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

Abstract

We performed hydration experiments of pure and Nb-, Cr-, and V-doped synthetic dry (H 2 O < 3 ppm) single rutile crystals. They were equilibrated with pure H 2 O 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 H 2 O-equivalent is uniformly distributed in the samples, but depends extremely on trace element contents. In pure rutile, the average H 2 O-content is 314 ± 50 ppm, the saturation level at these conditions. Rutile doped with 500 ppm Nb has a lower average H 2 O content of ~235 ppm, rutile with 2000 ppm Cr has ~900 ppm H 2 O, and rutile with 2000 ppm V does not incorporate H 2 O. 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 Ti 3+ +Ti 4+ → Ti 4+ +Ti 3+ . The reduction of Ti 4+ to Ti 3+ 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 Nb 5+ is charge balanced by Ti 3+ . 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 Ti 3+ reduced in the untreated rutile. Reacted Cr-rutile almost retains its brownish-orange color, but the spectrum shows a prominent Ti 3+ / Ti 4+ IVCT band at ~6400 cm -1 with moderate intensity considering the high-H 2 O contents of ~900 ppm. The high-H + contents are best explained by the reduction of Cr 4+ to Cr 2+ . 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 Ti 4+ to Ti 3+ for charge balance of V 5+ . 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 H 2 O (~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 H 2 O contents of the natural crystal in terms of f O2 or a H2O is not possible.