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).
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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.
- Fe-Ti oxides
- Oxygen fugacity