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Variations in Fe3+/FeT ratios in volcanic amphibole as a function of magmatic oxygen fugacity and mineral composition

Abstract

In volcanic rocks, magmatic oxygen fugacity (ƒO2) is generally determined through Fe-Ti oxybarometry1 and/or bulk rock/glass Fe2O3/FeO contents2. The first method is only applicable if Fe-Ti oxide pairs are present and have not equilibrated at subsolidus conditions. For the second method to be applied, bulk rock/glass compositions must have not been altered from magmatic values. The mineral amphibole, through the incorporation of Fe2+ and Fe3+ iron into its crystal structure and its common occurrence in hydrous magmas, has the potential to provide quantitative constraints on variations in magmatic ƒO2. However, the relationship between the Fe3+/FeT ratio in amphibole and ƒO2, Fe3+ incorporation into the amphibole structure, and amphibole chemistry are not well understood.

To address these questions, we will present Fe3+/FeT ratios, major and minor element composition, H2O contents, and crystal structure of amphiboles separated from andesitic to dacitic volcanic rocks of five well-studied localities (Mt. St. Helens and Lassen volcano, Cascades; Pinatubo volcano, Philippines; Mono basin craters, California and El Peñon area, Mexico). These volcanic rocks were chosen to cover a range of magmatic ƒO2 (DNNO = -1.0 to +2.5, log units below and above the Ni-NiO buffer). Individual amphibole grains have tschermakite to magnesiohornblende compositions. FeT/Mg in amphibole decreases with increasing ƒO2, consistent with the previously demonstrated dependency of FeT/Mg ratios in amphibole to ƒO23. However, no correlation in the amount of octahedral Al3+ (Al3+ C-site = Fe3+C-site) nor A site vacancy (Na,K+ A-site + Fe2+ C-site = ☐ A-site + Fe3+ C-site) is observed with changing ƒO2 , which may be sensitive to Fe3+ accommodation mechanisms. Petrography and H2O contents of amphiboles will be used to explore the possibility of eruption-related oxidation. Forthcoming Fe3+/FeT ratios measured using synchrotron-based Mössbauer spectroscopy will be related to sample ƒO2 constrained through Fe-Ti oxide oxybarometry. Amphibole composition, as well as, ferric iron accommodation in amphibole will be explored.

1 Ghiorso & Evans (2008): AJS, Vol. 308, no. 9, p. 957-1039

2 Kress & Carmichael (1991): Contrib Mineral Petrol, Vol. 108, no. 1-2, p. 82-92

3 Krawcyzynski et al. (2012): Contrib Mineral Petrol, Vol. 164, no.2, p. 317-339


Publication:
AGU Fall Meeting Abstracts
Pub Date:
December 2018
Bibcode:
2018AGUFM.V11D0059R
Keywords:
  • 1030 Geochemical cycles;
  • GEOCHEMISTRYDE: 1042 Mineral and crystal chemistry;
  • GEOCHEMISTRYDE: 3612 Reactions and phase equilibria;
  • MINERALOGY AND PETROLOGYDE: 3651 Thermobarometry;
  • MINERALOGY AND PETROLOGY

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