Isotopic, chemical and dissolved gas constraints on spring water from Popocatepetl volcano (Mexico): evidence of gas–water interaction between magmatic component and shallow fluids

doi:10.1016/j.jvolgeores.2004.09.006

Journal of Volcanology and Geothermal Research

Volume 141, Issues 1–2, 1 March 2005, Pages 91-108

https://doi.org/10.1016/j.jvolgeores.2004.09.006 Get rights and content

Abstract

Geochemical research was carried out on cold and hot springs at Popocatepetl (Popo) volcano (Mexico) in 1999 to identify a possible relationship with magmatic activity. The chemical and isotopic composition of the fluids is compatible with strong gas–water interaction between deep and shallow fluids. In fact, the isotopic composition of He and dissolved carbon species is consistent with a magmatic origin.

The presence of a geothermal system having a temperature of 80–100° C was estimated on the basis of liquid geothermometers. A large amount of dissolved CO₂ in the springs was also detected and associated with high CO₂ degassing.

Introduction

Popocatepetl (Popo) is a large andesitic stratovolcano (5452 m) near Mexico City, which has been erupting since December 1994. The fumarolic activity increased in the early 1990s and culminated in ash eruptions at the end of 1994 and in early 1995. Since 1996, a consecutive series of crater domes have been formed and destroyed explosively. During the previous eruptive activity (1918–1925), a small dome also grew on the crater floor.

Popo is potentially dangerous because of its explosive eruptive history and because millions of people live within 60 km of the volcano. A geophysical and geochemical monitoring network is maintained by UNAM-CENAPRED in order to evaluate changes in the eruptive activity.

Popo forms the southern part of the Sierra Nevada complex which includes the older volcano, Iztaccihuatl (Izta). The present-day Popo cone is also built on an older volcano that was destroyed in a Bezymmiany-type event (Robin and Boudal, 1987). To the south of Popo, directly at the foot of the volcano, Cretaceous limestones and granodioritic stocks crop out.

Several geochemical investigations have been carried out over the past years on the volcanic products and fluid emissions of Popo. On the basis of sulfur isotope data from the fumaroles, and the CO₂ and S budget of the volcano, Goff et al. (1980) hypothesized a minor assimilation of Cretaceous evaporitic wall rocks by the volcanic products of Popo, which is consistent with leachate studies of recent volcanic ash (Armienta et al., 1998).

Recent CO₂ and SO₂ budget estimates highlight the large amount of gas emitted by Popo volcano (Love et al., 1998, Delgado-Granados et al., 2001), which therefore represents one of the largest contributor of volcanic gases to the atmosphere. Spring water has been monitored for the last 13 years to detect changes related to the magmatic activity (Aguayo and Martin del Pozzo, 1994, Martin-Del Pozzo et al., 2002a). Recently, studies have been aimed at characterizing the isotopic composition and type of interaction between the volcanic gases and the spring water (Inguaggiato et al., 1999, Inguaggiato et al., 2001, Martin-Del Pozzo et al., 2002b).

On the basis of the previous geochemical studies, two different interpretations have been formulated regarding the circulation of fluids at Popo and the possible interaction between shallow waters and deep fluids of magmatic origin. In keeping with the first interpretation, based on the chemical composition of major, minor, and trace elements of the spring waters (Werner et al., 1997), each spring maintains a relatively constant composition over time, and this suggests that there is no interaction between spring waters and volcanic fluids.

Whereas, as indicated by second interpretation (Inguaggiato et al., 1999, Inguaggiato et al., 2001, Martin-Del Pozzo et al., 2002a, Martin-Del Pozzo et al., 2002b), based on the chemical and isotopic composition of both water and dissolved gases, there is a strong interaction between deep magmatic fluids and the cold spring waters circulating into the Popo.

The aim of this paper is to investigate the interaction processes between rocks, water, and deep gases at Popo volcano and explain the fluid circulation within the volcano. In order to reach this goal, in 1999 we studied 11 cold springs located near Popo and Izta volcanoes, as well as three hot springs to the south of Popo (Fig. 1). The cold springs on Popo are located between 7 and 22 km from the crater at altitudes between 3600 and 1900 m a.s.l. The hot springs, as well as the two springs from Izta were sampled for comparison. The hot springs, located about 40 km from the volcano at about 1000 m above sea level, were included in order to identify their possible relation either with the volcano or with the regional fault system. Previous reconnaissance and sampling allowed us to decide which springs were the most representative springs for this study. All samples of water and dissolved gases were analyzed for major and minor element compositions as well as for noble gas isotopes (³He/⁴He and ⁴He/²⁰Ne ratios) and stable isotopes (δ³⁴S, δ¹³C, δ¹⁸O, and δD).

Section snippets

Analytical methods

Groundwater samples were collected in polyethylene bottles for laboratory analyses while temperature, conductivity, and pH were determined directly in the field. Alkalinity was analyzed by titration with HCl 0.1 N, whereas major and minor elements were determined in the laboratory using a Dionex 2000i ion chromatograph with an accuracy of ±2%. A Dionex CS-12 column was used for the cations (Li, Na, K, Mg, Ca) and a Dionex AS4A-SC column for the anions (F, Cl, Br, NO₃, SO₄). The content of SiO₂

Geochemical framework

The chemical and isotopic compositions of the sampled water are reported in Table 1. Almost all the springs have low salinity and low outlet temperatures which suggests a low degree of water–rock interaction. Only the southern springs (i.e. AH, AT and IX samples) have relatively high salinity and T (Fig. 2). The Langelier Ludwig diagram (Fig. 3a) shows that many samples fall in the bicarbonate–alkaline–earth field except for the AH, AT and IX springs, located further south of Popo, which fall

Saturation index and geothermometer

The chemical composition and dissolved salt content in natural water result from the interaction between water, gas and host rock. Chemical equilibrium between water and rocks is not always attained since it depends on many chemical and thermodynamic conditions. In order to check whether equilibrium had been reached or not, the saturation index regarding possible mineral phases present in the host rocks was calculated for each spring. Said index in relation to a given mineral phase is defined

Dissolved gas contents

Dissolved gases in spring water are a useful tool for understanding gas–water interactions. High mobility in addition to different solubility makes gases excellent geochemical tracers. The amount and type of dissolved gases have been successfully used in geochemical investigations to solve hydrological, geothermal, and volcanological problems (Dyck, 1976, Capasso et al., 2000, Capasso et al., 2001, Inguaggiato et al., 2000, Taran et al., 2002, Carapezza et al., 2004).

The chemical composition of

Discussion

Since Popo volcano has gone beyond the immature stage, it should be characterized by the presence of a geothermal system linked to the volcano.

Previous studies hypothesized the absence of a geothermal system inside the Popo volcanic system, based on the absence of diffuse soil degassing from its flanks (Varley and Armienta, 2001, Varley and Taran, 2003) and on the lack of evidences regarding interaction processes between deep magmatic fluids and the groundwaters circulating in the volcano (

Conclusions

The geochemical investigation carried out on the dissolved gas species in this work highlights the following aspects:

(1) The chemical and isotopic composition of the Popo springs suggests that they discharge waters of meteoric origin that have undergone limited water–rock interaction processes This is also in line with the low salinity and temperature values that confirm our previous studies (Martin-Del Pozzo et al., 2002a, Martin-Del Pozzo et al., 2002b).

(2) High contents of magmatic gases (He

Acknowledgments

Authors wish to thank DGAPA (PAPIIT), Intercambio Academico (UNAM) and Istituto Nazionale di Geofisica e Vulcanologia Palermo for financing the research, as well as L. Marini, P.M. Nuccio and Y. Taran for their critical review and constructive comments for improving the manuscript. Ramon Espinasa, Fabiola Mendiola, Humberto Saenz, Francisco Sainz, Rita Fonseca, Miguel Angel Butron and Fernando Aceves assisted in field sampling and processing. Furthermore, we are indebted to Andrea Rizzo and

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Isotopic, chemical and dissolved gas constraints on spring water from Popocatepetl volcano (Mexico): evidence of gas–water interaction between magmatic component and shallow fluids

Abstract

Introduction

Section snippets

Analytical methods

Geochemical framework

Saturation index and geothermometer

Dissolved gas contents

Discussion

Conclusions

Acknowledgments

Appl. Geochem.

Appl. Geochem.

Geochim. Cosmoch. Acta

J. Hydrol.

Appl. Geochem.

Geochim. Cosmochim. Acta

J. Volcanol. Geotherm. Res.

J. Geophys. Explor.

J. Volcanol. Geotherm. Res.

Appl. Geochem.

J. Volcanol. Geotherm. Res.

Geothermics

Chem. Geol.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

J. Volcanol. Geotherm. Res.

Chem. Geol.

Chem. Geol.

Variaciones en los Manantiales de los Volcanes Popocatepetl y Colima

Interaction between fumarolic gases and thermal groundwaters at Vulcano Island (Italy): evidences from chemical composition of dissolved gases in waters

J. Volcanol. Geotherm. Res.

Interaction between the deep fluids and the shallow groundwaters on the vulcano island (Italy)

J. Volcanol. Geotherm. Res.