to 900 ppm H2O. The second sample contains a Ti-rich andradite garnet ranging from 200 to 8500 ppm H2O. For the grossular-rich garnet, a 1:1 correlation between Ti and H has been observed suggesting that H occurs as tetrahedral (2H)2+ point defect, charge compensated by 2 Ti4+ on the octahedral site. Based on this, a new hydrous garnet endmember with the formula Ca3Ti2H2Si2O12 is proposed. This 2TiVI↔ (2H)IV exchange mechanism is mainly responsible for the high amounts of TiO2 (up to 11 wt%) in the investigated Ti-andradite. The incorporation of (2H)2+ instead of (4H)4+ on the tetrahedral site has important consequences for the normalisation of the garnet and hence on the determination of Fe2+/Fe3+ based on stoichiometry. In the garnet from the metarodingite, a small-scale zoning in H2O contents of less than 100 µm can be resolved, indicating that the Ti–H defect is stable up to eclogite facies conditions and not modified by diffusion even at timescales of millions of years.