Diffusion in common mineral phases. Part 3: Chain silicates: pyroxene and amphibole

• Applicants: Dr. Ralf Dohmen; Professor Dr. Francois Holtz
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 405665352

Chain silicates like pyroxene and amphibole are common minerals in a wide range of magmatic rocks. Diffusion chronometry using major and trace element zoning in pyroxenes is being increasingly used in the last decade but this approach is hampered by the incomplete availability of diffusion data. In the first period of this research unit we have successfully developed new experimental strategies to measure diffusion rates of Fe-Mg and various trace elements in orthopyroxene. For Fe-Mg inter-diffusion we focused on the effect of Fe and Al-content as a function of temperature and oxygen fugacity. These data now allow to predict reliable diffusion coefficients for Fe-Mg at the relevant conditions and orthopyroxene compositions for diffusion chronometry in volcanic rocks. These new data could be only successfully determined by combining novel technologies. We combined pulsed laser deposition for the preparation of thin-film diffusion couples and special sample preparation methods after the diffusion experiments for chemical analysis by back-scattered electron imaging and time-of-flight secondary ion mass spectroscopy. For the measurement of diffusive fractionation of Li isotopes we used in addition the powder source technique to create diffusion profiles, which were analysed by femtosecond laser ablation inductively coupled mass spectrometry. In the second round of this research unit we aim to extend this approach to study diffusion of Fe-Mg and trace elements in clinopyroxene and amphibole. Likewise for orthopyroxene (ignoring our new data set) only limited diffusion data are available for clinopyroxen, which do not constrain for example the effect of the chemical composition on the diffusion properties of clinopyroxene. For amphibole, basically no relevant diffusion data exist so far. It is because of the unique combination of expertise and equipment in Bochum and Hannover that we can now address this fundamental gap in diffusion data. The newly acquired diffusion data can be directly used and field-tested in the accompanying case studies (TP7, TP8) of this research unit programme.

• DFG Programme: Research Units
• Subproject of: FOR 2881: Diffusion Chronometry of magmatic systems
• Co-Investigator: Professor Sumit Chakraborty, Ph.D.