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A novel in-situ method to track the redox evolution of magmatic systems

Charline Lormand 1, Enzo-Enrico Caccatiore1, Nils B. Gies2, David A. Neave3, Zoltan Zajacz1, Luca Caricchi1

  • Affiliations: 1Department of Earth Sciences, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerland (charline.lormand@unige.ch); 2Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, 3012 Bern, Switzerland; 3Department of Earth Sciences, University of Manchester, United-Kingdom 

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 224

  • Programme No: 1.9.31

  • Theme 1 > Session 9

Abstract

Magma redox, often expressed as oxygen fugacity (fO2), plays a significant role in metal and sulfur transport in magmas. Oxidised conditions increase (1) copper transfer efficiency from magma to fluid in mineralising systems, and (2) magma viscosity and explosive potential in volcanic systems. The ferric-to-total-iron ratio (Fe3+/ΣFe) is used as a proxy to estimate the magmatic fO2. However, quantifying Fe3+/ΣFe often involves high analytical costs, limited accessibility, and sample damage. The novel Soft X-ray Emission Spectrometer (SXES) implemented on an electron probe microanalyser enables precise measurement of Fe-La, Fe-Lblines, and thus Fe3+/ΣFe once the instrument is calibrated on matrix-matched materials measured by Mössbauer or X-ray absorption near edge structure (XANES) spectroscopy. Here, we present novel, in-situ Fe3+/ΣFe determinations from silicate minerals (clinopyroxene and amphibole) and glasses (natural and experimental) previously characterised by Mössbauer spectroscopy and XANES [1-4]. Silicate phases from controlled-fO2experiments of medium-K calc-alkaline basaltic and shoshonitic compositions are also presented to develop new oxybarometers. Caveats due to bonding effects such as non-linearity between the iron content and soft X-ray line properties will be discussed. This novel method allows the characterisation of the redox~ ~evolution of magmatic systems, adding a new dimension to our understanding of magmatic processes and the factors controlling the formation of ore deposits. [1] Cottrell & Kelley (2011), EPSL 305(3-4), 270-282. [2] Cottrell et al. (2009), Chem. Geol. 268(3-4), 167-179. [3] Kelley & Cottrell (2009), Science, 325(5940), 605-607. [4] Neave et al. (2024), CMP 179(5).