Abstract

Concentration profiles for Ca, Na, Al, K, Fe, Ti, Mg, Sr, and Ba obtained by electron microprobe and secondary ion mass spectrometry from plagioclase crystals, together with textural observations from interference contrast microscopy, are consistent with contrasting magma dynamics in two subvolcanic reservoirs from which silicic lavas erupted at the Tatara-San Pedro volcanic complex, Chilean Andes. The 1 km3 late Pleistocene (68 ka) Tatara dacite is chemically homogeneous, phenocryst-poor, and contains crystals of opa-citized hornblende, orthopyroxene, and titanomagnetite, plus 2 mm euhedral plagioclase phenocrysts with simple zoning patterns. Except at their rims, analyzed phenocrysts show nearly monotonie core-to-rim changes from An52 to An31, including slight decreases in MgO and FeO′ and slight increases in Ba, Sr, and K concentrations. Abrupt, but small, chemical shifts are associated with many subtle dissolution surfaces. In contrast, the 0.1 km3 of chemically and texturally heterogeneous Holocene San Pedro dacite contains crystals of clinopyroxene, orthopyroxene, hornblende, biotite, titanomagnetite, ilmenite, and abundant 2 mm euhedral plagioclase phenocrysts, plus plagioclase and olivine xenocrysts derived from fragmented quenched basaltic inclusions. Analyzed phenocrysts span the range from An72 to An32. Abrupt increases of 15 mol% An, large increases in MgO, FeO′, and Sr, and decreases in Ba and K2O concentrations are associated with a few major dissolution surfaces. Plagioclase xenocrysts of An74–86 have higher Sr, MgO, and FeO′ and lower Ba and K concentrations than the phenocrysts, reflecting crystallization from a basaltic melt. In both dacites, phenocryst rims decrease in Sr and increase in K and Ti relative to equilibrium values as a consequence of rapid crystal growth at high undercooling during magma ascent and eruption. Modeling of variations in apparent DSrplag/melt implies growth rates for the ≤100 μm phenocryst rims of 10−9 cm/s, suggesting that magma ascent may have taken several months. Provided these kinetic effects are identified, the remainder of the concentration profiles for Sr, Ba, Mg, and K in plagioclase phenocrysts can be inverted using partition coefficient expressions to monitor the temporal evolution of melts in the preemptive magma chambers.

Monotonie melt trends and periodic thermal dissolution of Tatara dacite phenocrysts imply a magma chamber closed to inputs of new magma and heat. Repeated cycles of crystal growth and weak dissolution occurred during phenocryst retention in thermally driven convection cells characterized by steady-state laminar flow. In contrast, dissolution events associated with large, abrupt shifts in melt Sr, Mg, and K/Ba toward basaltic values indicate that in the Holocene magma chamber heating of silicic melt adjacent to basaltic inclusions during magma mingling caused plagioclase dissolution. Renewed plagioclase growth from the resulting small volumes of hybrid melt recorded highly localized chemical mixing by diffusion and accompanying cycles of fractionation.

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