Abstract

The modern volcano Popocatépetl is 30 000 – 50 000 years old. Until 5000 years BP, its volcanic activity led to the construction of a 2000 m high cone, the El Fraile volcano. This edifice was later topped by the Popocatépetl summit. The volcanic activity was characterized by long-term construction by lava flows, alternating with periods of 1000–2000 years of mixed explosive and effusive activity. The El Fraile volcano experienced three periods of this type, marked by back-falling pyroclastic flows with heterogeneous magma products and thick air-fall deposits (ash and scoria). The first one occurred more than 10 000 years BP; the second, between 10 000 and 8000 years BP; the third, from 5000 to 3800 years BP. Each of these periods showed violent explosive episodes alternating with lava flows in cycles of 100 to several hundreds of years in duration. Whenever the explosive activity occurred, it destroyed the upper part of the volcano, opening large craters. After a ~ 2500 year period of lava-flow construction (from ~ 3800 to 1200 years BP), the Popocatépetl summit began a similar activity. The last event, producing pyroclastic flows, occurred just before me Hispanic Conquest, and since that time the activity has been effusive and Plinian.Heterogeneous to subhomogeneous pyroclastic flow products exhibit a complex mineralogy: Fe clinopyroxene, Mg clinopyroxene, Fe orthopyroxene, Mg orthopyroxene, plagioclase in equilibrium or disequilibrium, and scarce olivine. All lava flows show a similar paragenesis, suggesting magma-mixing processes. A model in which a basaltic magma is periodically injected in a differentiated chamber at the beginning of each explosive period (or each cycle?) is proposed to explain the heterogeneous products. However, calculations of mixing models do not agree with the high Mg and Ni values observed in some hybrid lavas. This excess is probably due to the remobilization of cumulative olivine by basic magma supplies in the lower part of the reservoir. On the other hand, lava flows emitted during the long phases of effusive activity correspond to evolution in a closed and zoned chamber, partly affected by convective movements. The convection explains the complex mineralogy of these lavas, which result from differentiation of a previously homogenized magma rather than directly from magma mixing.

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