Phenocryst complexity in andesites and dacites from the Tequila volcanic field,Mexico: resolving the effects of degassing vs. magma mixing |
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| Authors: | Holli M Frey Rebecca A Lange |
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| Institution: | (1) Department of Geology, Union College, Schenectady, NY 12308, USA;(2) Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1005, USA |
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| Abstract: | The petrology of five phenocryst-poor (2–5%) andesites and dacites, all of which were erupted from different short-lived,
monogenetic vents, is compared to that of phenocryst-rich (10–25%) andesites erupted from the adjacent stratovolcano, Volcán
Tequila, in the Mexican arc. Despite differences in phenocryst abundances, these magmas have comparable phase assemblages
(plagioclase + orthopyroxene + titanomagnetite + ilmenite + apatite ± augite ± hornblende), and similarly wide variations
in phenocryst compositions, coupled to complex zoning patterns. For the phenocryst-poor lavas, equilibrium pairs of two Fe–Ti
oxides lead to a narrow range of calculated temperatures for each sample that range from 934 (±24) to 1,073 (±6)°C and oxygen
fugacities that range from +0.1 to +0.7 log units relative to the Ni–NiO buffer. Application of the plagioclase-liquid hygrometer
to each sample at these calculated temperatures leads to maximum melt water concentrations of 4.6–3.1 wt% during plagioclase
crystallization, indicating that the magmas were fluid saturated at depths ≥6.4–4.5 km. There is a wide, continuous range
in the composition of plagioclase (≤44 mol% An) and orthopyroxene (≤16% Mg#) phenocrysts in each sample, which is consistent
with a loss of dissolved water (≤2.8 wt%) from the melt phase during degassing as the magmas ascended rapidly to the surface.
Evidence is presented that shows the effect of dissolved water is to reduce the activity of MgO relative to FeO in the melt
phase, which indicates that degassing will also affect the Mg# of pyroxene phenocrysts, with higher melt water concentrations
favoring Fe-rich pyroxene. Both plagioclase and orthopyroxene commonly display diffusion-limited growth textures (e.g., skeletal
and hopper crystals, large interior melt hollows, and swallow tails), which are consistent with large undercoolings produced
by degassing-induced crystallization. Therefore, degassing is proposed as a possible cause for the phenocryst compositional
diversity documented in the phenocryst-poor andesite and dacite lavas erupted from peripheral vents, including the coexistence
of normally zoned plagioclase and reversely zoned orthopyroxene. Degassing-induced crystallization may also explain some of
the phenocryst complexity in crystal-rich andesites erupted from large stratovolcanoes, including Volcán Tequila. |
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