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- Title
A framework for quantitative in situ evaluation of coupled substitutions between H+ and trace elements in natural rutile.
- Authors
Lueder, Mona; Tamblyn, Renée; Hermann, Jörg
- 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. H 2 O 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 H 2 O contents varying between ∼ 50–2200 µ g g -1 , with large intra-grain variabilities for vein-related samples with H 2 O contents between ∼ 500 and ∼ 2200 µ g 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 Ti 3+ , Fe 3+ , Al 3+ , Mg 2+ , Fe 2+ , and Cr 2+ , respectively. Rutile from eclogite samples displays the dominant exchange reactions of Ti 4+ → Ti 3+ , Fe 3+ + H + , whereas rutile in a whiteschist shows mainly Ti 4+ → Al 3+ + 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 Ti 3+ , Fe, and Cr are preferentially incorporated as Fe 3+ and Cr 3+ over Fe 2+ and Cr 2+ , and V most likely occurs as V 4+. 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 Ti 3+ - and Fe 3+ -related H + contents being most promising.
- Subjects
LASER ablation inductively coupled plasma mass spectrometry; RUTILE
- Publication
European Journal of Minerology, 2023, Vol 35, Issue 2, p243
- ISSN
0935-1221
- Publication type
Article
- DOI
10.5194/ejm-35-243-2023