A 2.5 ka History of Dacitic Magmatism at Nevado de Toluca, Mexico: Petrological, 40Ar/39Ar Dating, and Experimental Constraints on Petrogenesis

Abstract

After 11Á5 ka of quiescence (24Á5-13 ka), the Nevado de Toluca volcano started a 2500 year period of activity. This period was characterized by a dome destruction event at 13 ka, a small Plinian event at 12Á1 ka, and a large Plinian eruption at 10Á5 ka. About 10 km 3 of magma was erupted that was homogeneous in composition (63Á3-65Á7 SiO 2 wt % whole-rock) and in mineralogy. Pumice consists of plagioclase (An 30-59) > orthopyroxene (En 56-59) > hornblende) Fe-Ti oxides þ rare apatite (in opx) þ biotite, set in a rhyolitic matrix (72-76 SiO 2 wt %). 40 Ar/ 39 Ar analysis of single biotite crystals yielded ages (0Á81-4Á7 Ma), 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 4Á5-6 km below the summit at water pressures of 160-210 MPa and a temperature of 824 AE 12 C on the basis of Fe-Ti oxide thermometry, and under watersaturated conditions. To stabilize a homogeneous magma body of >10 km 3 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.

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

“…Ilmenite and minor titanomagnetite are present with homogenous composition averaging Ilm 0.88 and Usp 0.32 . By using the Fe-Ti oxide geothermometry (Andersen and Lindsley, 1988) an equilibrium temperature of 842-858°C and ƒO 2 of − 12.73 ± 0.2 for the MTP magma was estimated (after Arce et al, 2006).…”

“…Ilmenite and lesser titanomagnetite are presently attached to phenocrysts or as individual crystals in the matrix with Ilmenite as Ilm 0.83-0.87 and titanomagnetite Usp 0.27-0.48 . Fe-Ti oxide geothermometry (Andersen and Lindsley, 1988) gave an average temperature of 842°C and ƒO2 of − 12.0 (from Arce et al, 2006). Slightly different temperatures of 828°C for the UTP were obtained with the QUILF geothermometer (Andersen et al, 1993).…”

“…1) has erupted tens of cubic kilometers of mainly dacitic magma over a period of~42 ka (Cantagrel et al, 1981;Macías et al, 1997;García-Palomo et al, 2002;Arce et al, 2006), with similar mineral assemblages of plagioclase (with apatite inclusions) + amphibole + Fe-Ti oxides ± orthopyroxene. Among the products of this explosive period, three important sub-Plinian to Plinian eruptions have been documented: (i) the Lower Toluca Pumice (LTP: 21.7 ka; Capra et al, 2006); (ii) the Middle Toluca Pumice (MTP: 12.1 ka; Arce et al, 2005); and (iii) the Upper Toluca Pumice (UTP: 10.5 ka; Arce et al, 2003).…”

“…1981; Macías et al, 1997;García-Palomo et al, 2002;Arce et al, 2006). During that latter explosive period, the volcano produced block-and-ash flows, plinian fall deposits, pumice-rich pyroclastic flows, and pyroclastic surges that contain both dense blocks and highly vesicular pumice fragments (Bloomfield et al, 1977;Macías et al, 1997;García-Palomo et al, 2002).…”

“…Analysis of melt inclusions in plagioclase phenocrysts yields H 2 O contents of 2-3.5 wt.% for LTP and 1.3-3.6 wt.% 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 100 MPa and below 900°C; (ii) plagioclase crystallizes below 250-100 MPa at temperatures of 850-900°C; and (iii) pyroxene is stable only below pressures of 200-100 MPa and temperatures of 825-900°C.…”