Journal of Petrology Advance Access originally published online on November 7, 2005
Journal of Petrology 2006 47(3):457-479; doi:10.1093/petrology/egi082
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A 2.5 ka History of Dacitic Magmatism at Nevado de Toluca, Mexico: Petrological, 40Ar/39Ar Dating, and Experimental Constraints on Petrogenesis
1 INSTITUTO DE GEOLOGIA, UNAM, COYOACÁN 04510, MÉXICO D.F., MEXICO
2 INSTITUTO DE GEOFÍSICA, UNAM, COYOACÁN 04510, MÉXICO D.F., MEXICO
3 DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF TEXAS, AUSTIN, TX 78712-0254, USA
4 GEOPHYSICAL INSTITUTE AND DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF ALASKA, FAIRBANKS, AK 99775-7320, USA
RECEIVED AUGUST 27, 2003; ACCEPTED SEPTEMBER 29, 2005
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ABSTRACT |
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After 11·5 ka of quiescence (24·5–13 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 12·1 ka, and a large Plinian eruption at 10·5 ka. About 10 km3 of magma was erupted that was homogeneous in composition (63·3–65·7 SiO2 wt % whole-rock) and in mineralogy. Pumice consists of plagioclase (An30–59) > orthopyroxene (En56–59) > hornblende >> Fe–Ti oxides + rare apatite (in opx) + biotite, set in a rhyolitic matrix (72–76 SiO2 wt %). 40Ar/39Ar analysis of single biotite crystals yielded ages (0·81–4·7 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 4·5–6 km below the summit at water pressures of 160–210 MPa and a temperature of 824 ± 12°C on the basis of Fe–Ti oxide thermometry, and under water-saturated conditions. To stabilize a homogeneous magma body of >10 km3 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.
KEY WORDS: explosive volcanism; xenocrystic contamination; Nevado de Toluca, Mexico
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INTRODUCTION |
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Nevado de Toluca is a stratovolcano [19°06'30''N; 99°45'30''W; 4680 m above sea level (a.s.l.)] located 80 km SW of Mexico City in the central part of the Trans-Mexican Volcanic Belt (Fig. 1a). Volcanic activity commenced
2·5 Ma ago with the emplacement of andesitic lava flows, ending at about 1·3 Ma (Cantagrel et al., 1981
; Macías et al., 1997; García-Palomo et al., 2002). Renewed activity took place at about 100 ka through the emission of dacitic pyroclastic products. The most recent period of intense activity at Nevado de Toluca lasted for about 2500 years from 13 to 10·5 ka, after 11·5 ka of quiescence, during which three pyroclastic deposits were produced by three different volcanic eruptions. Samples analyzed from these deposits are relatively homogeneous in composition and mineralogy, which raises the question of whether the three magmas were fed from a single, large reservoir or from several small magma bodies. If they originated from a single reservoir, it is interesting to consider how much change occurred in the magma during the 2500 years.
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, some volcanoes in Mexico. (b) Map showing the distribution of the Upper Toluca Pumice (UTP), Middle Toluca Pumice (MTP), block-and-ash flow deposit of Several models have been proposed to explain the origin of dacitic rocks in volcanic systems; these include a simple crystal fractionation of basaltic or andesitic parental magmas (Berman, 1981
; Reid & Cole, 1983), mixing between rhyolitic and basaltic magmas (Eichelberger, 1975; Gerlach & Grove, 1982), crustal contamination of mafic magmas (Francis et al., 1980; Harmon et al., 1981; Grant et al., 1984), and partial melting of the lower crust (Reid & Cole, 1983; Grant et al., 1984). Recent petrological studies have suggested that silicic magmas are probably produced by a combined process of fractionation and assimilation (Hopson & Mattinson, 1990; Grove et al., 1997). However, the time required to produce and store such magmas in the crust is still unclear (e.g. Hawkesworth et al., 2000; Reagan et al., 2003).
The recurrence of silicic volcanism can be explained by a number of models; one of these considers that the magmas are fed from a single, long-lived reservoir (Halliday et al., 1989; van den Bogaard & Schirnick, 1995), whereas in another the magmas are generated by fast and repetitive partial melting of the lower crust (Huppert & Sparks, 1988; Sparks et al., 1990).
The Nevado de Toluca volcano offers an opportunity to investigate the mechanism responsible for generating eruptions of three, closely spaced in time, homogeneous dacitic magmas. The results of detailed petrological studies combined with hydrothermal experiments are used to investigate the evolution of the magmatic system during this period of its activity. All analytical methods are described in Appendix A.
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SUMMARY OF THE 2500 YEARS OF ACTIVITY |
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The late Pleistocene stratigraphic sequence of Nevado de Toluca volcano consists of three pyroclastic deposits informally designated BAF-13 (13 ka), MTP (12·1 ka), and UTP (10·5 ka), each of which was deposited to the NE–ESE of the volcano (Fig. 1b).
Block-and-ash flow deposit 13 (BAF-13)
The BAF-13 deposit, erupted between 13 160 ± 89 years BP (before present) and 13 870 +445/–420 years BP (Caballero-Miranda et al., 2001; García-Palomo et al. 2002), consists, from the base upward, of a grey laminated surge layer (10 cm thick), overlain by two 2 m thick, massive, grey, block-and-ash flow layers, each composed of light and dark grey dacitic blocks, grey and white dacitic pumice, and minor reddish altered blocks set in a coarse ash matrix (Fig. 2). The stratigraphy of BAF-13 suggests that a pre-existing dacitic dome in the crater was destroyed by magmatic explosions that produced pyroclastic surges and pyroclastic flows. The volume of material ejected during this volcanic event is not well constrained but its thickness and extent suggest a maximum of 0·5 km3 dense rock equivalent (DRE).
Middle Toluca Pumice (MTP)
The Middle Toluca Pumice consists of a complex pyroclastic sequence (Fig. 2) of five fall horizons generated by a small Plinian eruption dated at 12 040 ± 95 and 12415 ± 285 years BP (Macías et al., 1997; Cervantes, 2001; García-Palomo et al., 2002; Arce et al., 2003). The juvenile material of the MTP deposit consists of white, grey, and banded dacitic pumice. The volume of the magma ejected during this volcanic eruption is estimated to be 1·8 km3 (Cervantes, 2001; Arce et al., 2005).
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Upper Toluca Pumice (UTP)
The UTP deposit has been described by several workers (Bloomfield & Valastro, 1974
; Bloomfield et al., 1977; Macías et al., 1997; Arce et al., 2003). The UTP is a complex sequence composed of four fall horizons, four pyroclastic flow units, and two pyroclastic surge units, all erupted at 10·5 ka (Arce et al., 2003) (Fig. 2). The UTP deposit contains white, grey, and banded dacitic pumice, and dense, light grey, lithic fragments. These constituents vary through the stratigraphic sequence; at the base, grey pumice and dense lithic clasts are more abundant (F0, PC0 and base of PC1 units). Stratigraphically upwards, white pumice dominates the deposit. Minor amounts of banded pumice appear throughout the stratigraphy. The volume of the magma ejected during this eruption was estimated to be 8 km3 (Arce et al., 2003). The UTP sequence is capped by a dacitic central dome called El Ombligo, which is considered to be the last product of the UTP eruption (Arce et al., 2003). |
PETROGRAPHY OF THE DEPOSITS |
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Block-and-ash-flow deposit 13 (BAF-13)
White and grey pumice are highly porphyritic and dacitic in composition (Fig. 3), containing, in order of abundance, phenocrysts of plagioclase, orthopyroxene, hornblende, and minor Fe–Ti oxides and biotite, all set in highly vesicular glass (Fig. 4; Table 2). White pumice contains 53·54 vol. % phenocrysts, whereas grey pumice contains 46·50 vol. % (Table 1); both pumice types are relatively more crystalline than the UTP and MTP pumice (described below). We recognize two populations of crystals. The first includes large phenocrysts, all of which have resorbed margins and subhedral to anhedral shapes. The second population comprises small phenocrysts (<1 mm) that are all euhedral. All biotite crystals, regardless of size, are rimmed by reaction products. Plagioclase phenocrysts are weakly reversely zoned, with cores on average of An45±7 and rims of An47±1 (Table 3). Microphenocrysts are An45±6. Orthopyroxene is unzoned, averaging En58·8±0·3 (Table 4). Amphibole is edenite–pargasite hornblende. Biotite is ferroan–phlogopite, similar to that found in the UTP samples (Table 5). Ilmenite and rare titanomagnetite are present as isolated crystals in the matrix and as inclusions in the phenocrysts (biotite, orthopyroxene, and hornblende). The composition of ilmenite is Ilm 0·80 (mole fraction ilmenite) and that of titanomagnetite is Usp 0·22 (mole fraction ulvöspinel) (Table 6).
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Middle Toluca Pumice
White and grey pumice from the MTP are similar in composition (Fig. 3; Table 7), but differ significantly in crystallinity (Table 1). Phenocrysts consist of plagioclase > orthopyroxene > hornblende >> Fe–Ti oxides + biotite, all set in a vesicular rhyolitic glass (73 wt % SiO2) (Table 2). Larger phenocrysts (>1 mm diameter) have resorption textures; biotite crystals have reaction rims and resorbed margins (Fig. 5), whereas small phenocrysts (<1 mm diameter) are euhedral.
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Plagioclase is essentially unzoned with cores ranging from An45·8±6 to An48±4, and rims from An46·5 ±1 to An45±1. Microphenocrysts average An45±2 (Table 3). Orthopyroxene is homogeneous, averaging En58±2 (Table 4). Amphibole is edenite–hornblende (Table 5). Ilmenite and lesser amounts of titanomagnetite are present as single crystals, attached to rims, or as inclusions in other phenocrysts (biotite, orthopyroxene and hornblende). Both ilmenite and titanomagnetite are the same composition in both white and grey pumice (Table 6).
Upper Toluca Pumice
Juvenile UTP samples consist of white pumice, grey pumice, and fragments of the Ombligo central dome, all of which are dacitic in composition, with very little variation amongst them (Fig. 3). All samples are porphyritic, with pumice containing 31–37 vol. % phenocrysts (vesicle free), and the dome fragments containing up to 43 vol. % (Table 1). Pumice is highly vesiculated with rhyolitic glass (Table 2) forming the bubble walls, whereas the dome rock has a microlite-rich matrix and only minor glass (Fig. 4, Table 1). In order of abundance, phenocrysts are plagioclase, orthopyroxene and hornblende, with minor Fe–Ti oxides, biotite and rare apatite (as inclusions in orthopyroxene). As in the BAF-13 and MTP samples, two populations of phenocrysts were observed; one including relatively small and euhedral phenocrysts (<1 mm diameter, Fig. 4), the other large phenocrysts (>1 mm to 2·3 mm diameter, Figs 4 and 5)with reaction rims, and resorbed margins. Biotite appears in different proportions in the UTP samples, often <0·1 vol. %. It occurs only as large (>1 mm diameter) phenocrysts (never as microlites). Biotite appears either rimmed with small intergrowths of amphibole, orthopyroxene, Fe–Ti oxides, and plagioclase, or with resorbed margins (Fig. 5).
Plagioclase varies in abundance between UTP samples (Table 1). Euhedral, subhedral, and resorbed margins are observed in all samples. Plagioclase in the white pumice varies from An47 cores to An47±2, whereas in the grey pumice it varies from An44 cores to An40±8 rims (Table 3). Plagioclase in the dome samples varies from An33±2 cores to An40±10 rims. Small (<1 mm diameter) phenocrysts are An40±7 (Table 3).
Orthopyroxene represents 4–5 vol. % of the UTP samples (Table 1) and is present either as large (>1 mm diameter) phenocrysts with corrosion enbayments and resorbed rims, or small (<1 mm diameter), euhedral phenocrysts (Fig. 4). No substantial compositional variations exist, however; in the white pumice orthopyroxene is En56±1·5, in the grey pumice it is En57±3·5, and in the dome, En59±1·5 (Table 4). Amphibole phenocrysts are euhedral and range in size significantly. In addition, some of the larger phenocrysts have inner dissolution zones (Fig. 4). Compositions vary little between cores and rims (Table 5) and are generally edenite to pargasite–hornblende. Biotite represents <1 vol. % in the UTP samples, and was classified as ferroan–phlogopite (Table 5). Ilmenite and lesser titanomagnetite are present attached to phenocrysts (mainly of hornblende and orthopyroxene) or as individual crystals in the matrix. Ilmenite is Ilm 0·83–0·87 and titanomagnetite is Usp 0·27–0·48 (Table 6).
Summary of petrography
Juvenile fragments from the BAF-13, MTP, and UTP deposits have the same textures, mineral assemblage, and mineral composition. All samples contain two populations of phenocrysts: large phenocrysts with resorption textures and small, euhedral phenocrysts (Figs 4 and 5).Biotite is present only as large phenocrysts and is always partially resorbed or rimmed by reaction products. The lack of euhedral microphenocrystic biotite suggests that it is not stable in the three magmas.
Crystallinity varies amongst the samples; the Ombligo dome contains 43 vol. % crystals, followed by the grey pumice (38–48 vol. %), and the white pumice (34 vol. %) (Table 1). Glass composition also varies; the dome samples contain the most evolved glass (78 wt % SiO2), followed by the grey pumice (76 wt % SiO2), and the white pumice (72–73 wt % SiO2) (Fig. 3). The whole-rock compositions of the three deposits are very similar (63–65 wt % SiO2), however, suggesting that the variations in the glass composition are a function of the crystal content.
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GEOTHERMOMETRY |
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The presence of ilmenite and magnetite in all samples allows us to estimate pre-eruptive temperature (T) and oxygen fugacity (
), reported referenced to the fayalite–magnetite–quartz (FMQ) buffer. These parameters were calculated using the QUILF program of Andersen et al. (1993). Each titanomagnetite composition was combined with each ilmenite found in the same sample to obtain a range of T and .
For the BAF-13 sample one titanomagnetite crystal and four ilmenite crystals were analyzed (Table 6) and give an equilibrium temperature of 812–829°C (819 ± 7°C average) and 
log (FMQ) = 1·76–1·93 (1·84 ± 0·09 average) (Table 6; Fig. 6). For the MTP pumice, a total of three titanomagnetite and six ilmenite crystals in sample PBI2A and six ilmenite crystals in sample PBI29B were analyzed (Table 6). Assuming that the magnetites in PBI2A represent equilibrium compositions with ilmenite in the PBI29B pumice, we estimate the temperature and oxygen fugacity for the MTP as 796–809°C (802 ± 7°C average) and log (FMQ) = 0·82–0·96 (0·89 ± 0·07 average). For the UTP deposit, we obtain equilibrium temperatures of 812–837°C (824 ± 12°C average) and log (FMQ) = 1·31–1·40 (1·37 ± 0·07 average) (Table 6; Fig. 6) in white pumice and 829°C and log (FMQ) = 1·30 for the grey pumice. A much higher (and a large range) temperature of 846–877°C (865 ± 19°C average) and log (FMQ) = 0·70–0·90 (0·79 ± 0·09 average) was obtained for the dome (Fig. 6).
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There are some temperature variations between samples; the MTP magma was
20°C and the BAF-13 5°C cooler than the UTP, whereas the dome was 40°C hotter than the UTP magma. The high temperature of the dome (865°C average) is a puzzle, considering that slow cooling systems typically experience re-equilibration of the oxides with time (Hammond & Taylor, 1982). This temperature for the dome reflects the different composition of the titanomagnetite (Table 6). To constrain the extent of Fe–Ti oxide re-equilibration, we calculated maximum oxygen fugacities and temperatures for the three magmas utilizing the compositions of orthopyroxene, magnetite, and ilmenite in our samples, and assuming that the silica activity was nearly unity. Interestingly, the temperatures and oxygen fugacities obtained are very similar to those calculated with the two oxides, even for the dome (see Fig. 6), which again yields a higher temperature (869 ± 15°C). This indicates that orthopyroxene, titanomagnetite, and ilmenite are in equilibrium in all rocks, except for the dome, which displays a large temperature range (Fig. 6). The presence of large phenocrysts and microlites in the dome samples suggest that the magma underwent slow ascent and cooling, promoting rapid re-equilibration of the Fe–Ti oxides as proposed elsewhere (e.g. Frost, 1991; Lindsley et al., 1991; Venezky & Rutherford, 1999). This process modified the Fe–Ti oxide compositions resulting in an apparently higher temperature. The three magmas have a temperature range from 795 to 885°C (Fig. 6). log is highest in the MTP. |
40Ar/39Ar DATING AND BIOTITE SOURCE |
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Microphenocrysts of plagioclase, orthopyroxene, hornblende, and Fe–Ti oxides are euhedral and appear to be in equilibrium with the surrounding matrix. In contrast the larger phenocrysts (up to 2·3 mm diameter), including biotite, appear to be partially resorbed. Biotite has reaction rims of hornblende, orthopyroxene, and plagioclase (Fig. 5), suggesting that it may be xenocrystic. The presence of hornblende in the rims suggests that the reaction occurred at high water pressures, and thus did not occur as a result of decompression during magma ascent (Rutherford & Hill, 1993
). Instead, it probably occurred while the magma was stored in a deep reservoir.
To explore the possibility that the biotite is xenocrystic, we determined its Ar isotopic composition, because this technique has been used successfully to identify xenocrysts in other magmatic systems (e.g. Lo Bello et al., 1987; Nelson et al., 1992; Chen et al., 1996; Singer et al., 1998; Spell et al., 2001; Gardner et al., 2002).
Of the 30 biotite grains that were step-heated for 40Ar/39Ar dating (methods are described in Appendix A),only four grains showed fractions in the step-heating runs with apparent ages, ranging from 4·7 to 0·8 Ma, that are significantly older than the late Pleistocene (13–10·5 ka) age of the deposits. The detailed analyses of these four grains are reported in Appendix B and the age spectra are shown in Fig. 7. The age spectra for the remaining 26 analyses have integrated ages that are indistinguishable from the eruption age, and/or do not show any fractions that are significantly older than the eruption age. The weighted mean apparent age of these grains is –23 ± 109 ka. This age is essentially zero (within 1
error), but reflects that for some analyses, the apparent atmospheric argon percentage exceeded 100%, but not significantly. The poor precision of the analyses provides no additional information about the age of eruption or the potential presence of xenocrysts; consequently, the data are not included in the paper.
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Additionally, we used the composition of orthopyroxene and biotite determined by electron microprobe, with the model of Fonarev & Konilov (1986)
to test if these mineral phases are in equilibrium in our samples. The results show that at temperatures of 800–840°C the iron content of the biotite is too high (
) to be in equilibrium with orthopyroxene (
); this further supports the idea that biotite is xenocrystic.
There are no known biotite-bearing rocks cropping out in the Toluca area that are 4·7 Ma old or younger; thus we have no direct evidence that the biotite crystals are xenocrysts incorporated from the earlier products of the Nevado de Toluca volcano (which began erupting around 2·6 Ma; García-Palomo et al., 2002). The only known biotite-bearing rocks below the volcano are within the ‘Ixtapan–Teloloapan’ metamorphic sequence of Late Jurassic–Cretaceous age, which consists of a wide variety of lithologies (volcanic rocks, sandstones, schists, phyllites, tuffs) (Campa et al., 1974) and an 55 Ma felsic intrusion (De Cserna et al., 1974). If these rocks are potential sources of biotite xenocrysts, the age spectra could be consistent with incorporation of old biotite (>55 Ma) that would have lost significant amounts of argon (98–99%) by volume diffusion after incorporation in the magma (Fig. 7). Thus, our modeling implies an amount of argon loss that prevents us from determining the true source or age of the xenocrysts based on the age spectra alone.
As biotite xenocrysts would probably be highly reactive in the host dacitic magma they are likely to have only short residence time. The presence of reaction rims on the biotite crystals may have acted to reduce their contact with the melt, and hence extend their lifetimes. It is likely, however, that because biotite is present in all three of the eruptions, and those eruptions span about 2500 years, contamination by xenocryst incorporation was continual, and the biotite crystals in each of the deposits were introduced just prior to each eruption. At 800–850°C, which is the calculated range of temperature for the Toluca magmas (Fig. 6), 1 mm biotite crystals would be totally reset in about 1 year, based on the modeling approach of Gardner et al. (2002). It is, however, possible that some of the larger biotite xenocrysts would retain a trace of their original radiogenic argon, such as seen in the four crystals analyzed (Fig. 7). Relatively short times between incorporation and eruption have also been proposed for xenocrysts in other magmatic systems (e.g. Lo Bello et al., 1987; Gardner et al., 2002), although longer residence times have been proposed for plagioclase xenocrysts (Singer et al., 1998).
It is noteworthy that the pumice clasts in the three deposits have a population of large phenocrysts of plagioclase, pyroxene, and hornblende with disequilibrium textures. These mineral phases may also represent xenocrysts incorporated into the magma, but because the latter was saturated with these phases, they did not completely dissolve.
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EXPERIMENTAL RESULTS |
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The three magmatic units studied (BAF-13, MTP, and UTP) are dacitic and consist of plagioclase, orthopyroxene, hornblende, and Fe–Ti oxides in a rhyolitic groundmass. To constrain the stability field of this mineral assemblage, we performed hydrothermal experiments using natural samples of UTP (see Appendix A for methods). All experiments were run water-saturated and above the solidus (all experimental conditions are listed in Table 8), and so all runs contain melt and water vapor. Orthopyroxene and Fe–Ti oxides were present in all the charges. The stabilities of plagioclase and hornblende vary with temperature and pressure (Fig. 8). Plagioclase stability decreases in temperature as water pressure increases, such that it is stable below 880°C at pressures below 140 MPa, but only below 850°C at 250 MPa. In contrast, hornblende is stable at higher temperatures as pressure increases. Hornblende is not stable below
140 MPa or above 870°C. We note that the anomalously high temperatures of the dome sample obtained by Fe–Ti oxide geothermometry (846–877°C) are close to the upper stability limit of amphibole (Fig. 8). Above 860°C amphibole is unstable and does not coexist with plagioclase. In contrast, abundant hornblende and plagioclase exist together in the dome sample. Thus, those higher temperatures are not considered further. Despite the common presence of biotite in the natural samples, biotite did not crystallize in any of our experimental runs, and considering our petrographic observations that biotite is always resorbed or in reaction with the melt, the experimental results support the conclusion that biotite is xenocrystic. The experiments were run at FMQ + 1·5 (NNO + 0·5, where NNO is the nickel–nickel oxide buffer). It could be argued that biotite might be stable under more oxidized conditions (e.g. Carmichael, 1991). We note, however, that even in our most oxidized sample (BAF-13) biotite is in reaction with the melt, and thus if biotite was originally phenocrystic it would require all of the magmas to have been much more oxidized (up to one log unit greater). This would not, however, explain the presence of older 40Ar/39Ar ages in biotite. In addition, it would require that each magma became reduced just before erupting, because no biotite could survive 2500 years in a magma in which it is not stable. It has also been suggested that sulfur may help stabilize biotite (Costa & Pichavant, 2003; Costa et al., 2004). Electron microprobe analyses of biotite crystals in the Toluca samples did not detect sulfur, suggesting that its presence was not important in these magmas. We believe, therefore, that our experimental results are consistent with the conclusion that biotite is xenocrystic in the three magmas.
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). 
, runs that used natural powder. The arrowheads represent melting (pointing to the right) and crystallization (pointing to the left) experiments. All run conditions are listed in Residual melt (quenched to glass) is the most abundant phase in all the experimental runs and varies systematically in composition as a function of pressure and temperature (Fig. 9). With decreasing temperature at constant pressure, SiO2, Na2O, and K2O contents increase, whereas those of Al2O3, FeO*, MgO, and CaO decrease. With decreasing pressure at constant temperature, SiO2, Na2O, and K2O contents increase, whereas those of Al2O3, FeO*, MgO, and CaO decrease. These variations are expected, given that decreasing temperature and pressure trigger crystallization of plagioclase + orthopyroxene (±hornblende) + Fe–Ti oxides.
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DISCUSSION |
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Pre-eruptive conditions of the Nevado de Toluca magmas
Assuming that biotite is xenocrystic, the stable phase assemblage in the three Toluca magmas is rhyolitic melt (glass) + plagioclase + hornblende + orthopyroxene + Fe–Ti oxides. For the UTP magma at
824°C, those five phases occur together at water pressures >150 MPa (Fig. 8). When comparing natural and experimental glasses, we find that there is reasonably good agreement at pressures between 150 and 200 MPa (Fig. 9).
We did not perform experiments on MTP samples, but their bulk compositions are similar enough to the UTP pumice (Table 7) that it is unlikely that the results would be different. Again, assuming that biotite is xenocrystic, the similarities in glass composition and phase compositions to UTP samples, and the only slightly lower temperature, suggest that there was little difference in water pressures between the MTP and UTP magmas. We lack compositional data for matrix glass in BAF-13, but its similar phase assemblage and bulk composition to the MTP and UTP samples suggest that its storage conditions were also similar to those for the other magmas. These results imply that all three Toluca magmas were stored at similar conditions, which would have been 4·5–6 km depth (Fig. 10), assuming the magma was water saturated.
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Evolution of the Toluca magma chamber from 13 to 10·5 ka
The three eruptions represent a total of
10–11 km3 of magma, with BAF-13 constituting 0·5 km3 of magma, MTP 1·8 km3, and UTP 8 km3. All three magmas have similar whole-rock compositions, mineral assemblages, mineral compositions and pre-eruptive conditions, and contain biotite xenocrysts. There are two possibilities to explain how the plumbing system could have produced compositionally identical magmas over 2500 years of activity. First, each erupted magma could have been generated separately, by partial melting of the lower crust as attested by crustal xenoliths found in the area (Martínez-Serrano et al., 2004; Meriggi et al., 2004), and then ascended and erupted. This would require that each partial melting event occurred to essentially the same extent each time, involved the same crustal substrate, and that the resultant magma ascended to the same shallow level before erupting. The other possibility is that each eruption tapped the same magma body, and that the magma body did not evolve significantly for 2500 years. Although we cannot rule out the first hypothesis, we believe that the second is more likely, given that each magma appears to come from the same storage level, at about the same temperature, and that all contain biotite xenocrysts.
Given our hypothesis that the same magma body was tapped three times, minor differences in crystal content and glass composition between the grey and white pumice of the three eruptions could reflect their origin from different levels in the magma storage reservoir. In the case of the UTP samples, the grey pumice is slightly cooler than the white pumice (Table 7), suggesting that the grey pumice could have come from the upper, cooler part of the magma reservoir whereas the white pumice was derived from the main part of the magma body. The stratigraphic section through the UTP (Fig. 2) supports this idea, as grey pumice and dense lithic fragments are abundant at the base of the UTP sequence. The presence of white, grey, and banded pumice suggests that the two levels of the magma body (upper grey and lower white) underwent mechanical mingling during simultaneous tapping (Freundt & Tait, 1986).
Average temperatures of the magmas varied from 802 ± 7 to 824 ± 12°C during the 2500 year eruptive period. This may represent slight warming of the magma reservoir with time, probably by injections of mafic magma at the base of the chamber. The observed disequilibrium textures of the large phenocrysts may originate from such injections. These injections would have to be large enough to significantly raise the temperature (to cause resorption), but the magma body would then have to cool to almost the original temperature, for microphenocrysts to grow under the same conditions as the phenocrysts. Instead of such fluctuations in temperature, however, we believe that the slight variations in temperature may simply reflect the tapping of different levels in the reservoir, similar to the model proposed for the white vs grey pumice. This would also fit the trend of generally hotter magmas being erupted in larger volumes.
Changes in the magma reservoir must have occurred with time; we note that the oxygen fugacity of the magma lowered by one unit of log FMQ with time between BAF-13 and MTP, and a little higher for the UTP. This change may have occurred as a consequence of wall-rock assimilation.
Based on Sr, Nd, and Pb isotopic data, Martínez-Serrano et al. (2004) concluded that all the dacitic rocks of Nevado de Toluca have been produced by fractional crystallization and minor crustal contamination. However, our data indicate that crustal assimilation may have played an important role in the evolution of the magmatic system of Nevado de Toluca to produce dacitic magmas. Assimilation of relatively young plutonic bodies (e.g. >4 Ma), with similar whole-rock compositions to those of the Toluca magmas, could have occurred. This has been observed in other volcanic systems and called ‘cryptic’ assimilation (Reagan et al., 2003). This process can be difficult to detect, even with isotopic data such as those reported by Martínez-Serrano et al. (2004) for the Nevado de Toluca rocks. We propose that the dacites produced by the three eruptions were generated by combined ‘cryptic’ assimilation and crystal fractionation.
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CONCLUSIONS |
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After 11·5 ka of quiescence the Nevado de Toluca volcano started a new period of explosive activity between 13 ka and 10·5 ka. During this time
10 km3 of dacitic magma was ejected in three episodes. The products have similar whole-rock compositions (63·3–65·7 wt % SiO2) and mineral assemblages (Plg > Opx > Hb >> biotite xenocrysts >> Fe–Ti oxides). One of the most interesting aspects of these rocks is the common presence of biotite with reaction rims, as well as the presence of mineral phases with resorbed margins. Based on 40Ar/39Ar analyses of single biotite crystals, we believe they are xenocrysts. Thus, we show that although the magmas were similar in composition, assimilation of biotite-bearing wall-rocks with a minimum 40Ar/39Ar age of 4 Ma must have occurred throughout the 2500 years of activity. To maintain a near-constant temperature and composition of the magmas we propose that the dacitic reservoir was reheated by small batches of hotter magma. The magma that fed the three eruptions was stored at 150–200 MPa and was probably thermally zoned. |
APPENDIX A: METHODS |
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Whole-rock analysis
A total of nine whole-rock chemical analyses were carried out on samples of the three deposits, in addition to 12 analyses (Table 7) that have been reported in the literature (Macías et al., 1997
; Arce et al., 2003). Each analysis was made on individual pumice fragments by fusion-inductively coupled plasma emission spectrometry (F-ICPES) at Activation Laboratories, Ancaster, Canada. For the MTP and BAF-13 deposits, white and grey pumice were analyzed and for the UPT deposit we analyzed white and grey pumice, as well as dense, juvenile, lithic clasts and the central dome. In this paper, we report only major element data.
Modal analysis
The modal mineralogy was determined for seven representative samples of the deposits (Table 1). Two samples were analyzed for the BAF-13 deposit: a white pumice (264-wp-f), and a grey pumice (264-gp-f). Two samples were analysed for the MTP deposit: a white pumice (PBI-2A), and a grey pumice (PBI-29B). Three samples were analysed for the UTP deposit: a white pumice (PC2b), a grey pumice (PC0g), and the Ombligo central dome (9596). For each sample, we counted a minimum of 525 and a maximum of 1500 points, consisting of phenocrysts, groundmass (glass + microlites), and vesicles.
Electron microprobe
Microprobe analyses of mineral phases and glasses were performed at the University of Alaska Fairbanks, using a CAMECA SX-50 microprobe with four wavelength-dispersive spectrometers. A total of fifteen experimental samples (listed in Table 9), and eight natural samples from the deposits (Table 2) were analyzed: PC2A, PC1b, PC3, PC0g, and 9596 for the UTP; PBI-29b and PBI-2A for the MTP; 264 for the BAF-13. The analysis of the mineral phases was carried out with a focused beam, an accelerating voltage of 15 keV, and a beam current of 15 nA. For the glass analyses, a 10 µm defocused beam was used, with an accelerating voltage of 15 keV, and a beam current of 10 nA. Hydrous glasses (experimental samples) were analyzed using the Na-loss routine described by Devine et al. (1995).
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40Ar/39Ar analysis
Biotite phenocrysts (2 mm in size) were separated from two samples (MTP-45c and MTP-3I) and were analyzed by the 40Ar/39Ar method. The biotite separates were wrapped in aluminum foil and loaded into aluminum cans of 2·5 cm diameter and 6 cm height for irradiation at the McMaster Nuclear Reactor along with the standard mineral TCR-2 sanidine with an age of 27·87 Ma (M. Lanphere, personal communication, 2000). Upon their return from the reactor, the individual biotite grains and sanidine monitor minerals were loaded into 2 mm diameter holes in a copper tray that was then loaded in an ultrahigh-vacuum extraction line. The monitors were fused, and samples heated, using a 6 W argon-ion laser following the technique described by York et al. (1981)
, Layer et al. (1987) and Layer (2000).
Fifteen separate analyses were performed for each sample, each consisting of a single biotite crystal, which was step heated in 4–9 steps (fractions). Argon purification was achieved using a liquid nitrogen cold trap and a SAES Zr–Al getter at 400C. The samples were analyzed in a VG-3600 mass spectrometer at the Geophysical Institute, University of Alaska Fairbanks. The argon isotopes measured were corrected for system blank and mass discrimination, as well as calcium, potassium and chlorine interference reactions following procedures outlined by McDougall & Harrison (1999) and calculated using the constants of Steiger & Jaeger (1977). System blanks generally were 2 x 10–16 mol 40Ar and 2 x 10–18 mol 36Ar, which are 10–50 times smaller than fraction volumes. Mass discrimination was monitored by running both calibrated air shots and a zero-age glass sample. These measurements were made on a weekly to monthly basis to check for changes in mass discrimination.
Experimental methods
Powdered white pumice from the UTP deposit was used as starting material in all hydrothermal experiments. Each experiment consisted of 30–150 mg of powder plus distilled water, weighed into a Ag70Pd30 tube of 2 or 5 mm diameter. Enough water was added so that an excess fluid vapor was present at run conditions to ensure the sample was water-saturated, and hence water pressure equals total pressure. After weighing in the powder and water, the tube was weighed, welded shut with an oxyacetylene torch, and then weighed again to ensure that no water was lost.
All experiments were run in René-style cold-seal pressure vessels. Oxygen fugacity was buffered near NNO + 1 by using a Ni filler rod and water as the pressurizing medium (Geschwind & Rutherford, 1992; Gardner et al., 1995). Temperature and pressure are precise to within ±3°C and ±0·5 MPa, respectively. In three cases, two charges were run together to approach crystal–melt–vapor equilibrium from two directions. The powders used in these reversal experiments were aliquots of previously run experiments (Table 8), chosen so that one charge was a ‘melting’ experiment, using relatively crystal-rich material, and one charge was a ‘crystallization’ experiment, using relatively melt-rich material.
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APPENDIX B: 40Ar/39Ar ANALYTICAL DATA FOR FOUR BIOTITE GRAINS |
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 TOPABSTRACTINTRODUCTIONSUMMARY OF THE 2500...PETROGRAPHY OF THE DEPOSITSGEOTHERMOMETRY40Ar/39Ar DATING AND BIOTITE...EXPERIMENTAL RESULTSDISCUSSIONCONCLUSIONSAPPENDIX A: METHODS APPENDIX B: 40Ar/39Ar ANALYTICAL... REFERENCES |
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ACKNOWLEDGEMENTS |
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This research was partially supported by CONACyT grants (27993-T and 38586-T to J.L.M.), National Science Foundation grant (EAR-0229290 to J.G.) and a GSA ‘Lipman Research Fund’ (to J.L.A.). We are indebted to J. M. Espíndola, C. Siebe, J. C. Mora, and L. Capra for the constructive discussion of the ideas presented in this work. Thanks go to B. Brown, who performed the microprobe analyses of biotite xenocrysts. We are indebted to M. Dungan, B. Singer, S. Nelson, R. Frost (Editor) and M. Wilson (Executive Editor) for their careful reviews of the manuscript.
* Corresponding author. Telephone: 52-55-56224288, ext. 108. Fax: 52-55-562242889. E-mail: jlarce{at}geologia.unam.mx
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