Al and H incorporation and Al-diffusion in natural rutile and its high-pressure polymorph TiO<sub>2</sub> (II)

Author(s): Bastian Joachim-Mrosko, Jürgen Konzett, Thomas Ludwig, Thomas Griffiths, Gerlinde Habler, Eugen Libowitzky, Roland Stalder
Abstract: Rutile is an important accessory mineral in metamorphic rocks and is used as a geothermobarometer or geochronometer. This study aims to bridge the gap between diffusion studies in simplified and complex natural systems by investigating the incorporation and mobility of Al in natural rutile and its high-pressure polymorph TiO₂ (II).
Experiments were performed at 0.1 MPa to 7 GPa, 1223–1373 K, at buffered μ(Al₂O₃) and with fO₂ constrained to ≤CCO, which is the equilibrium between graphite and a CO-CO₂ gas phase. Based on electron probe microanalysis, secondary ion mass spectrometry and Fourier transform infrared analyses, we suggest a complex combination of mechanisms to explain the incorporation of Al and H in natural rutile and TiO₂ (II). This includes: (1) the incorporation of Al³⁺ on octahedral Ti-sites charge balanced by the formation of oxygen
vacancies; and (2) the incorporation of oxygen in interstitial positions charge balanced by hydrogen interstitials.
Determined Al-diffusivities in natural TiO₂ are approximately eight to nine orders of magnitude faster compared to previously published data. A possible explanation includes a significantly enhanced rate of ionic diffusion through the combined effect of hydrolytic weakening, enhanced Al-diffusion through extended defects and to a minor extent oxygen fugacity variations. Consequently, results of this study question that the inferred
high closure temperatures for the Al-in-rutile geothermobarometer can be applied to all natural systems.
Organisation(s): Department of Lithospheric Research, Department of Mineralogy and Crystallography
External organisation(s): Scientific Software Center, Leopold-Franzens-Universität Innsbruck, Russian Academy of Sciences
Pages: 123-147
Publication date: 09-2023
Peer reviewed: Yes
Austrian Fields of Science 2012: 105116 Mineralogy, 105113 Crystallography
Keywords
ASJC Scopus subject areas: Geochemistry and Petrology
Portal url: https://ucrisportal.univie.ac.at/en/publications/6966f319-c9e6-4b7b-8de6-de17941da84d

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