A 2.5 KA HISTORY OF DACITIC MAGMATISM AT NEVADO DE TOLUCA, MEXICO: PETROLOGICAL, 40AR/39AR DATING, AND EXPERIMENTAL CONSTRAINTS ON PETROGENESIS

Abstract

After 11.5ka of quiescence (24.5-13ka), the Nevado de Toluca volcano started a 2500 year period of activity. This period was characterized by a dome destruction event at 13ka, a small Plinian event at 12.1ka, and a large Plinian eruption at 10.5ka. About 10.km3 of magma was erupted that was homogeneous in composition (63.3-65.7 SiO2 wt % whole-rock) and in mineralogy. Pumice consists of plagioclase (An30-59) > orthopyroxene (En56-59) > hornblende ≫-Ti oxides + rare apatite (in opx) + biotite, set in a rhyolitic matrix (72-76 SiO2 wt %). 40 Ar-39Ar analysis of single biotite crystals yielded ages (0.81-4.7Ma), that do not correspond to eruption ages. The biotite represents partially assimilated xenocrysts, which could have resided in the magma for only a short period of time. Mineral chemical data, coupled with hydrothermal experiments, indicate that prior to eruption the dacitic magma stagnated at a depth of 4.5-6km below the summit at water pressures of 160-210MPa and a temperature of 824 ± 12 °C on the basis of Fe-Ti oxide thermometry, and under water-saturated conditions. To stabilize a homogeneous magma body of >10km3 at 824°C in the upper crust, we propose that reheating of the dacitic reservoir by hotter magma batches was able to maintain the equilibrium between the temperature of the magma and the assimilation of wall-rock over a period of 2500 years. Based on similarities among the juvenile products, we suggest that the three eruptions were fed from the same magma body. © The Author 2005 Published by Oxford University Press. All rights reserved.