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8/10/2019 Journal of Volcanology and Geothermal Research 141 (2005) 91 108.pdf
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Isotopic, chemical and dissolved gas constraints on spring water
from Popocatepetl volcano (Mexico): evidence of gaswater
interaction between magmatic component and shallow fluids
S. Inguaggiatoa,*, A.L. Martin-Del Pozzob, A. Aguayob, G. Capassoa, R. Favaraa
aIstituto Nazionale di Geofisica e Vulcanologia Sezione di Palermo, via Ugo La Malfa, 153, Palermo, 90146, ItalybInstituto de Geofisica UNAM, Ciudad Universitaria, Mexico DF, 04510 Mexico
Received 5 February 2004; accepted 1 September 2004
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 801008 C was estimated on the basis of liquid
geothermometers. A large amount of dissolved CO2in the springs was also detected and associated with high CO 2 degassing.
D 2004 Elsevier B.V. All rights reserved.
Keywords: popocatepetl volcano; helium isotope composition; carbon isotope composition; dissolved gases; gaswater interaction
1. Introduction
Popocatepetl (Popo) is a large andesitic stratovol-
cano (5452 m) near Mexico City, which has been
erupting since December 1994. The fumarolic activityincreased 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 (19181925), 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 geo-physical 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
0377-0273/$ - see front matterD 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jvolgeores.2004.09.006
* Corresponding author. Fax: +39 91 6809449.
E-mail address: [email protected] (S. Inguaggiato).
Journal of Volcanology and Geothermal Research 141 (2005) 91108
www.elsevier.com/locate/jvolgeores
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Fig.
1.
Locationmapofsampledsprings.
S. Inguaggiato et al. / Journal of Volcanology and Geothermal Research 141 (2005) 9110892
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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 volcanicproducts 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 CO2and SO2budget 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 ofvolcanic 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, 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 composi-
tion 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, 2001; Martin-Del Pozzo et
al., 2002a,b), based on the chemical and isotopic
composition of both water and dissolved gases, thereis 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 f luid
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 about1000 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
(d34S, d13C, d18O, and dD).
2. Analytical methods
Groundwater samples were collected in poly-
ethylene bottles for laboratory analyses while tem-
perature, 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
ofF2%. 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 was ascertained utilizing spectro-
photometric method based on yellow silicamolib-
date complex read at a wavelength of 650 nm.
Gases were analyzed using a Perkin Elmer 8500
gas-chromatograph equipped with a 4-m-long Car-
bosieve S II column and Ar as the carrier gas.
Helium, H2, O2, N2 and CO2 were measured by
means of a TCD detector while CH4 and CO were
determined through a FID detector coupled with a
methanizer. The detection limits were about 3 ppmvol. for He, 2 ppm vol. for H2 and 0.5 ppm vol. for
CO and CH4. The content of dissolved gas was
analyzed by the method described by Capasso and
Inguaggiato (1998) and Capasso et al. (2000) which
is based on the equilibrium partition of gas species
between a liquid and a gas phase after introduction
of a host gas into the sample.
Analyses of the dissolved He isotopic composition
were performed using the methodology proposed by
Inguaggiato and Rizzo (2004), which is based on
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isotope equilibrium between liquid and host gas
phases. The 3He/4He ratios were measured by a
VG-5400TFT double collector (accuracy F1%) and4He /20Ne ratios by a VG Masstorr FX quadruple
mass spectrometer (accuracy F5%).
The d13C of Total Dissolved Inorganic Carbon
(TDIC) and the d18O of H2O of spring waters were
analyzed by a Finnigan Delta Plus mass spectrometer.
Carbon isotopic values are expressed in dx vs. PDB,
with an accuracy of 0.2dx. Oxygen isotopic values areexpressed in dx vs. V-SMOW with an accuracy of
0.2dx. In particular, the chemical and physical
stripping of CO2 from the water samples was carried
out to determine the isotope ratios of TDIC, using the
Table 1
Chemical and isotopic composition of the water
Sample Date Altitude pH Electrical
conductivity
T Ca Mg Na K F Cl NO3 SO4 HCO3 SiO2 dD
(H2O)
d18O
(H2O)
d34S
(SO4)POPO
RIO Feb-99 8.2 152 18.1 8.4 6.0 12.3 3.0 0.29 8.8 2.5 18.3 64.1 44.5 78.0 11.0 n.d.TO Feb-99 2060 6.9 176 13.5 9.7 8.2 13.4 2.9 0.25 9.2 2.0 24.3 70.2 45.8 81.5 11.4 9.6AG Feb-99 2100 6.6 209 15.2 9.9 9.5 21.5 3.5 0.51 11.6 4.4 18.2 94.6 48.8 80.0 11.4 8.2AX Feb-99 1920 6.1 633 16.8 35.4 41.1 56.3 7.1 0.55 25.1 5.7 38.5 378 62.3 83.0 12.0 8.8SB Feb-99 2200 6.7 176 13.1 10.2 6.6 16.8 2.0 0.51 8.9 3.8 11.8 94.6 41.5 80.0 11.6 n.d.TG Feb-99 2140 6.9 202 17.1 9.7 7.8 20.0 3.7 0.80 9.7 9.9 20.5 88.5 43.6 82.0 11.5 6.7TL Feb-99 3620 7.4 72 7.6 4.7 2.0 6.7 2.5 0.13 2.1 1.3 3.3 42.7 54.0 79.0 11.5 n.d.AX Nov-99 5.9 680 19.1 39.9 43.3 65.8 7.8 0.76 21.9 2.0 37.7 421 n.d. n.d. 11.5 n.d.
IXTA
SP Feb-99 7.6 142 14.4 7.7 7.0 11.4 2.3 0.19 8.4 3.9 7.9 70.2 38.0 75.0 11.0 n.d.ZM Feb-99 7.8 113 8.7 9.2 3.0 6.2 2.9 0.21 7.9 5.7 14.0 39.7 51.6 84.0 12.0 6.5
CUAUTLA
AH Feb-99 1290 6.2 2443 26.2 411 122 112 11.0 3.50 192 21.8 1048 677 58.4 72.0 10.4 16.6IX Feb-99 1150 6.9 3088 51.5 495 77 326 14.7 4.31 564 20.7 1303 214 57.0 65.0 9.6 17.5AH Nov-99 6.3 1250 26.4 486 127 114 12.9 1.90 81 3.3 1225 833 n.d. n.d. n.d. n.d.
IX Nov-99 6.6 3400 52.1 516 87 372 28.5 3.61 585 7.8 1489 226 n.d. n.d. n.d. n.d.
AT Nov-99 1250 6.8 710 36.6 302 66.6 65.5 6.3 1.71 31 4.6 992 201 56.0 64.0 9.3 16.3
The values are expressed in mg/l. Temperature in 8C and electrical conductivity in AS/cm. The isotopic values are expressed in per mil vs. V-
SMOW for deuterium and oxygen and in per mil vs. CDT for Sulphur. Altitudes are expressed in m. n.d.=not determined.
Fig. 2. Temperature vs. TDS values of sampled springs.
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method proposed by Favara et al. (2002). Analytical
results are reported inTables 1 and 4.
3. 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
waterrock 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 bicarbonatealkalineearth field except for theAH, AT and IX springs, located further south of
Popo, which fall in the chloridesulphatealkaline
earth field. The relative SO4, Cl and HCO3 contents
of groundwaters (Fig. 3b) show that the collected
samples fall into two groups: one rich in HCO3(samples from the flanks of Popocatepetl) and the
other rich in SO4 (southern samples: AH, IX and
AT). Moreover, these two groups show different
SO4/Cl ratios, while the southern samples show
relatively high SO4/Cl ratios that are probably
linked to leaching of sedimentary gypsum.Conversely, the groundwater from the flanks of
Popocatepetl shows relatively low SO4/Cl ratios,
which are probably linked to the influence of
volcanic sources, such as volcanic gases that seep
through the volcano and interact with the rain-water
that feeds the springs (Martin-Del Pozzo et al.,
2002a). Alternatively, these low SO4/Cl ratios could
result from leaching of recently erupted ashes.
4. Waterrock interaction
4.1. Saturation index and geothermometer
The chemical composition and dissolved salt
content in natural water result from the interaction
betwee n wat er, gas and hos t roc k. 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 eachspring. Said index in relation to a given mineral
phase is defined as the logarithm of the ratio
between the ion activity product (I.A.P.), relative to
the mineral phase, and the corresponding solubility
product (Ksp):
S:I: log I:A:P:=Ksp 1
Computations were performed by means of the
WATEQP program (Appelo, 1988) (Table 2). An
aqueous solution is usually considered saturated by
Fig. 3. (a) LangelierLudwig diagram showing that all sampled
springs fall in bicarbonateearthalkaline field except the southern
samples (AH-AT-IX) that fall in chloridesulphatealkalineearth
field. (b) Triangular plot HCO3ClSO4. All the samples are shared
in two HCO3and SO4-rich groups.
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a given mineral phase when the S.I. ranges between
+5% and 5% of log Ksp (Jenne et al., 1980).Saturation indexes calculated for quartz, chalcedony,
calcite, gypsum and fluorite at outlet T, P are
plotted in Fig. 4a and b. All the samples are slightly
over-saturated as regards quartz and chalcedony but
almost all are under saturated as regards the other
mineral phases, except for samples AH, IX and AT
that are close to being saturated by the considered
minerals. This excludes the possibility that the water
samples from the flanks of Popocatepetl interact
with carbonate minerals.
The deep temperature of the aquifer feeding the
southern springs was estimated by several geo-
thermometers. These water geothermometers are
based on the following theoretical assumptions: (a)
the water and host rock are in equilibrium which
implies that the water is saturated by the mineral
phases governing the geothermometer; and (b)during the ascent towards the surface, the waters
did not undergo re-equilibration nor did they mix
with shallow fluids.
Deep temperatures at depth of the aquifer feeding
the southern springs were calculated by the quartz
geothermometer without any vapor loss (TQC,
Fournier, 1973), and chalcedony solubility. Equili-
brium temperatures obtained by these geothermom-
eters are about 120 8C for quartz and 80 8C for
chalcedony (Fig. 5).
The two geothermometers for waters flowing in
carbonate and evaporite aquifers (Marini et al.,
1986; Chiodini et al., 1995) provide deep temper-
atures (about 100 8C) consistent with the silicageothermometers (Table 3).
Fig. 4. Saturation index diagrams: (a) gypsumcalcite; (b) fluorite
quartz. These diagrams highlight that all sampled springs are
slightly over-saturated with respect to quartz and that only the
southern samples are close to the saturation with respect to gypsum,
fluorite and calcite.
Table 2
Saturation indexes table: saturation indexes calculated with respect
to (Quartz, Chalcedony, Calcite, Gypsum and Fluorite) at outletP,T
conditions
Sample Calcite Gypsum Fluorite Quartz Chalcedony
RIO 0.55 3.02 2.45 0.65 0.14TO 1.79 2.84 2.46 0.74 0.23AG 1.95 2.98 1.85 0.74 0.23AX 1.45 2.32 1.42 0.82 0.32SB 1.89 3.13 1.8 0.7 0.19TG 1.66 2.93 1.5 0.66 0.15TL 1.85 3.91 3.17 0.91 0.4SP 1.19 3.4 2.78 0.64 0.13ZM 1.24 3.04 2.52 0.87 0.36AH 0.15 0.34 0.72 0.65 0.2IX 0.35 0.22 0.63 0.19 0.2AT 0.00 0.27 0.41 0.71 0.25
Computation were performed by means of the WATEQP program
(Appelo, 1988).
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4.2. H, O and S stable isotopes
The dD and d18O isotopic data of the water
samples are plotted in Fig. 6 together with the
world meteoric water line (Craig, 1961) and the
local meteoric water line (Cortes et al., 1997). This
graph shows that all the water samples fall between
the two meteoric water lines, suggesting that the
spring waters are of meteoric origin or, at least,
controlled largely by meteoric recharge (Martin-Del
Pozzo et al., 2002a). Furthermore, the samples do
not show any isotopic shift in oxygen due to water
rock exchange. This suggests that the southern
waters come from low-temperature relatively
bdynamicQ geothermal reservoirs, characterized by
short residence time, as suggested by Giggenbach
(1991).
The isotopic composition of sulfur is plotted
against the total concentration of SO4 of the spring
waters in Fig. 7, along with the isotopic composi-
tion of the Cretaceous marine evaporites, and
PopoTs crater fumaroles. The samples fall into two
main groups: (a) one characterized by a high SO4content (10001300 mg/l) and high isotopic values,
close to 17 vs. CDT; and (b) another one with a
very low SO4 content (1439 mg/l) and relatively
low isotopic values, close to 8 vs. CDT.
The first group is related to the leaching of
Cretaceous evaporite beds underlying the southernarea, which are expected to have an average 734S
value of 15.6x (Nielsen et al., 1991), which is very
close to the isotopic values of S in these springs.
Furthermore, as previously discussed, these springs
are in equilibrium with gypsum. The second group
is probably related to volcanic SO2 that interacts
with the meteoric-waters soaking the Popo edifice.
The isotopic value of S in the fumaroles adsorbed
from alkaline traps is 3.35F0.92 (Goff et al., 1980).
This value is higher than that of the mantle sulphur
Fig. 5. Silica content vs. outlet T of sampled springs. Assuming equilibrium with chalcedony solubility, a process of cooling starting from about
80 8C, has been hypothesized for Cuautla springs.
Table 3
Estimated equilibrium temperatures computed by several liquid
geothermometers
Sample T-
vent
T-
Chalcedony
T-
Quartz t
TCa
Mg
TSO4
FHCO3
Cuautla
AH 26.2 80 132 75 98
IX 51.5 78 120 105 100
AT 28.7 78 130 52 99
The temperature is expressed in 8C.
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and is probably to be referred to the assimilation of
evaporite rocks by the magma (Goff et al., 1980).
The S isotopic values of the second group of
springs, ranging from 6.2x to 9.8x, are slightly
more positive than those of the fumaroles. This can be
explained at least in two ways: shallow contamination
with sulfur of evaporitic origin or fractionation
processes between SO2
and dissolved SO4
(Sakai,
1968). Considering that the SO4contents in the spring
waters from Popo are very low (14 to 39 mg/l), the
addition of very small amounts of evaporitic S would
be sufficient to move the isotopic composition of S
from 3.35 (average crater fumarole value) towards the
values found in the springs.
5. Gaswater interaction
5.1. Dissolved gas contents
Dissolved gases in spring water are a useful tool
for understanding gaswater interactions. High
mobility in addition to different solubility makes
gases excellent geochemical tracers. The amount
Fig. 6. dD vs. d18O diagram. The World Meteoric Water Line, and the Local Meteoric Water Line are also shown.
Fig. 7. d34S values vs. SO4 content of sampled springs. The values of the magmatic and evaporitic end-members are also indicated.
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and type of dissolved gases have been successfully
used in geochemical investigations to solve hydro-
logical, geothermal, and volcanological problems
(Dyck, 1976; Capasso et al., 2000, 2001; Inguag-giato et al., 2000; Taran et al., 2002; Carapezza et
al., 2004).
The chemical composition of the dissolved gases
in almost all the samples of the Popo spring water
shows high gaswater interaction (Table 4). In fact,
the amount of dissolved CO2 measured in the
spring waters (up to 336 cm3 STP/l) is several
orders of magnitude higher than that of water in
equilibrium with the atmosphere. Furthermore, high
concentrations of helium (up to 5104 cm3 STP/l)
were also detected in the southern springs (Fig. 8).The relative amounts of dissolved O2, N2, and CO2,
are shown in Fig. 9. All the samples have O2/N2ratios lower than the atmosphere and show different
N2/CO2 ratios, with a minimum of 0.03 to 0.10 for
samples AX and AH. These samples have a CO2partial pressure of up to 0.5 atm. Three groups of
samples can be identified in this graph: CO2dominated samples (AH and AX) that fall close to
CO2 vertex; samples with intermediate N2/CO2ratios, between 1.5 and 0.6; N2-rich samples (ZM,
SP, TL) that fall relatively close to the N2 vertex
(N2/CO2 ratio around 2.6); The chemical composi-
tion of the dissolved gases does not reflect the
geographic distribution of the sampled springs (i.e.
Popo flanks vs. southern area) and is related to
different degrees of gaswater interaction andvarying gas contents and composition. The IX and
Table 4
Chemical and isotopic composition of dissolved gases
Sample Data He H2 O2 N2 CO CH4 CO2 Log
(C/3He)
d13C
(CO2)
R/Ra He/
Ne
R/Racorr.
Diss. gases
POPO
TO Feb-99 1.6e04 b.d.l. 0.1 15.2 b.d.l. 5.63e04 9.9 11.1 19.6 1.11 0.62 1.37AG Feb-99 7.6e05 b.d.l. 0.2 13.6 b.d.l. b.d.l. 17.2 11.5 14.3 1.14 0.51 1.51AX Feb-99 7.2e05 b.d.l. 0.3 9.9 b.d.l. 3.38e04 336.6 12.3 6.9 1.78 0.60 2.63AX Jun99 n.d. b.d.l. 2.0 11.6 1.7e03 3.22e03 301.4 n.d. n.d. n.d. n.d. n.d.AX Nov-99 n.d. 3.5e04 0.1 15.3 7.8e05 n.d. 244.7 n.d. 7.2 n.d. n.d. n.d.SB Feb-99 n.d. b.d.l. 0.3 18.1 2.5e05 1.38e04 19.7 n.d. 15.6 n.d. n.d. n.d.TG Feb-99 1.6e04 b.d.l. 0.1 14.4 b.d.l. 3.10e04 11.4 11.2 22.1 1.02 0.66 1.30TL Feb-99 1.5e04 b.d.l. 0.1 14.4 b.d.l. 1.07e03 5.6 10.9 21.4 1.12 0.71 1.46
IXTA
SP Feb-99 4.9e05 b.d.l. 0.2 14.2 1.4e04 8.26e03 5.4 11.6 22.9 1.06 0.89 1.25ZM Feb-99 n.d. b.d.l. 1.1 11.1 n.d. 1.82e04 4.3 n.d. 17.5 n.d. n.d. n.d.
CUAUTLA
AH Feb-99 4.8e04 b.d.l. 0.1 16.5 1.84e04 3.25e02 398.9 12.5 7.8 2.74 6.43 2.84AH Jun99 n.d. 1.4e03 0.6 11.5 b.d.l. 7.29e03 284.6 n.d. n.d. n.d. n.d. n.d.AH Nov-99 n.d. b.d.l. 0.3 29.2 b.d.l. 1.64e02 302.0 n.d. 8.1 n.d. n.d. n.d.
IX Feb-99 n.d. 4.6e04 2.8 11.9 1.2e04 6.88e02 17.4 n.d. n.d. n.d. n.d. n.d.IX Nov-99 4.8e04 b.d.l. 2.9 12.7 b.d.l. 1.89e02 25.8 11.9 8.3 1.34 2.41 1.43AT Feb-99 1.8e04 b.d.l. 0.1 16.2 n.d. 2.50e02 15.0 11.4 12.0 1.17 1.69 1.36AT Jun99 2.4e04 b.d.l. 1.5 18.0 3.2e04 1.06e02 31.1 n.d. n.d. n.d. n.d. n.d.AT Nov-99 n.d. b.d.l. 1.2 12.6 b.d.l. b.d.l. 20.0 n.d. n.d. n.d. n.d. n.d.
A.S.W. 4.55e05 ***** 6.6 12.3 ***** ***** 0.3 1 0.286 1
Bubb. gases
IX Feb-99 290.0 5.0 5.0 90.5 1.4 6.30E+03 3.0 n.d. 9.9 n.d. n.d. n.d.IX Nov-99 366.0 9.0 0.3 92.0 b.d.l. 7.96E+03 9.0 8.1 8.5 1.48 55.28 1.48
The chemical values are expressed in cm3 STP/l for dissolved gases. The chemical values of bubbling gases are expressed in ppm vol for He,
H2, CO and CH4and in vol.% for O2, N2and CO2. The isotopic values are expressed in per mil vs. PDB for Carbon isotope and as R /Ra(where
Ra=1.36106) for Helium isotope. b.d.l.=below detection limits. n.d.=Not determined.
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AT samples are also characterized by high quantities
of dissolved He, with up to 366 ppm in the
equilibrated free gas phase. Two explanations can
be given for the origin of the dissolved gases in the
samples that plot relatively close to the N2 vertex:
(a) the chemical composition of the pristine gas is
N2-dominated with a high He content;
(b) the chemical composition of the pristine gas is
CO2-dominated; during ascent towards the surface it
loses CO2through water interaction. This is caused by
the high solubility coefficient of CO2, a n d a s a
consequence, the residual gas is enriches in the low-
solubility components such as He, CH4and N2.
5.2. Isotopic composition of dissolved gases
The helium and carbon isotopic compositions of
the dissolved gases in groundwater give useful
information about the origin of the gases and the
physico-chemical processes they undergo during
their rise towards the surface (Sano and Wakita,
1985; Nakai et al., 1997; Marty et al., 1994;
Capasso et al., 1997; Inguaggiato et al., 2000;
Inguaggiato and Rizzo, 2004; Capasso et al., 2004).
Moreover, because of their different mobility and
chemical reactivity, these gases can highlight differ-
ent degrees and kinds of interaction with shallow
fluids.
Helium is a non-reactive, chemically inert gas that
has low solubility in water, i.e. only 9 cm3/l STP when
the He partial pressure is equal to 1 atm. Furthermore,
it does not undergo any significant chemical or
isotopic modifications during its interaction with
shallow fluids, at least in the absence of diffusive
effects (Jahne et al., 1987; Chiodini et al., 2000).On the contrary, CO2 is a highly soluble and
chemically reactive gas that undergoes large chemical
and isotopic modifications when interacting with
shallow fluids. These modifications strongly depend
on the temperature and pH of the groundwater.
5.2.1. Carbon
To better define the origin of carbon, it is important
to consider that the isotopic composition of TDIC is
the result of the following chemical and isotope mass
Fig. 8. CO2vs. He contents of dissolved gases in the sampled springs. The ASW (Air Saturated Water) values are also reported for comparison.
Fig. 9. Relative amounts of dissolved of CO2, N2and O2. The ASW
values are also reported for comparison.
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balance, where M stands for molarity, and the activity
of CO3 is considered negligible for pHb8.3:
d13
CTDIC d
13
CCO2aq4
MCO2aq
d13CHCO34MHCO3 =MTDIC 2
MTDIC MCO2aq MHCO3 3
By utilizing the enrichment factors ea and eb(Mook et al., 1974; Deines et al., 1974),
ea d13CHCO3 d
13CCO2g 9552=TK24:1 4
eb d13CCO2aq d
13CCO2 g 0:91
0:0063*106
=T2
k 5Eq. (2) can be written as follows:
d13CCO2g d
13CTDIC eb*MCO2aq=MTDIC
ea*MHCO3=MTDIC 6
In this way, we can calculate the d13C value of the
pristine CO2 gas interacting with the groundwater
(Inguaggiato et al., 2000) the obtained values are
reported inTable 4.
The d13C computed values have been justified
utilizing two different models:
(1) Two end-members mixing (magmaticorganic):the d13C computed values have been plotted inFig. 10
against the total content of dissolved inorganic carbon.
Thed13CCO2g of the Popo crater area (Goff et al., 1980)
has also been reported for comparisons sake. A
theoretical mixing-curve between magmatic and
organic end-members was constructed considering
values of6.5x with 35 mmol of TDIC and 25xwith 1 mmol of TDIC, respectively. All the analytical
data for the water-samples fall along a theoretical
mixing-curve that strongly supports this hypothesis.
(2) A Rayleigh-type fractionation process by CO2removal: this hypothesis implies that the deep gas is
CO2-rich (9095 vol.%). In this case, the CO 2content
in the bubbling gas phase is the result of strong CO2removal caused by dissolution in the deeper aquifer.
This process virtually enriches the gain mixture in the
less-soluble components such as He and CH4. In a
situation like this, we can model a Raleigh-typefractionation process that removes about 95% of CO 2from the pristine gases and moves the original isotope
composition of Carbon towards the more negative
values. The isotope C value of pristine CO2, which
interacts with a deep aquifer, can be calculated by
using the following relation (Inguaggiato et al., 2000):
d13CCO2res 1000d
13CCO2ini *fa1 1000 7
where f is the fraction of the CO2 remaining in the
system, a stands for the isotopic fractionation factor
between HCO3 (aq) and CO2 (g), and subscripts res andini refer to residual and initial, respectively. The results
of this theoretical process, which has two different
starting values for the carbon isotope composition
Fig. 10. d13CCO2 gvs. TDIC plot. A theoretical mixing-curve between magmatic and organic end-members was constructed considering the
values of6.5x with 35 mmol of TDIC and 25x with 1 mmol of TDIC, respectively.
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(+2x, 6x) at two different temperatures (30 and508C, which is approximately theoutlet temperature of
springs IX and AT) are shown inFig. 11.According to
this type of process, the possible initial values of CO2for IX and AT samples range around +2. These
theoretical values are compatible with CO2 of carbo-
nate origin, considering that IX and AT springs are in
equilibrium with carbonate minerals sediments (see
above), which outcrop in this zone. The possible initial
value of CO2 for all the remaining samples is 6,which is compatible with a different degree of
magmatic CO2removal (0.15bfb0.85).
To investigate and support the existence of this
selective CO2dissolution process, we also modelized
the chemical effect linked to this process. During aselective CO2-dissolution in the groundwater, the less
soluble gases, as He, undergo a virtually process of
enrichment producing a change in the He/CO2ratio. In
graph ofFig. 12,the He/CO2ratio has been plotted vs.
the isotopic composition of CO2. This diagram high-
lights the good inverse correlation between these two
parameters showing a increase of He/CO2ratio and a
decrease ofd13CCO2g, confirming the existence of this
process. Only the AT and IX sample values lie out of
this trend, probably because of the different starting
point of isotopic CO2 value (+2) and also for the
differences in outlet temperatures.
5.2.2. HeliumThe isotope composition of dissolved He ranges
from 1.37 to 2.63 (R/Racorr.; R a=1.39106), which
indicates a magmatic contribution. Fig. 13shows the
isotopic composition of CO2 gas vs. the He isotopic
composition. The AX and AH samples show the
highest He isotope ratios (around 3 R/Ra) which
suggests that there is a high input of He, clearly of
magmatic origin. The remaining samples show values
ranging from 1.2 to 1.5R/Ra. The IX sample is the only
one in which we also sampled the bubbling gas phase.
It is characterized by a N2-dominant composition, lowCO2content (about 9%) and high He and CH4content
(366 and 7960 ppm vol, respectively). The isotopic
composition of the He of free gas (R/Racorr.=1.48)
confirms the value measured in the dissolved gas (R/Racorr.=1.43) which indicates that there is a good cor-
respondence between free and dissolved gases while
supporting the hypothesis of a significant deep, magma
or mantle contribution for this gas. The theoretical
amount of dissolved He in the IX spring, recalculated
from the He bubbling gas (30104 cm3/l), is
Fig. 11. d13CCO2 resgasvs. residual fraction (f). Theoretical Rayleigh-type fractionation process with two different starting values of carbon
isotope composition (+2x and6x) at two different temperatures (30 and 50 8C).
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compatible with the measured amount of dissolved
helium (8104 cm3/l) this also indicates that a partialchemical equilibrium between the dissolved and free
gas phases for this gas has been reached.
Assuming that the isotopic values of He are not
influenced by shallow interaction processes, and by
coupling this information with the carbon isotope
composition, a non-linear isotopic mixing of two
component end-members could be hypothesized to
explain the origin of the deep gases and the processesthey underwent. So as to test this last hypothesis, we
used the equation proposed byLangmuir et al. (1978):
if bMQ and bOQ are two end-members containing
different amounts of a given compound C and if IC
represents the isotopic ratio of C, it is possible to
calculate the isotopic value of the mixture S, ISC, by
utilizing the following equation:
ICS ICMXMyI
CO 1y
=XMyXO 1y 8
where ISC is the isotopic value of mixing, XM and
XO are respectively the contents of C in M and O,the magmatic and organic end-members, and y is
the molar fraction of end-member M in the mixture.
Fig. 12. d13CCO2 gas vs. He/CO2 ratio of dissolved gases showing a inverse correlation between these two parameters.
Fig. 13. d13CCO2 gas vs. 3He/4He diagram. Theoretical mixing lines of two components, organic and magmatic end-members, considering
different values ofK(0.1 to 100) are plotted along with a pair of isotopic values (He and C) of the sampled springs ratio of dissolved gases
showing a inverse correlation between these two parameters.
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To calculate the isotopic composition of two
elements (C and He) in a binary mixture, we need
to write Eq. (8) twice (for two different isotope), and
solve both of the (Langmuir et al., 1978) relationswith respect to y. The following equation is thus
obtained:
aICS bICS I
HeS cI
HeS d 0 9
where, in our case, a=IMC*CMHe OIO
C*COHeM;
b=C OHeMCMHeO; c=IOHe*CMHeOIM
He*COHeM;
d=IOC*IM
HeCOHeMIMC IHeO CMHeO.
To define the shape of the mixing line, we can use anew constant K=(C/He)O/(C/He)M. When K=1, b=0,
then the mixing line is a straight-line. On the contrary
whenKN1 orb1, the mixing line is a hyperbola andthe
curve increases for KH1 or Kb1. In Fig. 13, the
Fig. 14. (a) R/Ravs. log C/3He values for the sampled springs and fluids from other parts of the world including two Mexican volcanoes
(Ceboruco and Colima (data from Taran et al., 2002). (b) d13CCO2 gas vs. log C/3He. springs ratio of dissolved gases showing a inverse
correlation between these two parameters.
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theoretical mixing-line between organic and magmatic
end-members, considering different values ofK (1 to
100), has been plotted together with the pair of isotopic
values (He and C) of the sampled springs. A theoreticalmixing-curve was constructed considering values of
6.5x of carbon and 6 R /Raof He for the magmaticend-member, and values of25x of carbon and 1 R /Ra for organic end-member. The origin of He in the
organic end-member was mainly supposed to be of
recycled atmospheric origin. The sample points, which
fall along mixing lines with Kvalues ranging between
10 and 100, strongly support the hypothesis of a non-
linear mixing between magmatic and organic end-
members for these samples.
To better clarify the origin of these gases and theprocesses that they underwent during the ascent
towards the surface, the log of the C/3He ratio is
plotted vs. the isotopic composition of both He and C
(Fig. 14; Marty and Jambon, 1987; Varekamp et al.,
1992;Sano and Marty, 1995).
In Fig. 14a, the R/Ra vs. log C/3He values of the
sampled springs are plotted together with data from
other parts of the world including the two Mexican
volcanoes Ceboruco and Colima (data from Taran et
al., 2002).
All the sampled waters show log C/3He values
ranging between 10.8 and 11.6, withR/Ravalues above
1 and below 3, which are consistent with pristine
mantle deep fluids contaminated by carbon addition
linked to continental crust end/or carbonate sediments.
The higher R/Ravalues of AX and AH support the
hypothesis of a more He-mantle contribution for these
samples that could be referred to many causes: different
hydrological system, different tectonic system (magni-
tude and/or distance to fault), etc.
In the same graph, the analytical values of gaseous
phase of IX (IX bubbling) have been plotted. This point
shows the sameR/Ravalue but with a very different logC/3He ratio caused by selective CO2 removal processes
that virtually increase the He content. If we consider
that the CO2content decreases by about one order of
magnitude (90 to 9 vol.%) and consequently that the He
increase is of the same magnitude, the restored log
C/3He for these samples is 10.1 which is compatible
with mantle origin. The log C/3He differences observed
in the free and dissolved IX sample strongly support the
importance of the investigation of both phases which
give us a complete picture of both the shallow
interaction processes and the relative modifications of
thepristinechemical composition.
Fig. 14b highlights the scattering of the d13C
isotopic composition, caused by mixing or fractiona-tion processes of the original fluids (magmatic or
carbonate origin) through interaction with shallow
fluids. Moreover, the IX bubbling shows that neither
also for carbon in this sample underwent any isotope
fractionation process between free and dissolved IX
phases.
6. Discussion
Since Popo volcano has gone beyond the immaturestage, 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
(Werner et al., 1997).
The results obtained in the course of the present
geochemical studies in question contradict these
previous studies and confirm the hypothesis put
forward by Inguaggiato et al. (1999, 2001) and
Martin-Del Pozzo et al. (2002a,b), wherein spring
waters carry the signature of magmatic processes.
The different interpretations regarding the interac-
tion processes between deep magmatic fluids and
shallow fluids are probably linked to a different
geochemical approach. The investigated area is huge
and a large number of areas are difficult to access
especially with heavy scientific equipment. This makes
it extremely difficult to make a methodical survey ofdiffuse soil degassing. Due to the insufficient number
of measurements and the non-uniform distribution of
the aerial coverage, the geochemical interpretation
probably does not truly reflect the real degassing
situation. As a comparison, the diffuse degassing
studies of a Mt. Etna show anomalous zones with high
fluxes on its flanks located only in a few points that
represent a very small surface of the total area and are
hard to find (Giammanco et al., 1998a,b). For these
reasons, it is very likely that on the Popo flanks, there
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are zones with anomalous degassing that still have to be
identified and monitored. Instead, the geochemical
approach used in our work, on the contrary, was based
on the study of the dissolved gases in the groundwaterscirculating in the volcanic edifice. The main advantage
of this approach is that the chemical and isotopic
characteristics of the dissolved gases in spring waters
result from the overall deep degassing gathered by the
dynamic system along several rising pathways. On the
contrary, soil degassing has to be seen as a relic of deep
degassing after the interaction with groundwater.
To support this, the amount of CO2dissolved in the
aquifers of Popo area has been preliminarily estimated
considering a mean of total dissolved carbon of 10
mmol/l (this work) and a mean of annual rainfall ofabout 2 km3 (SPP, 1981; Cortes et al., 1997; CNA,
2001; Martin-Del Pozzo et al., 2002a) referred to a
surface of 800 to 3000 km2 (Popo cone onlyentire
area covered by the Popo deposits, respectively).
Therefore, an average amount of total dissolved CO2ranging approximately from 1 to 2 Mt/a has been
estimated for the groundwaters circulating in the Popo
area.
This value represents about the 10% of total
amount of CO2 emitted from the plume of Popo
volcano (14.536.5 Mt/a) as estimated fromDelgado-
Granados et al. (1998).
Furthermore, the geochemical approach employed
byWerner et al. (1997)did not allow them to identify
the chemical interaction processes occurring between
deep and shallow fluids, as the authors collected and
analyzed water samples for major and minor compo-
nents but did not investigate the chemical and isotope
composition of the dissolved gases. The Popo springs
show a very low salt content resulting from low
waterrock interaction, whereas the only way to
identify gaswater interaction is to investigate the
dissolved gas species.In fact, the high CO2and He contents measured in
the spring-samples, coupled with the respective iso-
topic compositionsclearly of magmatic origin
indicate a high gaswater interaction between deep
CO2-rich gases and shallow fluids. This process lowers
the pH of water and makes it more aggressive
promoting dissolution of sublimates and ash compo-
nents (Martin-Del Pozzo et al., 2002b). The possible
sources that determine the chemical composition of
springs-water are: (a) input of strongly acid magmatic
fluids (HF, HCl, H2S, and SO2); (b) sublimate
remobilization, ash solubilization, reaction with pyro-
clastic rocks; and (c) input of weak acidic magmatic
gases, mainly CO2.Process (a) can be ruled out because of the very
low salinity, low temperature, and not very acid pH of
the waters; the processes of group (b) are all possible
considering the low amount of dissolved salt in the
springs that is quite often less than 200 mg/l; process
(c) is very probable and well supported because the
quantity of dissolved carbon dioxide in the water is
very high (up to 336 cm3/STP liter) and its isotopic
composition suggests a magmatic origin. Moreover,
the isotopic composition of dissolved He also
supports a magmatic component in the waters.Despite the large difference in the chemical compo-
sition of the hot springs in the south with respect to
those located on the flanks of the volcano (i.e. linked to
different interacting lithologies), the isotopic character-
ization of the dissolved gases indicates a common
volcanic origin for the southern AH hot spring and the
ones located on the Popo flanks. However, further
researches are necessary in order to understand the
origin and possible mechanisms that determine the
chemical and isotopic composition of IX and AT
springs. Nevertheless, based on the isotopic composi-
tion of He and C of the dissolved gases, a connection
with volcanic activity cannot be ruled out.
7. 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 waterrock 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,b).
(2) High contents of magmatic gases (He and CO 2mainly as revealed by the chemical and isotopic
fingerprints) are present as dissolved gases in the cold
groundwaters located on the flanks of Popocatepetl;
(3) The flanks of the volcano release large amounts
of CO2 which have been detected as dissolved gases
in the groundwater, indicating the presence of
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important CO2 magma degassing. This is supported
by the estimated amount of CO2 released from the
Popo flanks effectuated by coupling the information
of volume of Popo groundwater and the amount oftotal carbon dissolved.
(4)The hot water (AH spring) emerging at about 40
km to the south of Popos crater is probably the only
surface manifestation of the geothermal system (i.e.
estimated temperature about 80100 8C) located
beneath the volcano. Further research on Cuautlas
thermal springs will help to clarify its origin.
(5)The characteristics of the IX and AT springs are
probably linked to both regional tectonic structures
and/or the volcanic activity of Popocatpetel. Even
though the isotopic composition of C suggests aprobable carbonate origin for these gases, the isotopic
composition of He reveals the presence of a mantle
component.
In conclusion, the chemical and isotopic composi-
tions of the water and dissolved gases in the Popo
springs have given us insight into the mechanism and
degree of interaction between the deep magmatic
fluids and shallow groundwaters.
Acknowledgments
Authors wish to thank DGAPA (PAPIIT), Inter-
cambio 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 com-
ments 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
Fausto Grassa for their isotopic analytical support.
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