Nevado de Toluca (19 06 30N; 99 45 30 W; 4680 m above sea level), also known as volcán Xinantéctal, is a long-lived stratovolcano located in the central Mexican highland about 80 km southwest of Mexico City (Macias and Arce, 2019). It is part of the Trans Mexican Volcanic Belt, a 1000 km long east-west trending volcanic arc of Miocene to Holocene age that developed in response to the subduction of the oceanic Cocos and Rivera plates beneath the North
American continental lithosphere at different angles (Pardo and Suárez, 1995; Ferrari et al., 2012). The volcanic edifice was built from various short silicic lava flows and domes with intercalated pyroclastic deposits, with the most prominent feature of the volcanoes morphology being a 2.5 ×1.5 km2 horseshoe shaped crater that opens towards the East in direction in the direction of the City of Toluca (Fig. 1.1, Fig. 1.2; Norini et al., 2004).
Detailed stratigraphic and radiometric dating has shown that volcanic activity at Nevado de Toluca started in the Early Pleistocene at 1.5 Ma and was continuous since then, frequently with long periods of dormancy of several thousand years between eruptions (Bloomfield and Valastro, 1974; Garcia-Palomo, 2002; Arce et al., 2003, 2005; Capra et al., 2006; Torres-Orozco et al., 2017). The interplay of three fault systems that intersect beneath the volcano potentially influenced the location and eruptive history of the volcano (Garcia-Palomo et al., 2000; Bellotti et al., 2006), especially several cone destruction episodes by sector collapses that gave rise to a set of widely dispersed debris avalanche deposits around the volcano (Capra and Macias, 2000;
Caballero and Capra, 2011). The volcanos history can be divided into several stages based on geomorphology and age relations (Fig. 1.1b; Torres-Orozco et al., 2017). During the early ‘Old
(Fig. 1.3a, b) between 1.5 and 0.27 Ma in the periphery of the main volcano. Only a few
monogenetic systems have been dated in the area (Bloomfield, 1975; Arce et al., 2013a; Torres-Orozco et al., 2017), but available data point towards a long age range between at least 0.86 Ma to 8.5 ka. The younger eruptive history can be dived into the 0.57 – 0.009 Ma ‘Recent Nevado’
stage, mostly characterized by silicic lava effusion in the crater area, and a sequence of
pyroclastic deposits ‘Young PD stage’ spanning ca. 0.114 to 0.003 Ma (Macias et al., 1997; Arce et al., 2006).
The young pyroclastic deposits (Fig. 1.3c-f) record a complex series of dome
destruction events that are preserved in block and ash flow deposits (Garcia-Palomo et al., 2002), as well as by a sequence of at least three Plinian eruptions (Arce et al., 2003, 2005, 2006; Capra et al., 2006). The oldest and least voluminous of these Plinian events is the 0.85 km3 DRE Lower Toluca Pumice (LTP) that was dated at 26 ka calibrated 14C years BP (Capra et al., 2006). In the following 13 ka activity at Nevado de Toluca was again dominated by dome formation and destruction, notably at about 16500 cal. 14C years BP when a massive block and ash flow of 0.5 km3 was produced (Arce et al., 2006). Renewed activity commenced at approximately 14
thousand years ago when a Plinian eruption of 1.8 km3 DRE gave rise to the Middle Toluca Pumice (MTP) that was distributed mainly to the north-east of the volcano (Arce et al., 2005).
The so far largest and last Plinian-type eruption from Nevado de Toluca, the 8 km3 DRE Upper Toluca Pumice (UTP), was erupted at about 12.5 ka and blanketed the area today occupied by Mexico City within the 10 cm isopach (Arce et al., 2003), demonstrating the large potential hazards emerging from this volcano. Following this event, the dacite dome ‘El Ombligo’ (Fig.
1.2c) was emplaced inside the crater at about 9.5 ka (Arce et al., 2003; Bernal et al., 2014). The last eruptive activity at Nevado de Toluca was described as a phreatomagmatic surge and dated by Macias et al. (1997) with a calibrated 14C age of 3768 years BP.
Nevado de Toluca is a remarkably uniform volcano in terms of its compositional diversity, which is restricted to minor andesite and subalkaline dacite throughout its lifetime (Torres-Orozco et al., 2017). Several detailed petrological studies have been conducted on the Plinian deposits (Arce et al., 2003, 2005, 2006; Martínez-Serrano et al., 2004; Capra et al., 2006;
Smith et al., 2009 Arce et al., 2013), which are summarized here as these eruptions are a major focus of this study. Petrographic analysis of the Plinian pumice showed that the UTP and MTP deposits are identical in terms of their mineral phase assemblage plag>opx>hbl>>ilm+mt+bt.
The LTP rocks, however, lack orthopyroxene as a major phenocryst phase. Notably, this eruption also differs from the preceding events by the presence of schist fragments that are not found in
the MTP and UTP, possibly reflecting different fragmentation or reservoir depth (Arce et al., 2013). Estimates of pre-eruptive conditions (P-T-X-fO2) for all three eruptions have been obtained by experimental phase equilibria and Fe-Ti oxide thermometry (Arce et al., 2006, 2013).
Figure 1.1. Topography and geology of Nevado de Toluca a) Satellite image and digital elevation model (DEM) of the Nevado de Toluca area (source: Google Earth). B) Geological map overlaying the DEM showing the major eruptive stages of the volcano (modified from Torres-Orozco et al., 2017).
Relatively high pre-eruptive temperature conditions have been estimated for the LTP oxide pairs ranging between 861–870°C, which are about 10 to 20°C higher than estimates for the UTP and MTP magmas (Arce et al., 2013). Experimental constraints on storage pressures are similar for the UTP and LTP (between 150 and 200 MPa) at water saturated conditions and are with 200 to 300 MPa slightly higher for the MTP magma (Arce et al., 2006, 2013). This shallow part of the magmatic system evolves as an open system, where fresh pulses of dacitic and
andesitic magma enter the preexisting reservoirs (Smith et al., 2009). Martínez-Serrano et al., (2004) used isotopic (Sr, Nd, Pb) and trace element evidence to show that even though Nevado de Toluca sits on thick continental crust of about 50 km, crustal melting plays only a marginal role in silicic magma genesis. Interestingly, trace element patterns of Nevado de Toluca and the neighboring monogenetics show an adakite-like signatures. The wealth of information from field, geochronologic and petrological studies obtained on Nevado de Toluca makes it an ideal target for more advanced studies to understand magma dynamics in volcanic arcs.
Two consecutive field campaigns have been carried out at Nevado de Toluca during the course of this study in August 2016 and January 2018. The principle goal of this fieldwork was to collect samples representative of the entire eruptive history of the volcano and surrounding monogenetic cones. Further, we focused on stratigraphic relations and sampling of young pyroclastic deposits. A total of 134 samples were collected, of which 97 were analyzed for this thesis. A description of the collected samples and their locations is attached in the electronic supplementary materials (Table E2.1).
Figure 1.2. Selected photographs of Nevado de Tolucas’ crater area from various perspectives.
a) View from south-west showing the main crater domes U31 and U30 and the peripheral dacite dome U27. b) Crater of Nevado de Toluca and Cerro Gordo dome seen from west. c) View into the crater looking southwards. The interior of the crater holds the rounded ‘Ombligo’ dome and two lakes: Laguna de la Luna and Laguna del Sol.
Figure 1.3. Selected outcrop photographs showing the range of eruptive styles and stages of Nevado de Toluca. a) Typical peripheral dacitic lava dome. b) Lava dome U27 in vicinity of the crater. c) Block-and-ash flow deposit. d) Sequence of wet surges and block-and-ash flow
deposits. e) Massive pyroclastic density current deposit (WQ) underlying a fallout layer (line above persons’ heads). e) Plinian fallout (upper coarse white layer), massive and dilute PDCs with cross bedding of the Upper Toluca Pumice eruption.