Coupled H and Nb, Cr, and V trace element behavior in synthetic rutile at 600 °C, 400 MPa and possible geological application

Friedrich Lucassen; Monika Koch-Müller; Michail Taran; Gerhard Franz

doi:10.2138/am.2013.4183

Abstract

We performed hydration experiments of pure and Nb-, Cr-, and V-doped synthetic dry (H₂O < 3 ppm) single rutile crystals. They were equilibrated with pure H₂O in hydrothermal experiments at constant conditions of 600 °C, 400 MPa, and fO2 near the Ni-NiO buffer, run time between ~25 min and 14 days. Slabs cut parallel to (110) of the reacted single crystals (1 to 2 mm3) were analyzed for H⁺ by FTIR. Hydration occurs almost spontaneously and the H₂O-equivalent is uniformly distributed in the samples, but depends extremely on trace element contents. In pure rutile, the average H₂O-content is 314 ± 50 ppm, the saturation level at these conditions. Rutile doped with 500 ppm Nb has a lower average H₂O content of ~235 ppm, rutile with 2000 ppm Cr has ~900 ppm H₂O, and rutile with 2000 ppm V does not incorporate H₂O. During stepwise heating at atmospheric pressure of a reacted Nbdoped rutile, H⁺ is quickly released between 450 and 550 °C. UV-VIS spectra of unreacted colorless and reacted deep blue pure rutile show the rutile-characteristic sharp absorption edge in the UV spectra. The reacted rutile has a broad absorption band at 6500 cm^-1 wavenumber attributed to intervalence charge transfer transition Ti³⁺+Ti⁴⁺ → Ti⁴⁺+Ti³⁺. The reduction of Ti⁴⁺ to Ti³⁺ is charge balanced by the incorporation of H⁺. The Nb-doped rutile changed its color from light greenish-blue (untreated) to deep blue. In the untreated Nb rutile, the UV-VIS absorption band at 6500 cm^-1 indicates that Nb⁵⁺ is charge balanced by Ti³⁺. In the reacted Nb-rutile the absorption band is more intense, but compared with the pure rutile, H+ incorporation is lower by the equivalent of Ti³⁺ reduced in the untreated rutile. Reacted Cr-rutile almost retains its brownish-orange color, but the spectrum shows a prominent Ti³⁺/ Ti⁴⁺ IVCT band at ~6400 cm^-1 with moderate intensity considering the high-H₂O contents of ~900 ppm. The high-H⁺ contents are best explained by the reduction of Cr⁴⁺ to Cr²⁺. The UV-VIS spectra of the dark-blue to opaque V-doped rutile show a very strong absorption toward low energies, which is likely caused by reduction of Ti⁴⁺ to Ti³⁺ for charge balance of V⁵⁺. This forms a deep narrow window of transmittance in the range 25 000-20 000 cm^-1, which causes the dark-blue color.

To explore the possible use of H-in-rutile as a geohygrometer, geothermobarometer, and oxybarometer, we measured the H⁺ content in a natural rutile crystal from a retrograded eclogite with a zoned trace element (Fe, Nb, and Zr) content. The crystal reveals a slight correlation between the variable H₂O (~200 to 900 ppm) and its trace element concentrations. The observations indicate that the preservation of H⁺ contents in this natural rutile is a complicated interplay of diffusive reequlibration of fast H⁺, slower Fe and very slow other trace elements. An interpretation of the H₂O contents of the natural crystal in terms of f_O2 or a_H2O is not possible.

Coupled H and Nb, Cr, and V trace element behavior in synthetic rutile at 600 °C, 400 MPa and possible geological application

Abstract

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