An evaluation of the effect of degassing on the oxidation state of hydrous andesite and dacite magmas: A comparison of pre- and post-eruptive Fe 2+ concentrations

Stephen M. Crabtree, Rebecca A. Lange

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24 Scopus citations

Abstract

The bulk (post-eruptive) wt% FeO concentration in each of 11 phenocryst-poor (<5%) andesite and dacite (60-69 wt% SiO 2) lavas from different monogenetic vents in the Mexican arc has been measured by titration, in duplicate. The results match, within analytical error, the wt% FeO content of the magmas during phenocryst growth (pre-euptive), which were calculated on the basis of oxygen fugacity and temperature results from Fe-Ti two-oxide oxygen barometry. The average deviation between the pre- and post-eruptive FeO concentrations is ±0. 15 wt%. Application of the plagioclase-liquid hygrometer shows that at the time of phenocryst growth, these 11 magmas contained from ~3-8 wt% H 2O, which was extensively degassed upon eruption. There is no evidence that degassing of ≤8 wt% H 2O changed the oxidation state of these magmas. Calculations of pre-eruptive and post-eruptive oxygen fugacity values relative to the Ni-NiO buffer (in terms of log 10 units) for the 11 samples span a similar range; pre-eruptive ΔNNO = -0. 9 to +0. 7 and post-eruptive ΔNNO = -0. 4 to +0. 8. The data further show that extensive groundmass (closed-system) crystallization had no affect on bulk Fe 3+/Fe 2+ ratios. Finally, there is no systematic variation in the range of pre-eruptive Fe 3+/Fe T values of the samples as a function of SiO 2 concentration (i.e., differentiation). Therefore, the results of this study indicate that the elevated Fe 3+/Fe T ratios of arc andesites and dacites, compared with magmas erupted in other tectonic settings, cannot be attributed to the effects of (1) degassing of H 2O, (2) closed-system crystallization, and/or (3) differentiation effects, but instead must be inherited from their parental source rocks (i.e., mantle-derived arc basalts).

Original languageEnglish (US)
Pages (from-to)209-224
Number of pages16
JournalContributions to Mineralogy and Petrology
Volume163
Issue number2
DOIs
StatePublished - Feb 2012
Externally publishedYes

Bibliographical note

Funding Information:
Acknowledgments This work was supported by the National Science Foundation: EAR-0409052 (research grant) and EAR-9911352 (equipment grant for the electron microprobe at the University of Michigan). This manuscript was substantially improved by the constructive comments of Ed Mathez, Liz Cottrell and one anonymous reviewer.

Keywords

  • Colorimetry
  • Degassing
  • Fe-Ti oxides
  • Oxygen fugacity
  • Titration

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