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

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).