Amphibole: a mineral to unravel magma storage and timescales of sub-volcanic processes and eruptions

• Applicant: Dr. Filippo Ridolfi
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 433318228

The continuation of this project involves three major goals. The first is to improve the accuracy of thermobarometry for using Mg-rich calcic amphiboles (subduction-related and intraplate volcanism)through steady-state crystallization experiments at high P-T conditions (Goal 1). The second is to investigate textural and compositional variations of amphibole as a function of cooling and decompression rates to achieve a timescale model for the magma interaction in the crust (Goal 2).The last is to unravel the thermodynamics, kinetics and reversibility of amphibole oxidation/dehydrogenation reactions and obtain speedometric equations for the timescale of Plinian eruptions. This information is used to obtain time scales of Plinian eruptions (Goal 3). Although some shortcomings occurred (related with technical issues), Goal 1 (amphibole thermobarometry) is almost completely achieved. Additional experiments were performed and two articles were published. The first concern with the revisions of the structure and composition of Smithsonian standard amphiboles while the second is on an application protocol of amphibole thermobarometry. This protocol paper contains application examples for the historical and recent volcanic products of Bezymianny (Kamtchatka). It was also applied to phenocrysts of Stromboli volcano (Italy) and recently sampled amphiboles (both phenocrysts and megacrystals) in volcanic rocks of the West Eifel (Germany), the results of which will be presented soon in an international congress. Although in progress, a new method was developed to determine the water content in the melt hrough Raman spectroscopy. The water content data obtained with this method will be used to retrieve chemometric and hygrometric equations for Amphibole bearing glasses (Goal 1). Goal 2 is in progress and will be achieved through dynamic (isobaric cooling and isothermal decompression) crystallization experiments at high P-T. For Goal 3, high-T redox experiments will be performed by thermogravimetric analysis of amphiboles using reducing and oxidizing gases (e.g. H2, air). Most of the Eifel megacrysts have been fully chemically characterized by electron microprobe, Karl Fischer titration and UV-Fe3+/Fe2+ determination. Some of these megacrystals wil be used, together with the previously characterzed Rosenberg megacrystals (Kassel, Germany) and Alpine amphiboles (Italy), as staring material for the Goal 3 experiments. The experimental amphiboles and the coexisting phases will be analyzed using optical and electron microscopy, electron microprobe (EMP) and micro-Raman spectroscopy. The final phase of the project (total duration 3 years) will continue to be dedicated to the publication of articles and the application of amphibole for thermobarometry and geospeedometry. The following volcanic systems will be examined: Eifel, Rosenberg (Hesse), Unzen (Japan), El Reventador (Ecuador), Apacheta (Chile) and Stromboli (Italy).

• DFG Programme: Research Grants