Olivine-melt thermometry, hygrometry and oxybarometry applied to lavas erupted from the Mascota volcanic field, western Mexico

Abstract

Olivine-melt thermometry, based on the partitioning of Ni, is applied to 13 lavas (absarokites, olivine minettes, basaltic andesites and a hornblende lamprophyre) from the Mascota volcanic field in western Mexico, to determine the onset temperature for olivine phenocryst growth during ascent. A test of whether the most Mg-rich olivine analyzed in each sample is a close approximation of the first olivine to crystallize from a melt composition that matches the whole rock was made using the thermodynamic MELTS model and shown to be consistent in all but one case. One sample with 16 wt% MgO, shows possible evidence of olivine accumulation, whereas the remaining 12 samples indicate that olivine crystallization began at ~1200 C for the absarokites (~11.7 wt% MgO), ~1150 C for the olivine minettes (~8.5 wt% MgO), ~1100-1030 C for the basaltic andesites (~ 8.7-7.0 wt% MgO), and as low as ~954 C for the hornblende lamprophyre. (~6 wt% MgO). These olivine-melt thermometry results are fully consistent with hydrous phase-equilibrium experiments from the literature. Moreover, all samples contain olivine phenocrysts with diffusion-limited rapid-growth textures, consistent with phenocryst growth driven by H2O degassing and cooling during ascent, which leads to large effective undercoolings that cannot be attained in a magma undergoing cooling by conductive loss of heat in a crustal chamber. In addition to Nibased olivine-melt thermometry, for which dissolved H2O in the melt has a negligible 2 effect, the application of Mg-based olivine-melt thermometry to the same samples allows the magnitude of the suppression of olivine crystallization temperatures due to dissolved water in the melt to be evaluated. The results show that olivine liquidus temperatures have been reduced by 95-234 ( 29) C, indicative of dissolved water concentrations that range from >3-8wt% H2O, consistent with hydrous phase-equilibrium experiments. Finally, application of Fe2+-Mg exchange coefficient (KD) values from the literature to the olivine-melt pairs used for thermometry constrain melt Fe3+/FeT ratios (0.25-0.49) at the onset of olivine growth; these values correspond to oxygen fugacities that range from 1 to 3 log units above that of the Ni-NiO buffer. The results of this study show that these 12 Mascota lavas, which display a strong arc geochemical signature (e.g., high Ba/Zr ratios) are both hydrous (≤ 8 wt%) and oxidized (ΔNNO ≤ +3).