Pre-eruptive conditions of dacitic magma erupted during the 21.7 ka Plinian event at Nevado de Toluca volcano, Central Mexico

Abstract

The Nevado de Toluca volcano in Central Mexico has been active over the last ca. 42ka, during which tens of km3 of pyroclastic material were erupted and two important Plinian-type eruptions occurred at ca. 21.7ka (Lower Toluca Pumice: LTP) and ca. 10.5ka (Upper Toluca Pumice: UTP). Samples from both the LTP and UTP contain plagioclase, amphibole, iron-titanium oxides, and minor anhedral biotite, set in a vesicular, rhyolitic, glassy matrix. In addition, UTP dacites contain orthopyroxene. Analysis of melt inclusions in plagioclase phenocrysts yields H2O contents of 2–3.5wt.% for LTP and 1.3–3.6wt.% for UTP samples. Ilmenite–ulvospinel geothermometry yields an average temperature of ~868°C for the LTP magma (hotter than the UTP magma, ~842°C; Arce et al., 2006), whereas amphibole–plagioclase geothermometry yields a temperature of 825–859°C for the LTP magma. Water-saturated experiments using LTP dacite suggest that: (i) amphibole is stable above 100MPa and below 900°C; (ii) plagioclase crystallizes below 250–100MPa at temperatures of 850–900°C; and (iii) pyroxene is stable only below pressures of 200–100MPa and temperatures of 825–900°C. Comparison of natural and experimental data suggests that the LTP dacitic magma was stored at 150–200MPa (5.8–7.7km below the volcano summit). No differences in pressure found between 21.7ka and 10.5ka suggest that these two magmas were stored at similar depths. Orthopyroxene produced in lower temperature LTP experiments is compositionally different to those found in UTP natural samples, suggesting that they originated in two different magma batches. Whole-rock chemistry, petrographic features, and mineral compositions suggest that magma mixing was responsible for the generation of the dacitic Plinian LTP eruption.