The ~ 2000 yr BP Jumento volcano, one of the youngest edifices of the Chichinautzin Volcanic Field, Central Mexico

Journal of Volcanology and Geothermal Research 308 (2015) 30–38

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The ~ 2000 yr BP Jumento volcano, one of the youngest edifices of the Chichinautzin Volcanic Field, Central Mexico J.L. Arce a,⁎, E. Muñoz-Salinas a, M. Castillo a, I. Salinas b a b

Instituto de Geología, UNAM, Cd. Universitaria Coyoacan, México, D.F. 04510, Mexico División de Ciencias de la Tierra, Facultad de Ingeniería, UNAM, Cd. Universitaria Coyoacan, México, D.F. 04510, Mexico

a r t i c l e

i n f o

Article history: Received 30 May 2015 Accepted 7 October 2015 Available online 22 October 2015 Keywords: Jumento volcano Chichinautzin Volcanic Field Radiocarbon dating Morphometric analysis Monogenetic volcanism

a b s t r a c t The Chichinautzin Volcanic Field is situated at the southern limit of the Basin of Mexico and the Metropolitan area of Mexico City, the third most populated city around the world. The Chichinautzin Volcanic field holds more than 220 monogenetic volcanoes. Xitle is the youngest of these with an estimated age of 1.6 ky BP. Xitle's eruptive activity took place during the Mesoamerican Mexican Pre-classic period and is related to the destruction of Cuicuilco Archaeological Site, the oldest civilization known in Central Mexico. However, there are still several regional cones that have not been dated. Based on 14C ages, stratigraphic and geomorphologic criteria, we conclude that the Jumento volcano, located to the west of Xitle, is one of the youngest cones of the Chichinautzin Volcanic Field. The Jumento volcano has a basaltic andesite composition, and its eruptive activity was initially hydromagmatic, followed by Strombolian and finally effusive events occurred recorded through: (1) a sequence of hydromagmatic pyroclastic surges and ashfall layers emplaced at a radius of N5 km from the crater with charcoal fragments at its base; this activity built the Jumento's cone with slopes of 32°; and (2) lava flows that breached the southern part of the cone and flowed for up to 2.5 km from the vent. The resulting 14C ages for this volcano yielded a maximum age of ~2 ky BP. Morphometric analysis indicates that the state of degradation of Jumento cone is similar to the Xitle, suggesting that the Jumento could be in the state of degradation of a volcanic structure of similar age or younger adding credence to the probable radiocarbon age of ~2 ky BP for the Jumento edifice. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The Trans-Mexican Volcanic Belt (TMVB) has captured the attention of the scientific community during the last decades for its unusual configuration (Ferrari et al., 2012). The TMVB runs through the central part of Mexico along the 19°N parallel as a result of the oblique subduction of the Rivera and Cocos plates beneath the North American Plate. Chains of mountains and high-elevated plateaus of ~ 2,200 m above sea level constitute the topography of the TMVB where some active stratovolcanoes are more than 4,000 m in elevation. Several monogenetic volcanic fields constitute the landscape of the TMVB, such as in the regions of the Michoacan-Guanajuato, Zitácuaro-Valle de Bravo, and Chichinautzin (Hasenaka and Carmichael, 1985). In central Mexico, the study of the eruptive activity of the TMVB has mainly focused on the Popocatépetl volcano. This edifice is an active stratovolcano considered as one of the most dangerous volcanoes around the world because it is placed less than 50 km away from Mexico City, which is the third most populated city on Earth with more than 20 million inhabitants (INEGI, 2010). Siebe et al. (1996) ⁎ Corresponding author. E-mail address: [email protected] (J.L. Arce).

http://dx.doi.org/10.1016/j.jvolgeores.2015.10.008 0377-0273/© 2015 Elsevier B.V. All rights reserved.

reported that the decline of the Cholula Civilization took place at 1.1 ky BP, due to the emission of pyroclastic flows and lahars from Popocatépetl volcano. Such catastrophic event can be compared in terms of human destruction as the annihilation of Pompey by a Plinian eruption in 79 AC from the Vesuvius volcano in Italy. Although the magnitude of Plinian eruptions from stratovolcanoes cannot be compared with effusive and Strombolian activity typical of monogenetic volcanoes, the latter can cause economical damage and destruction of villages and cities. For example, the lava flows emitted from Xitle volcano in central Mexico some 1.6 ky BP inundated the Cuicuilco Archaeological Site (Siebe, 2000), destroying buildings of the town site. In modern times, the destruction of the villages of Paricutin and San Juan Parangaricutiro, in the State of Michoacán, between 1943 and 1952 was caused by the emission of ash and lava flows emitted by Paricutin volcano (Foshag and González, 1956; Luhr and Simkin, 1993). The latter has been the first monogenic cone studied in detail by the scientific community from its birth in a corn field to its extinction (Luhr and Simkin, 1993; Erlund et al., 2010; Pioli et al., 2008). Having in mind the potential destructive capabilities of monogenetic volcanoes, both the Chichinautzin Volcanic Field (CVF) and Popocatépetl volcano must be considered when assessing the volcanic risk to inhabitants of Mexico City. Nevertheless, only a few works have focused on the

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study of the eruptive activity of CVF volcanism (Bloomfield, 1975; Martin del Pozzo, 1982; García-Palomo et al., 2002; Siebe, 2000; Márquez et al., 1999; Siebe et al., 2004a; Meriggi et al., 2008; Guilbaud et al., 2012, 2015). Considering the large number of monogenetic structures in the CVF (Siebe et al., 2005), more detailed research is needed for more accurate volcanic hazard predictions. In this paper, we focus on one of the monogenetic volcanoes located in the central part of CVF, the Jumento volcano (JV) reporting field descriptions, whole-rock chemical analyses and new radiocarbon ages. The importance of the results allow (1) a better understanding of the current volcanic activity in the TMVB and (2) better constraints on the volcanic risk to Mexico City associated with eruptive activity in the CVF.

2004a; and Agustin-Flores et al., 2011). However, this CVF age range (38–1.6 ky) was related to the 14C method limitations. In fact, older monogenetic structures have just recently been dated using the 40 Ar/39Ar method, yielding ages as old as ~1.2 Ma and spanning through late Pleistocene at 0.8, 0.4, 0.2, 0.09 Ma (Arce et al., 2013). Eruption rates at the Chichinautzin Volcanic Field have been estimated to be 0.6 km3/kyr (Siebe et al., 2005) for Holocene volcanism, including Xitle volcano, and an equivalent rate of 0.016 km3/kyr per 100 km2 was estimated for the whole volcanic field (Arce et al., 2013).

2. Study area

JV is located in the central part of the CVF where the field overlaps the polygenetic volcanoes of the N-S aligned Sierra de Las Cruces volcanic range (Fig. 1). Products from Sierra de Las Cruces are porphyritic andesite and dacite lava flows and pyroclastic deposits that range in age from ~ 3.7 to ~ 0.7 Ma (Osete et al., 2000; Mejia et al., 2005; García-Palomo et al., 2008; Arce et al., 2008). JV was formed inside a smooth and wide depression, surrounded by Sierra de las Cruces volcanics in the N, E, and W, and by products from other Chichinautzin volcanoes to the S. JV is a scoria cone with steep flanks (32°) and no signs of fluvial erosion on its slopes. The cone is opened to the south, where at least three lava flows are exposed (Figs. 2 and 3). These lava flows overlap each other and have steep slopes in their fronts and lateral levees averaging ~20 m in thickness; indicating relatively high viscosity during flowing. We distinguish three main lava flows (LF-1, LF-2, and LF-3) and small blocky lava flows defined as “breakout flows” (Fig. 2) this latter one probably was a product of reactivation of overlapped lava flows, as discussed later.

2.1. Chichinautzin Volcanic Field The CVF is constituted by more than 220 volcanic structures formed by either scoria cones, lava flows, or lava domes of a wide compositional range (i.e., basalt to dacite) (Márquez et al., 1999; Wallace and Carmichael, 1999; Meriggi et al., 2008; Siebe et al., 2004a; Straub et al., 2013). The area occupied by the CVF is elongated and is bounded to the east by Popocatépetl volcano and to west by Nevado de Toluca volcano (Fig. 1). The knowledge of the eruptive activity in the CVF has been restricted to the few published geological studies that contain radiocarbon dates from less than a quarter of the total number of the volcanic structures composing the whole volcanic field. According to these ages, it had been generally accepted that the CVF evolution ranged between ~38 ka BP at Coaxusco volcano to 1.6 ka BP, at Xitle volcano (Bloomfield, 1975; Siebe, 2000; García-Palomo et al., 2002; Siebe et al.,

2.2. Jumento volcano

Fig. 1. (A) Location map of the Chichinautzin Volcanic Field within the Trans-Mexican Volcanic Belt (TMVB), showing the most important volcanic edifices and cities. LTVF, Los Tuxtlas Volcanic Field. (B) Distribution of the Chichinautzin Volcanic Field products (modified from Bloomfield, 1975; Márquez et al., 1999; Arce et al., 2013) showing the distribution of dated volcanic products summarized in previous works (Márquez et al., 1999; Siebe et al., 2004a), 40Ar/39Ar ages reported in Arce et al. (2013). Abbreviations: SM, San Miguel; LC, La Corona; ZEM, Zempoala volcanoes form the Sierra de las Cruces range. Normal faults: La Pera (Siebe et al., 2004a, 2005); Tenango (García-Palomo et al., 2000); Xochimilco (García-Palomo et al., 2008).

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J.L. Arce et al. / Journal of Volcanology and Geothermal Research 308 (2015) 30–38

Fig. 2. (A) Digital elevation model of the Jumento volcano. The three main lava flow units (labeled LF), the breakout flows (Brk-F) in several parts on the flows, the main cone and an accumulation of scoria fallout probably related to the same eruption located immediately to the NE of the cone are shown. Rock sampling sites are also indicated. (B) Photograph showing the scoria fall deposit from the lobe-shaped, located just to the NE of the main Jumento cone. The person is 1.75 m, for scale. (C) Photograph showing one of the breakout flows, located on the LF-3. For scale, the tree is about 15 m high.

Fig. 3. Panoramic view to the north of the Jumento volcano cone and lava flows 2 and 3 (see Fig. 2 for the location of the lava units).

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The three lava flows show a plateau-like morphology with a gently sloping surface (Fig. 3). Because they are overlapped in some areas, it was difficult to determine individual areas and volumes. Total area of the JV is 2.8 km2 and considering an average thickness of 20 m, a total volume of 0.056 km3 is estimated for the three lava units, whereas the cone yielded a volume of 0.04 km3. These volumes demonstrate that the JV is one of the smallest volcanoes in the CVF, with only the lava flow volume of the Hijo del Cuauhtzin volcano, estimated in 0.04 km3 (Siebe et al., 2005) being comparable. Lava flow 1 (LF1) distributes ~ 2.5 km far away from the cone, was emplaced toward the S-SW, with 15-m high fronts. Lava 2 (LF2) has flowed a shorter distance of ~ 1.5 km from the crater and is also emplaced at the SW of the cone, and this lava flow has a mean thickness of about 15 m. Lava 3 (LF3) was emplaced centrally on top of Lava 1 and 2, it reached ~ 2.1 km from the cone, and has steep fronts of 20 m of thickness. A series of blocky, soil-free, small flows were identified on the surface of lavas, that were classified as “breakout lava flows” (Brk-F), based on the descriptions of such type of products in the Etna's 2001 lava flows (Applegarth et al., 2010). These breakout flows were emitted from fissures, observed at the southern base of the cone (Fig. 4). These breakouts flows are restricted to small depressions, the largest one is ~30 m wide and ~300 m in length (Fig. 4). It is highly possible that these breakout flows were emitted after the other three lavas, although it is not possible at this time to say exactly when. Because these breakout flows are soil (and vegetation)- free, it could indicate a large period of time between the emplacement of the last lava flow and the breakout flows, however it is unlikely because of the short eruptive histories of most monogenetic volcanoes. An alternate explanation could be that the breakout flows have a blocky morphology, being a difficult environment

Fig. 4. (A) Photograph showing a localized lava surface breakout, coming from vertical fractures. (B) Photograph of the breakout flow (Brk-flow) constituted of blocky lava. Notice the absence of vegetation compared to the previous lava flow 3 (LF-3).

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for the development of soil. Unfortunately, we do not have ages for each lava flow and the breakout flows, to precisely determine the elapsed time between them. Underneath the lavas we recognized a deposit formed by friable, alternated layers of coarse to fine ash, each layer is well sorted, containing subangular particles, with a constant thickness of ~40 cm (Fig. 5A). These features strongly suggest that it is an ashfall deposit. Fragments consist of juvenile basaltic andesite dense fragments and crystals of plagioclase and olivine plus vesicular glass shards. Just beneath the ashfall deposit we identified a series of undulated and hardened layers (compared to overlying ashfall layers), made of fine ash, developing undulated structures, poorly sorted, and contain some rounded, fine lapilli fragments (Fig. 5A and B). This sequence of layers has variable thickness (~ 7 cm) and shows an erosive basal contact. All of these diagnostic features support that this deposit was produced by a wet pyroclastic flow. This wet surge deposit eroded parts of the underlaid black-paleosoil, incorporating some charcoal fragments from this pre-existing deposit (i.e., the paleosol) (Fig. 5B). According to these observations, the stratigraphic sequence of the JV can be interpreted as follows: (1) the emission of a dilute pyroclastic density current (wet surge) that eroded pre-existing deposits (paleosol), (2) ashfall materials deposited from a vertical low-altitude pyroclastic column, and (3) emission of lava flows from the southern part of the cone. The eruptive event started with hydromagmatic explosions that produced wet surges, followed by the emission of fallout materials that built the main cone, and finally the southern sector of the cone was destroyed by the outflow of lavas.

Fig. 5. (A) Photograph of a representative section of pyroclastic deposits from the Jumento volcano. The sequence starts with a pyroclastic density current, followed by an ashfall layer and ends with a lava flow (LF-1). Location of the SM-1304A charcoal sample (1.1 ky BP) is shown. (B) Detail of the base of the Jumento volcano sequence. Notice the erosive nature of the pyroclastic density current (wet surge) incorporating paleosoil and charcoal fragments. The location of the SM-1304Bis charcoal sample (that yielded 2.3 ky BP radiocarbon age) is indicated by an arrow.

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3. Samples and analytical methods

Table 2 Whole-rock chemical analysis of the Jumento volcano samples.

3.1. Radiocarbon dating Fragments of charcoal were found and collected at the base of the wet surge deposit at two distinct locations in order to obtain radiocarbon ages. To collect the charcoal fragments, during fieldwork, ~ 10 kilos of ash sediment was extracted and placed on the surface of white paper sheets. Using hand lenses, we identified and picked up small fragments (about 1 mm in size) of charcoal until ~ 100 mg of charcoal was obtained, enough to be dated. Afterward, charcoal samples were stored in plastic bags. A total of four different charcoal samples were obtained from two locations (Table 1), from the same stratigraphic level, all within the basal wet pyroclastic surge deposit. In the lab, the fragments of charcoal were dried in an oven at a temperature of 35 °C for 24 h, and then they were packed in labeled plastic bags and sent to Beta Analytic Laboratory (USA) for radiocarbon dating (Table 1) by using accelerator mass spectrometry (AMS) technique. 3.2. Whole-rock chemical analysis We processed a total of six rock samples for whole-rock chemical analyses collected from the lava flows and from scoria taken from the cone (Table 2). The samples were crushed and cleaned to avoid altered material by using a brush and distilled water and then crushed. The cleaned fragments were dried in an oven at 100 °C for 24 h. Because quartz minerals were observed and determined as xenocrystic (see the results section), samples were also picked to remove all of quartz crystals manually, by crushing the samples and using hand lenses and stereoscopic microscope during picking. X-ray fluorescence analysis of major elements, with precisions of b1%, were carried out at the Laboratorio Universitario de Geoquímica Isotópica (LUGIS) at UNAM (México) following the methodology described in Lozano et al. (1995). Trace elements, including rare earth elements, were analyzed at Centro de Geociencias, UNAM, by the Inductively Coupled Mass Spectrometer (ICP-MS) method, following the procedures described in Mori et al. (2007). 3.3. Morphometric analysis for the JV and other young reported volcanoes We performed morphometric analysis of the JV cone and two other dated cone structures in order to evaluate and compare the relative ages of the JV. According to Wood (1980), the longer the time of erosion, the higher the reduction of the values of slope (S) and ratio of elevation (Hco) vs width (Wco) of the cone. Therefore, by plotting the value of Hco against Wco, and of S against the ratio of Hco/Wco for different scoria cones, the youngest structures will have higher values and the oldest ones lower values. The two volcanoes chosen for comparison are Xitle and Paricutin. Xitle volcano was selected because it is the youngest volcano reported in the CVF and Paricutin volcano was selected because is the youngest monogenetic volcano in the TMVB (1943–1952 A.C.). Xitle and Jumento cones are located in a similar regional area, with similar climatic conditions (temperature and rainfall distribution along the year) according to the National Weather Service of Mexico (http://smn.cna.gob.mx; 2015),

Sample

JU 1401

JU1402

JU1403

JU1404

JU1407

SM-1304

SiO2 TiO2 Al2O3 * Fe2O3t MnO MgO CaO Na2O K2O P2O5 LOI Total Li Be B Sc V Cr Co Ni Cu Zn Ga Rb Sr Y Zr Nb Mo Sn Sb Cs Ba La Ce Pr Nd Sm Eu Tb Gd Dy Ho Er Yb Lu Hf Ta W Tl Pb Th U

53.81 1.04 15.85 7.42 0.12 7.18 7.71 3.95 1.48 0.38 0.88 99.81 12.39 1.37 5.32 16.68 132.15 137.28 25.87 37.09 33.18 69.79 17.52 21.49 716.68 21.55 144.42 6.74 1.04 1.23 0.10 0.73 675.92 29.55 65.67 9.23 38.38 7.99 2.07 0.84 6.46 4.23 0.77 2.01 1.73 0.25 3.51 0.43 1.93 0.12 8.58 3.73 1.17

55.02 0.97 16.16 7.42 0.11 6.97 7.39 4.00 1.35 0.31 −0.07 99.63 12.13 1.32 6.12 16.49 128.70 127.05 24.90 35.46 31.10 66.90 17.65 20.56 607.94 19.79 134.23 5.49 1.01 0.85 0.09 0.70 469.05 22.22 48.33 6.82 28.82 6.17 1.67 0.71 5.24 3.78 0.71 1.88 1.66 0.25 3.36 0.35 1.88 0.13 6.42 2.87 0.94

54.44 1.07 16.14 7.57 0.12 7.10 7.44 3.95 1.36 0.33 0.14 99.66 12.46 1.41 6.45 17.82 134.54 157.92 27.02 36.26 36.62 70.32 18.04 20.96 646.45 21.33 146.92 7.51 1.24 0.98 0.11 0.74 557.07 24.43 53.62 7.47 31.37 6.70 1.80 0.77 5.66 4.08 0.76 2.02 1.81 0.26 3.54 0.47 4.70 0.82 8.67 3.19 1.04

55.23 1.07 16.05 7.48 0.12 7.13 7.42 3.97 1.40 0.33 −0.21 99.99 12.74 1.41 5.90 17.74 135.65 142.45 26.82 36.70 32.84 70.47 18.02 20.99 642.92 21.40 147.46 7.47 1.06 0.94 0.10 0.73 561.52 24.44 53.45 7.50 31.30 6.69 1.80 0.77 5.68 4.08 0.76 2.02 1.79 0.26 3.54 0.47 2.18 0.13 7.34 3.15 1.04

54.84 1.10 15.92 7.58 0.12 7.33 7.63 3.94 1.39 0.36 −0.18 100.03 11.72 1.38 5.78 17.50 135.30 142.11 26.94 37.90 33.73 75.53 17.70 20.38 688.11 21.77 148.83 7.17 1.06 0.92 0.10 0.67 622.13 27.08 59.77 8.40 35.09 7.42 1.95 0.82 6.11 4.23 0.78 2.05 1.79 0.26 3.60 0.44 2.03 0.14 7.82 3.41 1.08

55.74 1.24 17.43 7.15 0.12 5.48 6.33 4.36 1.74 0.35 −0.17 99.75 17.72 1.94 14.86 113.85 213.16 26.43 126.09 30.07 76.16 19.03 31.52 720.35 24.52 210.39 15.69 2.50 1.20 0.11 1.19 592.75 29.29 64.15 8.66 35.37 7.36 2.03 0.85 6.10 4.62 0.87 2.35 2.14 0.32 4.62 0.96 3.14 0.15 7.56 4.11 1.46

Values are reported in normalized wt.% (major) and ppm (trace) elements. LOI, loss on ignition; * Total Fe as Fe+2. For sample location, see Fig. 2.

and the three cones are built of scoria fragments. We also include in the comparison, the morphometric analysis results from volcanoes of the San Francisco Volcanic Field, Sunset crater and Merriam and Tappan volcanoes reported by Wood (1980).

Table 1 Results of the accelerator mass spectrometry (AMS) radiocarbon ages of the Jumento volcano. Sample

Conventional age

Calibrated age (2σ)

C-13/C-12 ratio

Lab. Code

Latitude N

Longitude W

Altitude (masl)

SM-1304A JU-1405Bis JU-1405 SM-1304-Bis

1160 ± 30 yr BP 2010 ± 30 yr BP 2010 ± 30 yr BP 2230 ± 30 yr BP

1170 to 1050 yr BP 2030 to 2025 yr BP 2035 to 2025 yr BP 2335 to 2150 yr BP

24.1 25.7 23.8 23.3

Beta-356313 Beta-396661 Beta-379430 Beta-396662

19°11′28″ 19°12′01″ 19°12′01″ 19°11′28″

99°19′25″ 99°18′26″ 99°18′26″ 99°19′25″

3503 3609 3609 3503

All analyses were done using charcoal fragments sampled from the wet pyroclastic surge deposit (see text). All ages were obtained by accelerator mass spectrometry method at beta analytic (Miami, Florida). Conventional ages were calibrated using Calib 7.0 (Stuiver and Reimer, 1993).

J.L. Arce et al. / Journal of Volcanology and Geothermal Research 308 (2015) 30–38

For obtaining the morphological parameters of S, Wco, and Hco, we used a digital elevation model of 10 m of pixel interpolated from a terrain model obtained from an airborne Light Detection and Ranging (LIDAR) technology that was downloaded from the website of the Mexican Institution in charge of the geographical and statistical analysis known as INEGI (http://www.inegi.org.mx; 2014). The morphometric analyses on the DEM were done by using the Geographic Information System Arc Gis 10.1. 4. Results 4.1. Radiocarbon dating results The four charcoal samples analyzed were collected from the same unit (basal wet surge deposit; Fig. 5), from two different locations. At one location (samples JU-1405 and JU-1405Bis), samples were taken about 10 m of distance from each other and yielded similar ages 2010 ± 30 yr BP, while in the other location (samples SM-1304A and SM-1304Bis), the samples were taken in two different dates and a different levels in the wet surge deposit and yielded different ages (Table 1; Fig. 5). The age obtained for sample SM-1304A of 1160 yr BP is younger than Xitle (~ 1600 yr BP, Siebe, 2000), and for sample SM1304Bis, the age is 2230 ± 30 yr BP. The data suggest a most probable age of ~ 2 ky BP, posing the JV as one of the youngest edifices of the Chichinautzin Volcanic Field. 4.2. Whole-rock chemistry and petrography The four lava flows are petrographically and compositionally similar. In hand samples, they are gray in color and exhibit a porphyritic texture

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with anomalous large phenocrysts of quartz and plagioclase (2–4 mm). These large quartz and plagioclase crystals are probably xenocrystic (see below). In contrast, the rest of phenocrysts with sizes of ≤1 mm in diameter, observed only by using hand lenses, consist of plagioclase, pyroxene, and olivine crystals. Under the microscope, samples display a mineral assemblage of plagioclase, olivine, and minor pyroxene phenocrysts, set in a microlitic and glassy matrix (Fig. 6). Small plagioclase phenocrysts are subhedral to euhedral, while the largest ones (2 mm) commonly show disequilibrium textures (i.e., sieve texture, corroded margins) consistent with their xenocrystic origin. Large phenocrysts of quartz (2–4 mm) are anhedral, with rounded borders and sometimes with a reaction rim made of microliths of clinopyroxene, attesting to a xenocrystic nature. In fact, crustal xenoliths have been reported elsewhere in the CVF rocks (Meriggi et al., 2008; Arce et al., 2013), consisting mainly of granodiorites and tonalites. Chemical analyses of the lava reveal a restricted compositional range, with SiO2 contents from 53.8 to 55.7 wt.% and alkalis (Na2O + K2O) varying from 5 to 6 wt.%. All samples lie in the limits between basaltic trachy-andesitic to basaltic-andesitic fields, in a total alkalis versus silica diagram (Fig. 7) belonging to the calc-alkaline suite. These compositions are typical since the basaltic andesites are the most common rock composition in the Chichinautzin Volcanic Field (Siebe et al., 2004b; Meriggi et al., 2008; Straub et al., 2013; Arce et al., 2013). On the trace elements diagrams (Fig. 7), samples have the same behavior, with negative anomalies of Nb, Ta, and Ti, and positive in Pb, K, and Cs, typical of volcanoes in subduction zones. A weak negative Eu anomaly suggests early plagioclase fractionation (Holme et al., 1982). In summary, the Jumento composition is typical of CVF products.

Fig. 6. Representative photomicrograph of Jumento volcano samples. Mineral assemblage is constituted by phenocrysts and microphenocrysts of plagioclase, olivine, clinopyroxene, Fe-Ti oxides, set in a microlitic and glassy matrix. Large quartz crystals (N1 mm), surrounded by clinopyroxene microcrysts rim, are considered xenocrysts showing disequilibrium textures.

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Fig. 7. (A) Total alkalies vs silica (TAS) classification diagram of Le Bas et al. (1986) for the Jumento volcano samples. Major elements in wt. % (normalized on anhydrous basis). Alkaline versus subalkaline fields are separated by the Macdonald and Katsura (1964) line. (B) Trace element concentrations in whole-rock samples from the Jumento volcano samples, normalized to primitive mantle (Sun and McDonough, 1989). (C) Chondrite-normalized (Sun and McDonough, 1989) rare earth element concentrations for the Jumento volcano samples. Previously analyzed samples from the CVF (gray field) are those summarized in Agustin-Flores et al. (2011) and Arce et al. (2013).

Fig. 8. (A and B) Shaded maps of the Jumento and Xitle volcanoes respectively, where transect profiles (A–A′ and B–B′) are traced, showing their morphometric characteristics: Hco, cone height; S, slope; Wco, base cone diameter.

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4.3. Morphometric results for the JV and other young reported volcanoes New morphometric results for the Jumento, Xitle, and Paricutin cones are shown on Fig. 8A and B and compared with existing values calculated for San Francisco Volcanic Field, Sunset crater, and Merriam and Tappan volcanoes (Moore et al., 1976; Wood, 1980) on Fig. 9A and B. In these plots, JV (~ 2 ky BP) has a very similar morphology to Xitle (1.6 ky BP) and is significantly smaller and more eroded (lower Hco/Wco) than Paricutin, indicating that the cone of the JV is very similar in age to the age of the Xitle cone. Additionally, in Fig. 8, an incipient erosion sign (drainage) is observed for the Xitle cone, yet the Jumento cone does not show signs of this erosional signal yet. However, this can be related to the more mafic composition (and vesicularity of scoria) of Xitle, as well as to human activity at the Xitle cone. 5. Discussion 5.1. Radiocarbon and morphometric age significance According to our results we propose that JV is one of the youngest cones of the CVF, and probably but not conclusive, younger than Xitle volcano, as supported by the comparative morphometric analysis. However, it is necessary to use other types of methods to date the lavas directly, to prove or discard the younger age interpretation for the JV. With the available radiocarbon data (Table 1), the ~ 2 ky BP represents the most probable age of the Jumento eruptive event. The dilute pyroclastic density current (wet pyroclastic surge; b100 °C in temperature) was not able to burn vegetation but eroded the underlying paleosoil containing charcoal fragments. A possible explanation for the incorporation of charcoal fragments clustered to ~2 ky BP age, could be a fire that burned the vegetation and produced charcoal before the eruption. Fires in the past have been reported in many

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investigations (Higuera et al., 2010; Power et al., 2008) even in prehuman epochs. Evidences of erosion by the wet surge were observed at several localities (i.e., SM-1304; Figs. 2 and 5B), where the incorporation of soil into the surge deposit occurs; thus, the ~2 ky BP age would be a maximum age of the pyroclastic surge and marks the starting point of the depositional sequence of the JV. In addition, the morphometric analysis (Figs. 8 and 9) also suggests that JV could be as young or younger than the Xitle volcano. Although this hypothesis is dubious, since the human activity could affect the morphometric parameters for Xitle cone, the younger age of Jumento volcano cannot be discarded with the available data up to now. The age of Jumento volcano is of great relevance because Xitle's age (~1.6 ka BP) has been used to calculate the eruptive recurrence interval of 1250 yr for the CVF. The age of JV (~2 ky BP) as one of the youngest volcanic structure in the CVF supports the 1250 yr eruptive recurrence interval proposed by Siebe et al. (2005). However, small-size volcanoes like Jumento, have been understudied, and hence not considered for published calculated recurrence intervals. This fact is of great importance for the studies of volcanic risk for the Metropolitan area of Mexico City. 5.2. Evolution of the Jumento eruption Based on the stratigraphic record, the eruption of Jumento started with a hydromagmatic activity. The ascending trachyandesitic magma interacted with external water producing a hydromagmatic explosion (Sheridan and Wholetz, 1983) that emplaced dilute pyroclastic density currents (wet surge deposits) at least to the south of the volcano, where the outcrops are preserved. It is likely that these deposits are preserved because they were immediately covered by the lava flows in this area, while in other sectors they were eroded. This explosion cleared the conduit from which immediately afterward formed a low-altitude vertical column that lasted a few hours or days in an intermittent fashion. Apparently, this vertical column was dispersed to the south, depositing a stratified ash and lapilli-size scoria fallout that edified the main cone. Then, the volcanic activity changed to an effusive fashion, destroying the south portion of the cone and emplacing three trachy-andesitic lava flows (LF-1 to LF-3). The volcanic activity ceased at this point for some time. However, a reactivation of the buried lava flow occurred to emplace the breakout flows (Fig. 2). The volume of the breakout flows is small, and apparently they were emitted from fractures (Fig. 4B), where compression cracks produced during the emission are present. This reactivation could have happen by the superimposed flows that pressurized the underlying lavas, leading to crust rupture and the subsequent extrusion of this small-volume breakout lava flows, by squeezing the still hot flow core away from the site of loading (Applegarth et al., 2010) as observed at Mount Etna volcano. The mineralogical and chemical composition of the breakout flows is the same as the entire Jumento samples (Fig. 7). 6. Conclusions

Fig. 9. (A) Height (Hco) vs cone diameter (Wco) diagram for Jumento, Xitle and Parícutin volcanoes, compared to San Francisco Volcanic Field cones (Moore et al., 1976; Wood, 1980). Notice that Paricutin is the largest cone, and Jumento is volumetrically between Xitle and Paricutin. (B) Slope (S°) vs Hco/Wco morphometric data for Paricutin, Xitle and Jumento volcanoes, compared to San Francisco Volcanic Field cones (Wood, 1980). Jumento is similar to Xitle values, suggesting a close, but younger age.

In this paper, we present new radiocarbon ages, whole-rock chemical data, and morphometric analysis of the JV. This volcano is a monogenetic structure similar in mineralogy and geochemistry to other volcanic structures of the CVF, which is composed by a scoria cone produced mainly by a Strombolian type eruption. This eruption started with a hydromagmatic explosion, producing dilute pyroclastic density currents (wet surges) that eroded the underlying paleosoil and incorporated charcoal and soil fragments. Immediately after, low-altitude pyroclastic columns were developed, depositing ashfall. The emission of three lava flows ended the initial activity of the JV, although reactivation happened some time later, leading to the development of breakout flows probably related to a superimposed lavas (LF-1 to LF-3) that pressurized the underlying lavas and squeezed the still hot flow core away from the site of loading. We propose in this paper that the

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maximum age of the JV is ~2 ky BP, being one of the youngest volcanoes in the CVF and supports the idea of periods of intense volcanic activity intercalated with long periods of quiescence at Chichinautzin Volcanic Field. Thus, recurrence intervals at this volcanic field are meaningless, then unpredictable. Acknowledgements We thank P. Girón who performed XRF analysis at LUGIS, UNAM, and O. Pérez-Arvizu who conducted ICP-MS analysis at Centro de Geociencias, UNAM. Many thanks to E. Rangel, J.L. Alvarez-García, and D.A. Sánchez-Salgado for their assistance during fieldwork. Discussions with J.L. Macias improved a first version of the manuscript. Thanks to J. Benowitz for his review of the final version of the manuscript. This research was supported by PAPIIT-IN104214 grant (to J.L. Arce). Constructive comments by two reviewers (C. Siebe and M.N. Guilbaud) greatly improved the final version of this paper. References Agustin-Flores, J., Siebe, C., Guilbaud, M.N., 2011. Geology and geochemistry of Pelagatos, Cerro del Agua, and Dos Cerros monogenetic volcanoes in the Sierra Chichinautzin Volcanic Field, south of México City. J. Volcanol. Geotherm. Res. 201, 143–162. Applegarth, L.J., Pinkerton, H., James, M.R., Calvari, S., 2010. Lava flow superposition: the reactivation of flow units in compound 'a'a flows. J. Volcanol. Geotherm. Res. 194, 100–106. Arce, J.L., Macías, R., García-Palomo, A., Capra, L., Macías, J.L., Layer, P., Rueda, H., 2008. Late Pleistocene flank collapse of Zempoala Volcano (Central Mexico) and the role of fault reactivation. J. Volcanol. Geotherm. Res. 177, 944–958. Arce, J.L., Layer, P., Lassiter, J.C., Benowitz, J.A., Macías, J.L., Ramírez-Espinosa, J., 2013. 40Ar/39Ar dating, geochemistry, and isotopic analyses of the quaternary Chichinautzin Volcanic Field, south of Mexico City: implications for timing, eruption rate, and distribution of volcanism. Bull. Volcanol. 75, 774. http://dx.doi.org/10. 1007/s00445-013-0774-6. Bloomfield, K., 1975. A late-Quaternary monogenetic volcano field in central Mexico. Geol. Rundsch. 64, 476–497. Erlund, E.J., Cashman, K.V., Wallace, P.J., Pioli, L., Rosi, M., Johnson, E., Delgado-Granados, H., 2010. Compositional evolution of magma from Paricutin Volcano, Mexico: the tephra record. J. Volcanol. Geotherm. Res. 197, 167–187. Ferrari, L., Orozco-Esquivel, T., Manea, V., Manea, M., 2012. The dynamic history of the Trans-Mexican Volcanic Belt and the Mexico subduction zone. Tectonophysics 522–523, 122–149. Foshag, W.F., González, J., 1956. Birth and development of Parícutin volcano Mexico. U.S. Geol. Surv. Bull. 965, 355–489. García-Palomo, A., Macías, J.L., Garduño, V.H., 2000. Miocene to recent structural evolution of the Nevado de Toluca volcano region, Central Mexico. Tectonophysics 318, 281–302. García-Palomo, A., Macias, J.L., Arce, J.L., Capra, L., Garduño, V.H., Espíndola, J.M., 2002. Geology of Nevado de Toluca volcano and surrounding areas, central Mexico. Geological Society of America Map Series, pp. 1–48. García-Palomo, A., Zamorano, J.J., López-Miguel, C., Galván-García, A., Carlos-Valerio, V., Ortega, R., Macías, J.L., 2008. El arreglo morfoestructural de la Sierra de las Cruces, México central. Rev. Mex. Cienc. Geol. 25, 158–178. Guilbaud, M.N., Siebe, C., Layer, P.W., Salinas, S., 2012. Reconstruction of the volcanic history of the Tacámbaro-Puruarán area (Michoacán, México) reveals high frequency of Holocene monogenetic eruptions. Bull. Volcanol. 74, 1187–1211. Guilbaud, M.N., Arana-Salinas, L., Siebe, C., Barba-Pingarrón, L.A., Ortíz, A., 2015. Volcanic stratigraphy of high-altitude Mammuthus columbi (Tlacotenco, Sierra Chichinautzin), Central México. Bull. Volcanol. 77, 17. http://dx.doi.org/10.1007/s00445-015-0903-5. Hasenaka, T., Carmichael, I.S.E., 1985. A compilation of location, size, and geomorphological parameters of volcanoes of Michoacan-Guanajuato volcanic field, central Mexico. Geofis. Int. 24, 577–607. Higuera, P.E., Gavin, D.G., Bartlein, P.J., 2010. Peak detection in sediment-charcoal records: impacts of alternative data analysis methods on fire-history interpretations. Int. J. Wildland Fire 19, 996–1014. Holme, P.M., Sten, H., Nielsen, A., 1982. The geochemistry and petrogenesis of the lavas of the Vulsini District, Roman Province, Central Italy. Contrib. Mineral. Petrol. 80, 367–378.

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