A Temporal Record of Magma Accumulation and Evolution beneath Nevado de Toluca, Mexico, Preserved in Plagioclase Phenocrysts
1Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK
2Instituto De GeologÍa, Universidad Nacional AutóNoma De México, Coyoacán 04510, México D.F., México
RECEIVED AUGUST 13, 2008; ACCEPTED JANUARY 20, 2009
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Abstract |
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Plagioclase crystals from the 8 km3 10·5 ka Upper Toluca Pumice (UTP) eruption from Nevado de Toluca provide a detailed temporal record of pre-eruptive magmatic processes. The crystals display a range of textures and major and trace element concentrations. A distinct feature of the crystals is the presence of several sharp increases in MgO, FeO and TiO2, which occur at some of the numerous resorption horizons and coincide with increases in XAn and in some cases Ce and La. These abrupt compositional peaks, and the associated textural variations, reflect recharge events of more mafic melt. Three distinct recharge events can be recognized. The general compositional trends suggest that crystallization occurred within a common chamber. However, the crystal cores display a range of textures and a wide range of compositions suggesting that they are relics of earlier crystallization episodes within the upper crust. The temporal record of Sr and Ba melt contents recorded by the crystals, calculated using partition coefficients, fluctuates significantly within single crystals. Overall, two trends of Sr–Ba variation in the melt are apparent. The first trend involves decreasing Sr and slightly increasing Ba, consistent with plagioclase crystallization. The second trend involves an increase in both Sr and Ba that is not consistent with crystallization of plagioclase alone. This trend is ascribed to chemical variations within the deeper magma reservoirs from which the various magma batches entering the UTP shallow reservoir were ultimately sourced. The magmatic system under Nevado de Toluca was open and received intermittent, but relatively small, pulses of magma from a deeper source. Dacite was the predominant recharge composition, although some pulses were distinctly more mafic and hotter. These processes led to the accumulation of a large volume of dacitic magma in the upper crust. One of the more mafic pulses appears to have triggered the eruption.
KEY WORDS: plagioclase; dacite; trace elements; open-system behaviour; Nevado de Toluca; Trans-Mexican Volcanic Belt
*Corresponding author. Present address: University of Oxford, Research Laboratory for Archaeology and the History of Art, Dyson Perrins Building, South Parks Road, Oxford OX1 3QY, UK. Telephone: +44-1865 285222. E-mail: victoria.smith{at}rlaha.ox.ac.uk