No Evidence of Degassing-Induced Oxidation of Hydrous Andesites and Dacites: An Analytical Study

There has been a long-standing discussion in the literature on the question of whether erupted lavas record the same (or similar) oxidation state as that of their source regions. For a melt that forms by equilibrium partial melting, it is required to have the same oxygen fugacity as that of its source. After it leaves its source region, however, the oxygen fugacity of the liquid can vary due to the effects of crystallization and/or degassing, with implications for the direction of its differentiation path and the potential to develop an ore deposit. In this study, we focus on the effects of degassing on the redox state of dacite and andesite magmas. Any effect of crystallization is eliminated by only considering samples that are remarkably crystal-poor (2-5% total crystals; 45-88% of which is plagioclase). The 12 andesite and dacite samples used for this study were all erupted from small, monogenetic, short-lived vents (Quaternary) in the western Mexican volcanic arc; they are all glassy and black, with no evidence of post-eruptive alteration. Total iron concentrations (FeOT) range from 3.1 to 6.5 wt%. All 12 samples are saturated with equilibrium pairs of titanomagnetite and ilmenite, which permits temperature and oxygen fugacity to be calculated (Ghiorso and Evans, 2008). Temperatures range from 760 (±18) to 1085 (±16) °C, and ΔNNO values (log10fO2 of sample - log10fO2 of Ni-NiO buffer) range from -0.9 to + 0.7. These ΔNNO values allow the ferric-ferrous ratio in the liquid to be calculated, using the experimental calibration of Kress and Carmichael (1988), which relates melt composition, oxygen fugacity and temperature to melt ferric-ferrous ratios. With temperature known, the plagioclase-liquid hygrometer of Lange et al. (2009) was applied and maximum melt water concentrations range from 8.3 to 2.8 wt%. Therefore, both the oxidation state and water concentration in the pre-eruptive magmas are known at the time of phenocryst growth. After eruption, the magmas lost nearly all of their volatiles, as indicated by the low loss on ignition values (LOI ≤ 0.31 wt%) on the bulk samples. In order to test how much oxidation of ferrous iron occurred as a consequence of that degassing, we measured the ferrous iron concentration in the bulk samples by titration. By comparison to a USGS standard (MRG-1; 8.66 wt% FeO) measured 63 times over 30 months, the average reproducibility is ± 0.33 wt% FeO at the 95% confidence level. A comparison between the pre-eruptive and post-eruptive FeO concentrations (obtained by two Fe-Ti oxides and titration, respectively) reveals close agreement across the range FeOtotal values (3.1-6.5 wt%) with a mean difference of -0.06 wt% FeO and an average absolute difference of 0.23 wt%, which is well within analytical error. Therefore, there is no evidence that degassing of up to 3-8 wt% H2O has led to any measurable oxidation of ferrous iron in the andesites and dacites from this study.