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

Abstract

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.

H2O-rich rutile as an indicator for modern-style cold subduction

“…Peak deconvolution is used to identify different defects contributing to the OH-bands of the FTIR signal. Lueder et al ( 2023 ) identified six different trace-element related H + -defects in natural rutile: three narrow bands at ~ 3278 cm −1 , ~ 3295 cm −1 , and ~ 3323 cm −1 related to Ti 3+ , Fe 3+ and Al 3+ , respectively and three wide bands at ~ 3345 cm −1 , ~ 3370 cm −1 , and ~ 3390 cm −1 , related to Mg 2+ , Fe 2+ , and Cr 2+ , respectively. Based on these observations we performed peak deconvolution on all measured spectra.…”

“…We performed polarized transmission FTIR spectroscopy at the University of Bern, following the measurement protocol and quantification described in Lueder et al ( 2023 ). Selected grains were mapped to evaluate intra-grain variability of H 2 O contents and H + diffusion.…”

“…To identify different OH-band positions related to different cations, we applied a deconvolution to the OH-range of each individual spectrum from single spot analyses and maps. The peak heights and width of fixed position Lorentzian peaks were fitted to trace-element dependent OH band positions at 3280 cm −1 for Ti 3+ , 3295 cm −1 for Fe 3+ , 3323 cm −1 for Al 3+ , 3350 cm −1 for Mg 2+ , 3370 cm −1 for Fe 2+ , and 3390 cm −1 for Cr 2+ (Lueder et al 2023 ). OH bands related to Cr 3+ were described for synthetic rutile (e.g.…”

“…Johnson et al 1968 ; Vlassopoulos et al 1993 ; Bromiley and Hilairet 2005 ). Natural rutile from different lithologies and P–T-conditions has been shown to incorporate H + linked to six main di- and trivalent impurities, most commonly Ti 3+ and Fe 3+ , to a lesser degree Al 3+ and Mg 2+ , as well as Fe 2+ and Cr 2+ in reduced conditions (Lueder et al 2023 ). However, the dominant H + -defects in rutile from different lithologies have not been evaluated.…”

“…As is pressure dependent, it may translate to pressure-dependent H 2 O contents in rutile. To investigate this, the use of polarised Fourier Transform Infrared Spectroscopy (FTIR) is a simple and precise tool to quantify H 2 O contents in randomly oriented rutile grains and can be applied to both metamorphic samples in-situ and single detrital grains (Lueder et al 2023 ).…”