Intermittent mixing processes occurring before Plinian eruptions of Popocatepetl volcano, Mexico: insights from textural-compositional variations in plagioclase and Sr-Nd-Pb isotopes

Volcan Popocatepetl has explosively erupted in Plinian style at least five times in the last 23,000 years. Extreme deviations in composition and the occurrence of dissolution features in plagioclase and pyroxene, and the occasional presence of xenocrysts of Cr-rich Fe-Ti oxides and Mg-rich olivines and pyroxenes indicate that magma mixing has been a major process affecting the magmatic system. The nearly invariant composition of the erupted products (andesitic-dacitic) suggests, however, that mixing is not acting alone and must be balanced by assimilation and/or crystallization. To investigate the magmatic processes that have modified the Plinian magmas, textural and compositional variations and growth rates in plagioclase were used to approximate the frequency of mixing events affecting each magma. Systematic analysis of Sr, Nd, and Pb isotopes was carried out on plagioclase, pyroxene, and pumice matrix glass to constrain the extent of assimilation of upper crustal rocks. Additionally, a series of phase equilibrium experiments were carried out to constrain the depth where such mixing and assimilation occurred. We find that magma was stored at one of two different depths beneath Popocatepetl with magma mixing acting in both reservoirs. Mixing frequency and the relative impact on mineral compositions and textures has varied with time. Assimilation of calcareous rocks underneath Popocatepetl has not been pervasive and does not contribute significantly to the evolution of the Plinian magmas. The similar compositions of magmas with diverse mixing histories suggest that fractional crystallization, and possibly assimilation of deep crust, takes place at depth and that intermediate magmas ascend into the upper crust already differentiated.