A framework for quantitative in situ evaluation of coupled substitutions between H⁺ and trace elements in natural rutile

Abstract

The coupling behaviour of H+ and trace elements in rutile has been studied using in situ polarised Fourier transform infrared (FTIR) spectroscopy and laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) analysis. H2O contents in rutile can be precisely and accurately quantified from polarised FTIR measurements on single grains in situ. The benefits of this novel approach compared to traditional quantification methods are the preservation of textural context and heterogeneities of water in rutile. Rutile from six different geological environments shows H2O contents varying between ∼ 50–2200 µg g−1, with large intra-grain variabilities for vein-related samples with H2O contents between ∼ 500 and ∼ 2200 µg g−1. From FTIR peak deconvolutions, six distinct OH absorption bands have been identified at ∼ 3280, ∼ 3295, ∼ 3324, ∼ 3345, ∼ 3370, and ∼ 3390 cm−1 that can be related to coupled substitutions with Ti3+, Fe3+, Al3+, Mg2+, Fe2+, and Cr2+, respectively. Rutile from eclogite samples displays the dominant exchange reactions of Ti4+ → Ti3+, Fe3+ + H+, whereas rutile in a whiteschist shows mainly Ti4+ → Al3+ + H+. Trace-element-dependent H+ contents combined with LA–ICP–MS trace-element data reveal the significant importance of H+ for charge balance and trace-element coupling with trivalent cations. Trivalent cations are the most abundant impurities in rutile, and there is not enough H+ and pentavalent cations like Nb and Ta for a complete charge balance, indicating that additionally oxygen vacancies are needed for charge balancing trivalent cations. Valance states of multivalent trace elements can be inferred from deconvoluted FTIR spectra. Titanium occurs at 0.03 ‰–7.6 ‰ as Ti3+, Fe, and Cr are preferentially incorporated as Fe3+ and Cr3+ over Fe2+ and Cr2+, and V most likely occurs as V4+. This opens the possibility of H+ in rutile as a potential indicator of oxygen fugacity of metamorphic and subduction-zone fluids, with the ratio between Ti3+- and Fe3+-related H+ contents being most promising.