Journal of Volcanology and Geothermal Research 205 (2011) 67–83
Contents lists available at ScienceDirect
Journal of Volcanology and Geothermal Research
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s
Fluid origin, gas fluxes and plumbing system in the sediment-hosted Salton Sea
Geothermal System (California, USA)
Adriano Mazzini a,⁎, Henrik Svensen a, Giuseppe Etiope b, Nathan Onderdonk c, David Banks d
a
Physics of Geological Processes, University of Oslo, Sem Sælandsvei 24, Box 1048, 0316 Oslo, Norway
Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 2, Via V. Murata, 605, 00143, Italy
Department of Geological Sciences, California State University, Long Beach, 1250 Bellflower Blvd., Long Beach, CA 90840, USA
d
School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
b
c
a r t i c l e
i n f o
Article history:
Received 12 January 2011
Accepted 31 May 2011
Available online 12 June 2011
Keywords:
Salton Sea Geothermal System
hydrothermal seeps
gas and water geochemistry
flux measurements
mantle
a b s t r a c t
The Salton Sea Geothermal System (California) is an easily accessible setting for investigating the interactions
of biotic and abiogenic geochemical processes in sediment-hosted hydrothermal systems. We present new
temperature data and the molecular and isotopic composition of fluids seeping at the Davis-Schrimpf seep
field during 2003–2008. Additionally, we show the first flux data for CO2 and CH4 released throughout the
field from focused vents and diffuse soil degassing.
The emitted gases are dominated by CO2 (~ 98%) and CH4 (~ 1.5%). By combining δ13CCO2 (as low as − 5.4‰)
and δ13CCH4 (−32‰ to − 17.6‰) with 3He/4He (R/Ra N 6) and δDCH4 values (− 216‰ to − 150‰), we suggest,
in contrast to previous studies, that CO2 may have a significant Sub-Continental Mantle source, with minimal
crustal contamination, and CH4 seems to be a mixture of high temperature pyrolitic (thermogenic) and
abiogenic gas.
Water seeps show that δD and δ18O increase proportionally with salinity (Total Dissolved Solids in g/L)
ranging from 1–3 g/L (gryphons) to 145 g/L (hypersaline pools). In agreement with elemental analyses, the
isotopic composition of the waters indicate a meteoric origin, modified by surface evaporation, with little or
no evidence of deep fossil or magmatic components. Very high Cl/Br (N 3,000) measured at many seeping
waters suggests that increased salinities result from dissolution of halite crusts near the seep sites.
Gas flux measurements from 91 vents (pools and gryphons) give a conservative estimate of ~ 2,100 kg of CO2
and 11.5 kg of CH4 emitted per day. In addition soil degassing measured at 81 stations (20x20 m grid over
51,000 m 2) revealed that 7,310 kg/d CO2 and 33 kg/d CH4 are pervasively released to the atmosphere. These
results emphasise that diffuse gas emission from soil can be dominant (~75%) even in hydrothermal systems
with large and vigorous gas venting. Sediment-hosted hydrothermal systems may represent an intermediate
class of geologic methane sources for the atmosphere, with emission factors lower than those of sedimentary
seepage in petroleum basins but higher than those of traditional geothermal-volcanic systems; on a global
scale they may significantly contribute to the atmospheric methane budget.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Sediment-hosted hydrothermal systems occur in a range of
geological settings, especially in active rift basins both onshore and
offshore (e.g. Helgeson, 1968; Von Damm et al., 1985; Simoneit, 1988;
Jamtveit et al., 2004; Zarate-del Valle and Simoneit, 2005). The fluids
involved may include mixtures of biotic gases, due to rapid thermal
maturation of kerogens, with possible oil generation (e.g. Welhan and
Lupton, 1987) and abiogenic gases derived by late- or post-magmatic
polymerization synthesis and Fischer-Tropsch Type (FTT) reactions
⁎ Corresponding author.
E-mail address: adriano.mazzini@fys.uio.no (A. Mazzini).
0377-0273/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jvolgeores.2011.05.008
(e.g. Simoneit, 1988; Potter et al., 2004) or from magma degassing
(e.g. Werner et al., 2008).
A key example of this type of setting is the Guaymas Basin in the
Gulf of California (e.g. Simoneit and Galimov, 1984; Welhan and
Lupton, 1987), where shallow sill intrusions associated with the East
Pacific Rise have led to hydrothermal fluid circulation, rapid
maturation of organic matter, oil generation, and seeps at the sea
floor. Following this trend further north in the Salton Trough (Fig. 1),
sedimentary rocks are deposited in a pull-apart basin intruded by
Quaternary basalt and rhyolite bodies that provide the heat
characterizing the Salton Sea Geothermal System (SSGS) (e.g.,
Helgeson, 1968; Elders et al., 1972; Younker et al., 1982).
Although the hydrothermal system has been thoroughly investigated (e.g. numerous confidential prospects for geothermal energy
production, drilling site “State 2.14”, Elders and Sass, 1988; Fig. 1),
68
A. Mazzini et al. / Journal of Volcanology and Geothermal Research 205 (2011) 67–83
Wister field
Mullet Island
State 2-14
B
DSF
A
Red Hill
Power plants
A
Davis Rd.
N33 12'6.12''
1 km
W115 34'38.1''
W115 34'43.5''
B
North pool
C
Central gryphon
Central East gryphon
East gryphon
West pool
South gryphon
oil pool
West gryphon
East pool
Central pool
Artificial lake
Ditch
120m south
Sulphate field
N33 11'58.92''
50 m
Fig. 1. (A) Location map of the south eastern Salton Sea. The main known faults (red lines) and seepages (black dots) are marked as well as the intensity of the thermal-gradient
anomaly that has the highest peaks at warmer colours. Compiled after Lynch and Hudnut (2008); Hulen et al. (2002). (B) Google Earth image of the SE Salton Sea where the
Davis-Schrimpf seep field (DSF), the Wister field, power plants and rhyolite extrusions are present. Line A-B refers to the section sketched in Fig. 7; (C) overview of the DavisSchrimpf seep field and the sampling stations.
there are only few studies specifically dealing with the SSGS surface
fluid manifestations. Published works on gas and water geochemistry
from the Davis-Schrimpf field and the State 2.14 hole suggest that the
gas seeping in the SSGS originates from 1) active decarbonation
reactions involving sedimentary carbonates (generating CO2 with a
δ 13C value close to the original carbonate composition, ~ -5‰ VPDB),
2) rapid high temperature maturation of organic matter (generating
CH4 with a δ 13C of −32‰ VPDB); the seeping water instead seems to
be compositionally modified by surface evaporation, but a deep origin
can not be excluded (Clayton et al., 1968; Muffler and White, 1968;
Williams and Mckibben, 1989; Sturz et al., 1992; Svensen et al., 2007).
The gas venting is mainly driven by the very steep geothermal
gradients at the South-East end of the Salton Sea (e.g., Helgeson,
1968) where most of the geothermal energy extraction occurs (Towse
and Palmer, 1975). Lynch and Hudnut (2008) hypothesized that faults
associated with the San Andreas fault system control the location of
the seeps. The regional CO2 degassing from the Salton Trough is poorly
quantified; based on theoretical estimates of convective heat flow
data, Kerrick et al. (1995) suggested that about 10 9 mol/y of CO2 is
released to the atmosphere.
A. Mazzini et al. / Journal of Volcanology and Geothermal Research 205 (2011) 67–83
69
In this paper, we present the results of a four year investigation
and sampling program of seep gas and water from the main seep field
in the SSGS, the Davis-Schrimpf field (DSF), in order to give more
constrained answers to the following main questions: 1) What is the
ultimate source of the seeping gas? 2) What is the origin of the
seeping water? 3) How is the structure of the subsurface plumbing
system affecting the surface seep temperatures? 4) Is the seepage
confined to the vents or is soil degassing a significant component of
the total gas release? 5) What is the total emission of the two
greenhouse gases, CO2 and CH4, to the atmosphere?
3.1. Water density and salinity
2. The geothermal field
Water temperature was measured with a hand held TFX 392 SK-5
thermometer with a precision of 0.1 °C. Temperature monitoring
methodology and results are described in Svensen et al. (2009),
additionally here we report new time series monitoring over a
sulphate-rich area in the south east of the field where a thermometer
was positioned 0.1 m underground through the period 06 December
2007 to 05 April 2008. A night survey for thermal imaging and filming
of different vents was carried out using a Flir System P640HS camera
in December 2007.
The SSGS is situated in the Salton Trough in Southern California, a
high heat flow area with abundant surface manifestations of
hydrothermal seeps. The hydrothermal activity in the Salton Trough
occurs in a pull-apart setting where Quaternary magmatic intrusions
are responsible for high heat flow of up to 600 mW/m 2 (e.g. Helgeson,
1968; Robinson et al., 1976; Lachenbruch et al., 1985; Williams, 1997).
Although the shallow stratigraphy of the basin is well known due to
extensive hydrothermal energy exploration and drilling in the 1980's,
in particular the Salton Sea Scientific Drilling (Elders and Sass, 1988),
only thin sill intrusions have been intersected. Meta-sediments are
present throughout the deepest drilled intervals (~3000 m) where
the temperature reaches 350 °C, and the heat source is likely to be
related to deeper intrusions.
Inferred hot intrusions at depth produce contact metamorphism of
predominantly fluvial and lacustrine sediments, and result in
temperatures exceeding 350 °C at a depth of 1400 m in the highest
heat flow zone (e.g. Helgeson, 1968; Muffler and White, 1969; Elders
and Sass, 1988; Haaland et al., 2000). Both the salinity and
temperature distribution of hydrothermal fluids from boreholes
suggest that there is a fluid density interface between deep saline
brines and less saline shallow waters (Williams and Mckibben, 1989;
Williams, 1997). The high salinity of the hydrothermal reservoir fluids
(up to 150,000 ppm total dissolved solids) is attributed to dissolution
of evaporites, modified by high temperature fluid-rock interactions
(Helgeson, 1968). Seeps and fluid expulsion features are likely a
consequence of the active contact metamorphism. Two seep fields are
located close to the south-eastern shore line of the Salton Sea (the
Davis-Schrimpf and the Wister fields), and seep activity is aligned
along an inferred fault zone (Lynch and Hudnut, 2008). This makes
the Salton Sea area a highly dynamic fluid expulsion zone, an
attractive area for studying long term seep dynamics, and is
furthermore a relevant onshore analogue of the offshore Wagner
Consag and Guaymas Basin hydrothermal seeps (e.g. Simoneit, 1985;
Canet et al., 2010).
At our study area, the Davis-Schrimpf field (Fig. 1C), seeps cover an
area of about 22,000 m 2. Here, gas, mud, and water are expelled from
gryphons, pools, and gas vents (Muffler and White, 1968; Sturz et al.,
1992; Svensen et al., 2007). Although the DSF is commonly referred as
a “mud volcano field”, we stress that both the surface expressions
(lack of main craters or large scale mud breccias) and the system
driving the seeps (hydrothermal CO2-dominated gases) are very
different from those typical of hydrocarbon-bearing sedimentary
volcanism (e.g. Etiope et al., 2007b; Mazzini et al., 2009). For some
specific vents releasing gas, water and mud we use, however, the term
“gryphons” in analogy to similar morphologic manifestations occurring in actual mud volcanoes.
3. Fieldwork and methods
Four fieldwork campaigns in December 2003, 2006, 2007, and
2008 included extensive sampling of the seeping fluids in the DSF. A
total of eleven gas seeps and seventeen water seeps were sampled for
in situ and laboratory analyses.
The density of expelled mud and waters was measured with a
commercial electronic scale, with accuracy better than ~ 2% for the
relevant mass of the measured samples.
Water salinity (Total Dissolved Solids) was measured in situ using
a VWR EC300 conductivity sensor; pH was measured by L.T. Lutron
YK-2001 pH meter.
3.2. Temperature measurements
3.3. Gas Geochemistry
Gases expelled from the seeps in 2003, 2006, and 2007 were
sampled into brine-filled glass bottles that were subsequently
hermetically sealed. Gases (N2, O2, CO2, C1-C4 hydrocarbons) were
analyzed at the Institute for Energy Technology (IFE), Norway.
0.1-1 ml of gas were sampled using a Gerstel MPS2 autosampler and
loop injected into a splitt/splittless injector on a Hewlett Packard 5890
Series II GC equipped with Molsieve and Poraplot Q columns, 1 flame
ionization detector (FID) and 2 thermal conductivity detector (TCD).
Hydrocarbons were measured by FID. H2, CO2, CO, N2, H2S and O2/Ar
were measured by TCD. The precision of the gas composition is better
than 3% (1 σ) for all compounds.
The stable carbon isotopic compositions of methane and carbon
dioxide were determined by sampling aliquots with a Gerstel MPS2
autosampler, then injected into a split/splittless injector on an Agilent
7890 gas chromatograph with a Poraplot Q column, connected to a
Horizon IRMS from NU-Instruments. After separation, the components were burnt to CO2 and water in a 1000 °C furnace over Cu/Ni/Pt.
The water was removed by Nafion membrane separation. Repeated
analyses of standards indicate a reproducibility of δ 13 C values better
than 0.3‰ VPDB (2 sigma). The hydrogen isotope analysis of methane
were completed by sampling aliquots from the exetainers with a
GCPal autosampler and injected into a split/splittless injector on a
Trace GC2000 from Thermo, equipped with a Poraplot Q column,
connected to a Delta + XP IRMS from Thermo. The components were
decomposed to H2 and cooked in a 1400 °C furnace. The international
standard NGS-2 and an in-house standard is used for checking
accuracy and precision. Repeated analyses of standards indicate a
reproducibility of δ D values better than 5‰ PDB (2 sigma).
The gas samples collected in December 2008 were analyzed at the
Isotech Labs Inc. (Illinois, USA) for C1-C6 hydrocarbons, He, H2, Ar, O2,
CO2, N2, (Carle AGC 100–400 TCD-FID GC; detection limits of CO2, Ar,
O2: 50 ppmv; N2: 100 ppmv; H2S: 300 ppmv; He H2: 10 ppmv; C1-C5
alkanes: 5 ppmv; precision 2% (1 σ), and isotopic compositions δ 13C1,
δD1, δ 13CO2, δ 15N (Finnigan Delta Plus XL mass spectrometer,
precision ± 0.1‰ (1 σ) for 13 C and ± 4‰ (1 σ) for 2H). The hydrocarbon components were separated by GC, and CH4 was thermally
converted to CO2 and H2 in the pyrolysis furnace at 1450 °C. The
resulting H2 was introduced directly into the mass spectrometer.
Results are reported in δ notation, as per mil (‰) deviation relative to
the VPDB (for carbon), VSMOW (for hydrogen) and atmosphere (for
nitrogen) standards.
Samples for He analyses were collected in annealed copper tubes
which were sealed in the field using a cold welding clamp, and
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A. Mazzini et al. / Journal of Volcanology and Geothermal Research 205 (2011) 67–83
analyzed at the Scripps Institution of Oceanography (USA) using
extraction, purification and measurement protocols described in e.g.
Shaw et al. (2003). The helium, neon and carbon dioxide aliquots were
captured in glass break-seals and transferred to a MAP noble gas mass
spectrometer and a manometric measurement/purification line
respectively. In the case of the He + Ne fraction, the gas was further
purified using a Ti-getter, charcoal trap (at liquid nitrogen temperature) and a He-cooled refrigeration trap designed to separate He from
Ne prior to inlet into the mass spectrometer. Aliquots of air were used
to calibrate the sensitivity and the mass fractionation. Following
purification and manometric measurement, an aliquot of the CO2 was
transferred to a Thermo-Finnigan DeltaPlus XP mass spectrometer for
13
C/ 12 C analysis. The results are given in the R/Ra notation, where R is
the sample 3He/ 4He ratio and Ra is the atmospheric 3He/ 4He ratio,
1.39 × 10 -6).
3.4. Water geochemistry
Water and mud from the seeps were collected in plastic vials and
filtered. Anions were analyzed using a Dionex DX-500 ion chromatograph and major cations analysed with a Perkin-Elmer Optima ICP-OES
and trace cations analysed with a Perkin-Elmer Elan DRC-e ICP-MS at
the School of Earth Sciences, University of Leeds (UK). The results are
reported as parts per million (ppm) (Table 2A). For oxygen isotopic
analyses, H2O was equilibrated with CO2 at 25 °C for 24 hours. The δ18O
composition of CO2 will then reflect the isotope composition of the
original water sample. The impurities were separated from CO2 on a
Poraplot GC column before on-line determination of δ18O using a
Finnigan DeltaXP isotope mass spectrometer at IFE. The average value
for GISP from IAEA is δ 18O VSMOW = −24.80 ± 0.10‰ (1 σ). “True” value
is −24.78 ± 0.08‰. Before measuring the δD composition H2O was
reduced with Zn to H2 and ZnO in sealed, evacuated quartz vessels at
900 °C. The δD composition was then determined by dual inlet, using a
Micromass Optima isotope mass spectrometer. Average value for GISP
from IAEA is δD VSMOW = −189.71 ± 0.89‰ (1 σ). “True” value is
−189.73 ± 0.9‰.
3.5. Flux measurements
In December 2008, CO2 and CH4 fluxes from focused venting
(macro-seepage) and diffuse soil degassing (mini-seepage, as defined
in Spulber et al., 2010) were measured throughout the DSF. Gas flux
from 86 vents (pools or gryphons) was measured by volumetric flux
meter techniques, time monitoring the funnelled expelled gas in a
1.5 l calibrated vial. Flux measurements from 5 smaller and weak
vents and in 81 soil degassing sites (invisible miniseepage) were
performed with a closed-chamber system (West Systems srl, Italy)
equipped with portable CH4 and CO2 detectors and wireless data communication to a palm computer (Etiope et al., 2010). The CH4 sensor
includes semiconductor (range 0–2,000 ppmv; lower detection limit:
1 ppmv; resolution: 1 ppmv), catalytic (range: 2,000 ppmv - 3% v/v),
and thermal conductivity (3% to 100% v/v) detectors. The CO2 detector
has a double beam infrared sensor (Licor) with an accuracy of 2%,
precision ±5 ppmv and a full scale range of 20,000 ppmv. The gas flux
is calculated through a linear regression of the gas concentration
build-up in the chamber. An accumulation time of ~10 minutes
allowed us to detect fluxes down to 30 mg m -2 d -1. Shorter accumulation times were applied at locations where high fluxes rapidly
saturated the chamber. These soil degassing measurements covered
an area of ~20,000 m 2 following a 20 × 20 m grid. Eight additional
measurements where made in order to extend the 20 × 20 m grid to
80 m from each of the corners. The total area covered thus extends to
over 51,000 m 2. Gas flux data were elaborated with commercial
kriging interpolation software in order to provide 3D maps and an
estimate of the total gas miniseepage output. The sum of miniseepage
and macro-seepage fluxes gives the total emission of CH4 and CO2 into
the atmosphere.
4. Results
4.1. Morphologies and seepage types
Gryphons and pools releasing a mixture of water, gas, and sediment
are commonly clustered in 10–20 m diameter calderas. The venting sites
can be divided into four main groups depending on the temperature and
the density of the expelled water or mud: gryphons, oil pools, water
pools, and thermally anomalous dry zones (Fig. 2, Table A1).
a) Gryphons: conical features resulting from superimposed mud flows
that commonly reach the height of 2–2.5 meters where the hot
(up to 70 °C) dominating muddy component is forcefully erupted by
constant or intermittent surges of gas from a central crater.
Gryphons can be isolated structures or grouped and are commonly
surrounded by subsiding calderas that host pools (Fig. 2A).
Pools are topographically negative structures that can also be found
as isolated features. Two types of pools can be distinguished.
b) Oil pools where gas is vented through greyish clay-dominated
mud. The grey mud (reaching temperatures up to 46.6 °C) is the
same erupted at gryphon sites suggesting that the oil pools are also
fed with sediments from deeper units. This mud is mixed with a
significant amount of oily components that commonly form a
foamy layer floating on the surface of the pools (Fig. 2B).
c) Water pools where gas is vented through reddish-brownish water
dominated mud. The reddish colour of the mud suggests oxidation
occurring on the surface and thus that there is little (if any?)
expulsion of clay from deeper strata. Their average temperatures are
between 20 to 30 °C (Fig. 2C).
d) A thermally anomalous dry zone in the south-eastern part of the field
is characterized by large patches of yellowish sulphate mineralizations (blodite) (Fig. 2D). These features consist of circular dry
depressions ranging in size between 1–2 meters. In 2003 a 0.5 m
deep trench was made into one of these circular zones and the
stratigraphy compared to the background stratigraphy from a trench
made 10 meters away Figs. A2 and A3. Locally nodular calcite and
anhydrite was discovered in the mud. The surface temperature was
N60 °C but precise measurements are lacking. When investigated
the following year, the thermal anomaly decreased in intensity and
the calcite and gypsum nodules had dissolved.
4.2. Density, temperature and pH
The density of the expelled water and mud mixtures from 18
individual seeps range from 1.1 to 1.7 g/cm 3 (Table 1). This is within
the same range as the data from December 2003 (Svensen et al.,
2007). The gryphons muds have densities generally higher than 1.4 g/
cm 3, while the pools release fluids with a density lower than 1.4 g/cm 3
(Fig. A1A, Table 1). Fig. A1B shows the correlation between the hotter
and denser gryphons and the colder and less viscous pools. The in situ
pH (Fig. A1C) varies between 5.6 and 6.8, which is a narrower range
compared to the 2003 measurements (pH between 5.2 and 6.3). In
addition to the differences in pH between the 2003 and 2006
sampling, the seep field was drier in 2006, with lower water tables
and fewer individual seeps. In general the gryphons with higher
temperature also have a higher pH (Fig. A1A).
Infrared imaging (Fig. 3) showed that adjacent seepage sites can
have broad variations in temperature even when the seeps are part of
the same structure (e.g. Fig. 3A-D). Additionally it was noticed that the
hot mud erupted from gryphons looses heat very rapidly after flowing
along the flanks of the structures.
A. Mazzini et al. / Journal of Volcanology and Geothermal Research 205 (2011) 67–83
A
B
C
D
71
Fig. 2. (A) Gryphons and caldera's pools. Gryphons commonly reach the height of 2–2.5 meters and erupt hot (up to ~ 70 °C) mud, gas and water. Most gryphons are grouped in
clusters surrounded by subsiding calderas. These caldera structures contain cold ~ 20-30 °C water-dominated pools seeping gas; (B) Oily pool seeping mud, water, and gas at ~ 40 °C
(diameter ~ 2.5 meters). The depth of the pool was measured as up to four meters; (C) Water dominated isolated cold (~20 °C) pool (i.e. north pool) reaching approximately three
meters in depth and a diameter of ~ 2 meters. Note the brownish colour compared to hotter oil pools and gryphons; (D) Two of the gentle depressions present in the sulphate field
where blodite mineralization are distributed over the warm (up to 34 °C) soil and where a higher gas flux is present.
4.3. Water geochemistry
The field measurements of the water TDS showed a wide spread in
solute content (Table 1), with the four measured gryphons having
values from 1.5 to 4.5 g/L whereas mud pools have salinities that
range from 17.6 to 134 g/L. The field measurements were corroborated with laboratory measurements of the same samples, and the
discrepancy between the two methods (cf. Tables 1 and 2A; the R 2 of
the correlation is 0.56 when excluding the high salinity sample) is
likely due to difficulties analysing solutes in clay-rich water. The
highest TDS, 134 g/L measured in the field, was recorded in a pool
with salt crust along the rims, in agreement with the onset of halite
precipitation at ~ 145,000 ppm Cl (Fontes and Matray, 1993). Nearby
surface water reservoirs (artificial lake directly east of the seeps, dyke
with running water, and the Alamo River) had measured chloride
concentrations of 1386, 873, and 518 ppm Cl respectively, overlapping the lower range of the gryphon waters (N768 ppm Cl)
(Fig. 4A-B). All cations except Ba and Br show a positive correlation
with the Cl concentration. The ‘oil pool’ was more saline in 2006,
71,042 ppm Cl, compared to 44,692 ppm Cl in 2003, and the Cl/Br
ratio increased from 3575 to 14,208 over the same period (cf. Svensen
et al., 2007). The sulphate concentrations were high, with up to
7549 ppm in the ‘oil pool’. Compared to the surface water reservoirs,
most of the gryphons and pools have very high Cl/Br ratios (Fig. 4A).
The highest value is from the pool water in contact with halite (Cl/
Br = 35,600), and even a relatively low chlorinity gryphon
(Cl = 15,658 ppm) has a ratio of 32,620. Note that the Cl/Br ratios
are considerably higher compared to the 2003 water data presented
by Svensen et al. (2007), where the Cl/Br in pools was between 1941
and 5099.
The δ 18O values of both the seep water and the nearby surface
waters are in the range − 15.1 to +0.7‰ VSMOW (Table 2A) and the
range in δD values is from −45.2 to − 112.9‰. The pool samples are
more 18O-enriched than the gryphon waters and when plotted
relative to the global meteoric water line (Craig, 1957), all but one
sample plots along a trend commonly attributed to evaporation
(Fig. 4C). The Alamo River and the dyke water are among the most 18O
depleted, whereas the seeps and the nearby artificial lake waters are
18
O enriched. The seep water with the highest measured salinity is
enriched in 18O (i.e. δ 18O N 0.7‰) and has a relatively high δD
(−52.3‰), while the most 18O depleted values are from the low
salinity gryphons (Fig. 4B-C). Note that the deep hydrothermal
reservoir brines (Elders and Sass, 1988) are more 18O-enriched (δ 18O
of 1.5-4.0‰) than any of the seep waters, and that most pool waters
plot between the evaporation trend values and the reservoir brine
values.
4.4. Gas geochemistry
The main gas component analysed in the seeps is CO2, ranging
from 97.8 to 99.5 vol. %. Additionally, the seep gas contains CH4 (0.21.9 vol. %), N2 (b0.76 vol.%) and trace amounts of ethane, propane and
butane (Table 2B). Stable carbon isotopic compositions of CO2 and CH4
range from − 5.4‰ to 0.4‰ and from − 32.7‰ to − 17.6‰
respectively (Table 2B). Isotopic composition of N2 was determined
only in one sample (δ 15N = −0.52‰), as for ethane (δ 13C2 = − 20‰).
The molecular composition and the 13C of CO2 and CH4 (Fig. 5A) are
independent of the type of venting structure (gryphons or pools). The
He isotopic ratio, R/RA, ranged from 6.08 to 6.64 (Table 2B).
4.5. Macroseepage flux at vent sites
Volumetric fluxes of CO2 and CH4 from gryphons and pools have
been normalized to the average gas composition measured during the
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A. Mazzini et al. / Journal of Volcanology and Geothermal Research 205 (2011) 67–83
Table 1
Field measurements and sampling. *Measurements from Svensen et al. (2007).
Sample ID
Structure
Comment
Level
(cm)
CA03-3*
CA03-5*
CA06-38
CA06-22
CA06-32
CA06-35
CA06-40
CA06-28
CA06-21
CA06-37
CA06-36
CA06-27
CA06-24
CA06-30
CA06-31
CA06-23
CA06-34
CA06-26
CA06-25
CA06-29
CA06-39
CA06-46
CA06-33
CA06-44
CA06-45
CA06-41
CA07-01
CA07-02
CA07-03
CA07-04
CA07-05
CA07-06
CA07-07
CA07-08
CA07-09
CA08-01
CA08-02
CA08-03
CA08-04
CA08-05
CA08-06
CA08-07
CA08-08
CA07-09
Pool
Pool
Gryphon
Gryphon
Gryphon
Gryphon
Gryphon
Pool
Pool
Gryphon
Pool
Pool
Pool
Pool
Pool
Pool
Pool
Pool
Pool
Pool
Pool
Water
Pool
Water
Water
Water
Pool
Pool
Gryphon
Gryphon
Gryphon
Gryphon
Pool
Pool
Pool
Pool
Pool
Gryphon
Gryphon
Gryphon
Gryphon
Pool
Gryphon
Pool
North pool
Oil pool
Splattering flank
East gryphon. 15 cm diameter gryphon close to oil pool
Big gryphon in westernmost caldera
Same structure sampled in CA06-34
Only gas seepage, but water inside
Double pool west of oil pool
Oil pool
Splatter cone
Small pool, 1.5 × 2 m
Double pool west of oil pool
West pool
Pool in westernmost caldera, grey, in triple pool
Pool in westernmost caldera, grey, forming three seepage sites
Muddy, west end, possible oil present (?)
Pool north of oil pool
North pool with brownish water
Pool in westernmost caldera, brown, in triple pool
Structure present at the base of the gryphon sampled at CA06-38
Salton Sea, tip of boat launching place, red hill, west side
Brown, rimmed with salt crusts
Flowing water in ditch (1 m across) just south of seep field
Alamo River, at bridge close to red hill marina
Artificial lake next to duck blind
Oil pool
North pool
East Gryphon
West Gryphon
South Gryphon
Central Gryphon
Central pool
Artesan pool with multiple seeping points (2 km NW from the SD field)
Artesan pool with multiple seeping points (2 km NW from the SD field)
Oil pool
North pool
East Gryphon
West Gryphon
South Gryphon
Central Gryphon
Central pool
East central gryphon
West pool
different field campaigns (i.e. 98% CO2, 1.5% CH4). Gas outputs from
individual vents ranged from 1.4 to 254 kg/day for CO2 and 0.01 to
1.4 kg/day for CH4. Integrating all the measurements performed from
the 86 vents, a total of 2,046 kg/day of CO2 and 11 kg/day of CH4 are
released from the DSF. If we include the output from five weak vents
(see Section 3.5) measured with the closed chamber (62 kg/day for
CO2 and 0.5 kg/day for CH4), then the total macro-seepage output
would be: 2,108 kg/day (~769 ton/year) of CO2 and 11.5 kg/day
(~4 ton/year) of CH4 (Fig. 6 A-B).
Note that these values represent a conservative estimate as the
transition from macroseepage to miniseepage is gradual and some
less obvious vents have been neglected both at gryphons and pool
sites.
4.6. Diffuse gas flux from soil (miniseepage)
Fig. 6C-D shows the spatial distribution derived by kriging
interpolation of diffuse CO2 and CH4 soil degassing measured with
the closed chamber system. CO2 fluxes from soil ranged from 1 to
3700 g m -2 d -1. While CO2 seepage was essentially detected at all
sampling stations, this was not the case for CH4. The CH4 fluxes from
Diam.
(cm)
30
120
135
-45
-50
-65
40
70
-35
-50
-70
-65
-65
-28
-80
-20
-67
-65
60
150
40
40
20
30
50
200
100
200
40
130
-85
400
70
40
T
(°C)
15.0
33.9
68.2
64.3
58.5
37.0
36.0
35.0
34.6
34.4
33.0
32.2
30.8
25.1
24.9
23.3
23.1
19.4
19.1
18.0
14.1
13.7
13.6
12.6
12.0
11.7
38.3
19.4
54.3
47.8
55.2
62.2
18.0
16.3
21.6
46.6
23.7
45.5
46.0
36.3
56.3
17.2
55.4
pH
TDS
(g/L)
Density
(g/cm3)
6.2
6.3
6.3
6.0
4.5
3.1
1.6
1.5
1.6
1.7
1.7
1.7
6.2
5.9
33.4
36.4
1.3
1.7
6.0
6.2
6.1
6.0
6.1
6.1
5.8
6.1
6.2
6.0
5.8
8.3
5.6
7.9
7.7
6.8
49.7
39.0
19.5
54.3
45.6
23.2
50.3
17.8
17.6
51.2
33.2
6.0
134.0
2.9
1.9
3.5
1.3
1.2
1.3
1.3
1.2
1.1
1.6
1.2
1.2
1.3
1.1
1.0
1.3
1.0
1.0
1.0
soil above the detection limit (~ 30 mg m -2 d -1, imposed by a
maximum accumulation time of 10 minutes) were measured in 45%
of locations (37 sites ranging from 30 to 8190 mg m -2 d -1). Measurements of soil degassing from the sulphate field rapidly saturated the
chamber indicating a substantial flux that gradually decreases radially
outward from the dry thermal anomaly zones.
The closed chamber measurements also reveal a direct correlation
between the CH4 and the CO2 fluxes (Fig. 6C-D-E). The average weight
ratio between the CO2 and CH4 flux is 228. This value is close to the
average compositional CO2/CH4 weight ratio, 186 (ranging from 145
to 231; Table 2B) measured in the laboratory. Therefore gas venting at
gryphon or pool sites has roughly the same CO2 and CH4 proportions
of that released by soil degassing. Applying kriging interpolation to
the miniseepage fluxes gives a total emission of 7310 kg/day
(~2,700 t/y) of CO2 and 33 kg/day (~ 12 t/y) of CH4.
4.7. Temperature monitoring
The temperature monitoring of the sulphate field revealed
temperatures varying from 34.1 °C in early December to a minimum
of 30.2 °C in early February and rising to 33.9 °C at the end of
A. Mazzini et al. / Journal of Volcanology and Geothermal Research 205 (2011) 67–83
A
B
C
D
E
F
73
Fig. 3. Infrared images (A, C, E) highlight the different temperatures of the seeping fluids even at closely spaced locations indicating a complex plumbing system in the near
subsurface. (B, D) Isolated pools (ca. 1.5 m in diameter) where focused seepage of fluids occur at several locations. The images also highlight that different water levels are present at
closely spaced sites. (F) Isolated gryphon (~ 2 m high) with hot mud seeping from the top of the crater and flowing along the flanks of the structure. Although the temperature of the
mud is higher than 60 °C in the crater, once the gaseous phase is released to the atmosphere, the mud cools down very rapidly as it flows.
Table 2A
Composition of waters at vent sites. All values in ppm. Isotopic composition is reported in per mil vs SMOV; nd: not determined.
Sample ID
Structure
Cl
Br
SO4
Cl/Br
Ca
Na
K
Mg
Sr
B
Ba
δ18O
δD
CA06-21
CA06-39
CA06-23
CA06-24
CA06-25
CA06-26
CA06-30
CA06-33
CA06-34
CA06-38
CA06-22
CA06-32
CA06-35
CA06-41
CA06-44
CA06-45
CA06-46
CA07-1
CA07-2
CA07-3
Oil pool
Pool
Pool
West pool
North pool
Pool
Pool
Pool
Pool
Gryphon
East gryphon
Gryphon
Gryphon
Water
Water
Water
Water
Oil pool
North pool
East Gryphon
71042
11368
15658
15086
13330
10361
26712
178000
46818
4685
6923
7758
768
1386
873
518
2371
nd
nd
nd
5.0
20.9
0.5
31.2
91.3
2.5
2.5
5.0
240.0
0.4
1.5
2.5
2.5
21.9
6.0
0.3
2.6
nd
nd
nd
7549
20
441
905
56
322
1539
5965
1272
1134
475
20
515
1173
1266
916
2375
nd
nd
nd
14208
544
32620
483
146
4144
10685
35600
195
12393
4569
3103
307
63
146
1990
912
nd
nd
nd
2093
146
291
351
48
127
629
1169
772
157
87
215
9
176
231
176
275
nd
nd
nd
44320
6860
9555
8817
7970
6875
16355
100700
29390
2967
4320
5160
881
915
672
443
1848
nd
nd
nd
1323
364
590
507
408
402
470
2843
766
174
228
264
79
23
15
3
36
nd
nd
nd
1197
136
110
116
127
106
697
7058
1088
50
68
122
23
168
148
99
263
nd
nd
nd
66
26
41
23
18
29
31
48
40
8
16
23
6
6
6
2
8
nd
nd
nd
186
62
63
78
61
65
88
601
143
18
28
33
6.5
2.7
2.4
4.8
5.4
nd
nd
nd
0.3
0.4
0.4
0.3
1.7
0.4
0.2
0.4
0.3
0.8
0.5
0.2
0.2
0.0
0.1
0.1
0.1
nd
nd
nd
-5.4
-2.0
-2.6
-2.9
-5.7
-4.2
-4.0
0.7
-4.2
-8.5
-8.2
-6.1
-15.1
-4.2
-8.1
-10.4
-8.9
-13.8
-10.7
-10.2
-81.9
-55.3
-73.1
-65.6
-74.3
-72.8
-45.2
-52.3
-52.5
-76.1
-86.3
-72.1
-112.9
-60.5
-81.7
-90.0
-82.8
-90.5
-80.4
-79.8
74
A. Mazzini et al. / Journal of Volcanology and Geothermal Research 205 (2011) 67–83
100000
2003 Gryphons
2003 Pools
‘oil pool’ 2006
2006 Gryphon
10000
2006 Pools
Cl/Br
Halite
‘oil pool’ 2003
2006 Artificial Lake
2006 Dike
2006 Alamos River
1000
5.1. A revised interpretation of the gas origin
100
A
10
0
50000
100000
150000
200000
Cl ppm
2
0
-2
18
δ O (VSMOW)
5. Discussion
2006 Salton Sea
Seawater evaporation trend
-4
-6
-8
-10
-12
-14
B
-16
100
1000
10000
100000
1000000
Cl (ppm)
-30
)
2006 Gryphon
10
-40
*δ 18
O+
2006 Pools
2007 Gryphon
(δD
=8
-50
ine
2007 Pools
2006 Artificial Lake
Wa
te
rL
-60
2006 Dike
SSGS
deep brine
2006 Alamos River
2006 Salton Sea
ba
-80
lM
ete
oric
-70
Glo
δD (VSMOW)
monitoring in early April. Although the total temperature variation is
only 4 °C, the trend reflects variations in air temperature with a
10 days delay (Fig. A4). These limited temperature variations are
remarkable considering that the air temperature had 40.5 °C range
(i.e. -1.5 °C in mid January up to 39 °C in mid March).
SSGS shallow brine
-90
‘North pool’ dilution
‘Oil pool’ dilution
-120
-16
Ev
-110
ap
ora
tio
n
-100
C
-12
-8
-4
0
4
8
18
δ O (VSMOW)
Fig. 4. Seep water and reference water geochemistry. A) Chloride and bromide data
compared to the evaporation trend for seawater (Fontes and Matray, 1993) and halite
composition (Mccaffrey et al., 1987). Note the large range in Cl/Br ratios and that high
Cl/Br cannot be obtained by evaporation. B) δ18O and Cl of seep waters. Note that all
seep waters shown here are enriched in Cl compared to the reference samples but that
the δ18O values overlap. C) Hydrogen and oxygen isotope systematic of the studied
waters together with the Global Meteoric Water line (Craig, 1957), the evaporation
trend for waters in the Salton Sea area and the hydrothermal reservoir brines of the
SSGS (Mckibben et al., 1987). Note that our reference samples plot close to the
evaporation trend, and that most of the pool seep waters are enriched in 18O. The
arrows show the geochemistry of the oil and north pool waters between sampling in
2006 and 2007 (arrows pointing towards the 2007 samples). Meteoric dilution can be
correlated to heavy rainfall prior to the 2007 sampling campaign.
The large gas-geochemical database obtained in the four field
campaigns allows to better constrain the interpretation of the origin
of gas and to re-evaluate the conclusions suggested by previous
studies. The stable carbon isotopic composition of seeping CO2
(δ 13CCO2 from −5.4 to + 0.4‰, with a mean of −3.4‰) is in the
range of typical CO2 derived from magmatic sources (− 6 to −3‰;
Kyser, 1986; Sheppard, 1986) and thermal decarbonation reactions
(−5 to +4‰). The biotic CO2 contribution from organic matter in
sediments (kerogen decarboxilation typically from −10 to −25‰,
e.g. Jenden et al., 1993) seems therefore to be negligible.
Svensen et al. (2007) suggested a CO2 origin from decarbonation
reactions on the basis of the 2003 field data (δ 13CCO2 = −5.4‰) and
earlier work based on gas geochemistry and petrography of the
metamorphic sediments (Muffler and White, 1968; Cho et al., 1988;
Shearer et al., 1988). It is known, however, that the carbon isotopic
composition of CO2 alone is not an exhaustive diagnostic tool, as both
metasediments and the mantle result in overlapping δ 13C values.
Furthermore, the ongoing decarbonation reactions in the SSGS
involves recrystallized and modified calcite cement originally derived
from lacustrine pore waters where the calcite δ 13C can be significantly
13
C depleted compared to marine carbonates. Moreover, we cannot
exclude that metamorphic marine sediments may be present at
greater depth in the Salton Trough.
Clayton and Muffler (1968) report the values of the carbon stable
isotopes of carbonate cuttings recovered from a borehole in the SSGS.
The δ 13C of calcite as pore filling and in veins ranges from −1.4‰ to
−7.8‰ (mean value − 4.4‰) in the top 2.5 km represented by
lacustrine sediments. For a CO2-calcite fractionation factor of 2–3‰ at
350 °C (Sheppard, 1986; Chacko et al., 1991), the CO2 derived from
either marine or deltaic lacustrine carbonates (e.g. δ 13C between −4.4
to 2.2‰) would range between −2 and 5‰. These values partly
overlap the range obtained from the seeping gases in the DSF.
However the δ 13C values of the lacustrine carbonate cuttings also
overlap the commonly used MORB values (Kyser, 1986), thus not
helping to differentiate the original source of gas. To sum up, an
unambiguous discrimination of carbon sources cannot be deduced
from the sole δ 13CCO2 values.
The helium isotope data indicate a strong mantle component, with
R/Ra from 6.08 to 6.64 (Fig. 5B top), which is within the range of
typical Sub-Continental Mantle (SCM; Gautheron and Moreira, 2002),
including that of western USA (Dodson et al., 1998). The δ 13C values
would also be consistent with a dominant mantle origin. In the
CO2/ 3He vs δ 13C plot (Fig. 5b bottom) Salton gas appears to be close to
typical SCM values which are CO2-enriched in comparison to MORB
(Sano and Marty, 1995; Kampf et al., 2007; Hahm et al., 2008; Tedesco
et al., 2010). Also, increased CO2/ 3He ratios can be due to fractionation
during gas migration (Hilton, 1996) so that the original deep gas may
be well within the SCM field, and the limestone-derived fraction could
be minimal or nil. To summarize, the origin of CO2 and He of Salton
Sea is similar to the magmatic gas in the Long Valley Caldera (Fig. 5b),
which is less than 600 km north of Salton (R/Ra: 6–6.7; CO2/ 3He:
10 10-10 11; Hilton, 1996). Motyka et al. (1989) reported similar results
showing that the Kalawasi group mud volcanoes in Alaska have CO2
dominated seep gases with δ 13CCO2 values (− 3.1 to 4.8‰) overlapping those of the DSF and high 3He/ 4He ratios (i.e. between 2.1 and
4.1). These mud volcano gases likely derive from a magmatic intrusion
A. Mazzini et al. / Journal of Volcanology and Geothermal Research 205 (2011) 67–83
75
Table 2B
Molecular and isotopic composition of gas from vents. Gas composition is in vol. %. Isotopic data: δ13C: ‰, VPDB; δD: ‰, VSMOW; δ15N: ‰, atm. R/Ra = (3He/4He)sample/(3He/4He)
atmosphere; Ra = 1.39 × 10-6; bdl: below detection limit, nd: not determined. Samples from CA07 have only hydrocarbons molecular composition reported.
Sample ID
Structure
C1
C2
C3
iC4
nC4
CO2
N2
δ13C1
δD1
δ13CO2
δ15N
δ13C2
3
CA03-3
CA03-5
CA06-21
CA06-22
CA06-35
CA06-37
CA06-40
CA06-32
CA06-23
CA06-24
CA06-25
CA06-29
CA06-34
CA07-01
CA07-02
CA07-03
CA07-04
CA07-05
CA07-06
CA07-08
CA07-10
CA08-01A
CA08-02A
CA08-03A
CA08-04A
CA08-05A
CA08-06A
CA08-07A
CA08-08A
Welhan, 1988
North pool
Oil pool
Oil pool
East gryphon
Gryphon
Gryphon
Gryphon
Gryphon
Pool
Pool
North pool
Pool
Pool
Oil pool
North pool
East Gryphon
West Gryphon
South Gryphon
Central Gryphon
South artesan pool
West pool
Oil pool
North pool
East Gryphon
West Gryphon
South Gryphon
Central Gryphon
Central pool
East central gryphon
Deeep high-T well
0.51
0.49
1.20
1.24
1.12
1.18
0.91
0.86
1.43
1.00
1.40
1.08
1.26
1.07
1.36
1.04
1.06
0.84
0.27
0.25
nd
1.67
1.86
1.21
1.45
1.17
1.81
1.48
1.37
0.01
0.02
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.04
0.01
0.02
0.01
0.03
0.01
0.01
0.01
0.00
bdl
nd
0.01
0.03
0.01
0.01
0.01
0.01
0.01
0.01
bdl
bdl
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
bdl
bdl
nd
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
0.00
bdl
0.00
0.00
0.00
bdl
bdl
nd
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
0.00
bdl
0.00
0.00
0.00
bdl
bdl
nd
0.00
bdl
0.00
0.00
7.72
0.00
0.00
0.00
99.49
99.49
98.79
98.74
98.86
98.81
99.07
99.13
98.55
98.99
98.55
98.91
98.71
nd
nd
nd
nd
nd
nd
nd
nd
98.31
98.11
98.08
97.78
98.82
98.18
97.16
98.96
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
nd
0.00
0.00
0.70
0.76
0.00
0.00
0.35
0.66
-32.7
-32
-24
-24.1
-21.9
-23.7
-23.8
-18.9
-24.2
-24.2
-27.6
-24
-17.6
-23
-26.4
-23.5
-23.1
-23.2
nd
nd
nd
-24.4
-26.5
-24.2
-23.6
-23.2
-24.8
-23.5
-23.6
-26
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
-171
-216
-159
-158
-160
-150
-156
-174
-270
-5.3
-5.4
-4.7
-4.1
-4.4
-4.6
-3.6
-3
-3.6
-4.5
-5
-3.9
-3.8
-4.7
-2.9
-4.3
-3.9
-4.4
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
-0.52
nd
nd
nd
nd
nd
nd
-20.1
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
6.64
nd
6.55
6.6
6.08
nd
nd
6.52
nd
nd
nd
nd
nd
nd
nd
nd
with additional contributions of CO2 from contact metamorphisms of
deep sites limestone beds, thus comparable to the outlined DSF
scenario.
The methane stable carbon isotopic ratio measured in 2003
(δ 13CCH4 ~ − 32‰) suggested a thermogenic origin (Svensen et al.,
2007). Our data collected between 2006–2008 gave considerably
higher values (from − 17.6 to − 26.5‰) suggesting oxidation of
thermogenic gas, high-temperature pyrolisis of bituminous material
or an abiogenic source (Fig. 5C). Welhan (1988) reported δ 13CCH4
values of −26‰ for unoxidised gas and values from −16 to −0.6‰
for fractionated gases in deep wells from the SSGS (T = 320 °C ±20).
The CH4 concentration (b1.5 vol.%) is much lower than that of the
Guaymas Basin (about 61 vol.%; Welhan and Lupton, 1987) where
light hydrocarbons were considered to be mainly thermogenic
(δ 13CCH4 from − 43 to −51‰). A simple calculation of Rayleigh
fractionation with bacterial oxidation factors (Coleman et al., 1981)
suggests that starting from a CH4 with δ 13CCH4 ~ −45‰ (as that of
Guaymas Basin), the oxidation of 97% of CH4 (necessary to get 2 vol.%
from an initial concentration of 60 vol.%, as that of Guaymas Basin)
would produce a residual CH4 with δ 13CCH4 much higher than
−20‰, similar to the oxidised residual gases reported by Welhan
(1988). The isotopic composition of DSF seeping CH4 is similar to
that of gas in deep SSGS wells (Fig. 5C); consistent with the carbon
isotopic composition of the evolved CO2, this suggests the lack of
significant microbial oxidation during migration to the surface; a
complete thermogenic source is however improbable. The presence
of a mantle helium (R/Ra ~ 6) and additional mantle CO2 together
with the high seep water temperatures (up to ~ 70 °C) suggests that
also CH4 might have a mantle component (Welhan and Craig, 1983),
in addition to the thermogenic one. By using the 13CCH4 vs R/Ra
diagram (Fig. 5D; Jenden et al., 1993) it appears that Salton has a
nd
-4.0
-1.6
-2.4
-0.2
0.4
0.2
-2.4
2.3
He/4He (R/RA)
(He/Ne)/(He/Ne)air
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
1671
nd
1563
456
29.7
nd
nd
107
nd
nd
nd
nd
nd
nd
nd
nd
higher magmatic component than the other large sediment-hosted
hydrothermal system of Guaymas. However, the occurrence of
abiogenic non-magmatic (crustal) CH4 components, due for example
to thermal reduction of CO2 (from mantle and limestones) to CH4,
metamorphism of graphite, thermal decomposition of carbonates
and FTT reactions (e.g. Potter et al., 2004; Fiebig et al., 2007) is
entirely possible. The presence of oil in some seeps suggests, in any
case, that high-T pyrolysis of petroleum has also taken place.
Multiple origins and consequent mixing of CO2 and CH4 in the Salton
gas prevent to quantify the actual sources without significant
uncertainty.
The two available isotopic data of N2 (δ 15N: -0.52‰) and ethane
(δ 13C2: -20.1‰) are typical of deep crust or mantle (δ 15N from − 2
to + 1‰; Zhu et al., 2000) and of highly mature thermogenic or
abiogenic gas (typically with δ 13C2 values higher than −25‰; Jenden
et al., 1993; Potter et al., 2004). However, additional isotope data on
ethane coupled with those of propane and butane are needed to better
assess the thermogenic vs abiogenic contributions.
5.2. Water source
The major and trace element geochemistry of the seep waters can
be interpreted to be controlled by a combination of in situ evaporation
and halite dissolution in addition to dilution by meteoric waters
during periods of rain. The elevated Cl/Br ratio of most seeps is best
explained by input from halite dissolution, as halite has Cl/Br N 3000
(Mccaffrey et al., 1987). Even rain water spilling off into the pools
would result in high Cl/Br waters in the seeps, as halite crusts are
abundant throughout the area. During dry periods, evaporation will
increase the pool salinities without changing the Cl/Br ratio until
halite precipitates (Fig. 4A). These interpretations are supported by
76
A. Mazzini et al. / Journal of Volcanology and Geothermal Research 205 (2011) 67–83
Fig. 5. Gas analyses results and interpretation. (A) Combined δ13CCH4 and δ13CCO2 with results sub-divided by year of sampling and type of venting structure; (B) He isotopic ratios vs.
13
CCO2 (top) and CO2/3He vs δ13CCO2 (bottom). Sub-Continental Mantle (SCM) and arc volcanoes (ARC) are from Gautheron and Moreira (2002), Kampf et al. (2007), Hahm et al.
(2008); Mid-Ocean Ridge Basalt (MORB), marine limestone and organic sediments are from Sano and Marty (1995). Long Valley Caldera from Hilton (1996). Mixing lines are among
the SCM, limestone and sediment endmembers. (C) Methane δ13C vs δD. Salton Sea data (seeps) are compared with another major sediment-hosted hydrothermal field (Guaymas
Basin) and abiogenic gases. Salton wells: Welhan and Lupton (1987); high-T well: Welhan (1988); Cerro Prieto: Welhan and Lupton (1987); EPR: East Pacific Rise: Welhan and Craig
(1983); Guaymas Basin: Welhan (1988); Socorro: Taran et al. (2010); Lost City: Proskurowski et al. (2008); Lovozero: Potter et al. (2004); Zambales, the Philippines: Abrajano et al.
(1988); Chimaera, Turkey: Hosgormez et al. (2008). (D) Plot of methane δ13C vs 3He/4He (as R/Ra). Dashed lines encompass commercial biotic gas from America, Asia and Europe as
reported by Jenden et al. (1993). Continuous lines are the mixing paths between hypothetical crustal and magmatic endmembers with ratios (CH4/3He)crustal / (CH4/3He) magmatic
from 1 to 106 (Jenden et al., 1993). East Pacific Rise: Welhan and Craig (1983); Guaymas Basin: Lupton (1983) and Welhan (1988).
the deuterium and oxygen isotopic data, showing a distinct
evaporation signature for most of the samples, and of meteoric
dilution in two cases (the ‘oil pool’ and the ‘north pool’). When
comparing the ‘oil pool’ and ‘north pool’ geochemistry from 2006 and
2007, both seeps were strongly depleted in 18O in 2007, which can be
explained by dilution by rain water in the weeks prior to sampling.
The salinity response to this dilution is not available as major element
analyses were not performed on the 2007 samples. The geochemical
signature of the deep geothermal reservoir brines, as reported by
Elders and Sass (1988), are enriched in 18O compared to pool waters
A. Mazzini et al. / Journal of Volcanology and Geothermal Research 205 (2011) 67–83
77
CH4 flux
Northing
CO2 flux
kg/d
B
0.
5
1
0.
1
0.
2
0
15
0
80
A
10
60
20
40
kg/d
Easting
CH4 flux: mg m-2 d-1
CO2 flux: g m-2 d-1
3674550
100
0
50
100
00
50
500
50
0
10
3674500
0
10
00
3674550
3674500
500
0
50
100
10
00
0
10
Northing
100
3674450
3674450
500
00
10
500
50
10
0
0
50
100
0
500
0
10
10
3674350
0
3674350
0
00 50
00
0
100
10
3674400
10
3674400
D
C
632450
632500
632550
632600
632450
632500
632550
632600
Easting
rth
No
g
in
g
in
st
Ea
E
Fig. 6. (A-B) CO2 and CH4 fluxes measured from the vent sites (pools or gryphons) throughout the field. A total of 96 seepage points have been measured; (C-D) 2D plots of CO2 and
CH4 with flux distribution of miniseepage from soil (vents excluded); (E) 3D plots of CO2 and CH4 flux distribution of miniseepage from soil (vents excluded). UTM coordinates
(zone 11).
78
A. Mazzini et al. / Journal of Volcanology and Geothermal Research 205 (2011) 67–83
by at least 4‰. A key question is: do the pool waters contain a
signature of this deep brine, or are all waters meteoric or mixtures of
meteoric waters affected by shallow water-rock interactions? Since
the temperature of the seeps at the surface can be as high as 70 °C
(i.e. in the gryphons, see Tables A1 and 1), even shallow subsurface
interactions between water and clay minerals could result in a 18Oenrichment (Savin and Hsieh, 1998). Moreover, oxygen isotope
exchange between CO2 and H2O at high temperature at depth in the
system may lead to 18O depletion of the water (Chiodini et al., 2000).
Although the seepage of deep brines has been suggested previously
(Sturz et al., 1992), and the new gas geochemistry suggests the
involvement of gases with a very deep origin, our water data remains
inconclusive.
5.3. Seep dynamics and their shallow plumbing system
Our data suggest the existence of a complex subsurface plumbing
system at the DSF. Although the gas composition is consistent
throughout the field, there are significant variations of water content
and temperature at different sites, in some places between vents as
close as 0.2 meters (e.g. Fig. 3).
Carbon dioxide flows to the surface together with Na-Cl brine,
mud, and petroleum, with the individual seeps having differing water
tables, gas fluxes, and fluid exit temperatures. Petroleum is present in
only one seep (i.e. oil pool, Table 1), and is likely sourced from high
temperature maturation of organic matter (Svensen et al., 2007).
Gryphons are mud-rich and normally hot (22-69 °C) whereas the
water-rich seeps are generally colder (b32 °C) and form pools. We
conclude that the water has a mainly meteoric source, and that hot
mud and a perturbed geothermal gradient in the vicinity of the seep
field are responsible for the up to 70 °C exit temperatures. This
hypothesis is supported by previous data by Svensen et al. (2009).
These authors conducted broad temperature monitoring of the vents
in the field showing that diurnal air variations control the temperature in the water pools, whereas the gryphon temperatures do not
correlate with air temperature and are instead characterized by
abrupt spikes interpreted as surges of hot fluids. Although these sites
have large temperature variations, their gas composition remains
consistent at most locations. This suggests that although the CO2 flux
is driving the seep activity at both the pools and gryphons, the
recorded temperature variations are related to different proportions
of water in the erupted mud. Seeps with higher percentages of water
may allow faster cooling, where hot gas is quickly cooled when
bubbling through a colder high viscosity media (i.e. water) (Svensen
et al., 2009).
Infrared images (Fig. 3) and field observations show that isolated
neighbouring pools often have different water tables and temperatures. Onderdonk et al. (2011) monitored pool levels over a 28 month
period and showed that there are consistent variations of the Salton
Sea water level and some of the pools. This suggests that some of the
pools may be connected to a shallow water table. Additionally,
significant variations in water levels (up to 0.5 m) were observed in
pools that were less than 0.3 m apart. This relationship, along with the
reported temperature differences indicate that single seeps have a
completely independent plumbing system that extends to significant
depth despite their narrow spacing.
As reported earlier, the soil degassing flux is stronger on the
periphery of active vents and in particular around gryphon sites. This
observation supports the hypothesis of Mazzini et al. (2009) that
gryphons and neighbouring pools (i.e. caldera collapse pools) are part
of the same plumbing and seepage system where a deep rooted
conduit bifurcates at shallow depth before reaching the surface. We
elaborate on this model here by suggesting that around the gryphon
clusters, where seepage is focused, a system of shallow radial fractures
produces pools on the surface or, when the flux is less vigorous, an
enhanced diffuse release through soil degassing. However, the precise
subsurface structure of these gryphon clusters and associated calderas
is still unknown. Svensen et al. (2009) suggested two models that
imply the gradual collapse around gryphons where deep (model A) or
shallow (model B) mud is forcefully erupted and mobilized by the
migrating hot gas. The second model is favoured by Onderdonk et al.
(2011) who interpreted that the positive volume of mud forming the
gryphons is constantly balanced through time by negative volume
(subsidence?) of the surrounding caldera by documenting coeveal
gryphon growth and caldera collapse. This observation may imply an
entrainment of shallow mud through the conduit that is periodically
erupted forming positive gryphons and subsequently subsided and
recycled for a new eruption. None of these models however seems to
be applicable to explain why the isolated pools in the field become
gradually wider and deeper. Two mechanisms are proposed: a) the
rise of fluids along the feeder conduits allows a continuous readjustment of the sedimentary textural mesh resulting in continuous
compaction and collapse at the surface; and b) the rise of CO2-rich
fluids promotes the continuous dissolution of anhydrite-rich units
that are abundantly distributed at least throughout the first 3,000 m
(Herzig et al., 1988). Several isolated pools are also visible 2 km to the
north of the DSF where the casing of abandoned exploration wells
have created the ideal pathway for CO2-rich fluids to reach the
surface.
Elders and Sass (1988) reported data from the borehole “State
2-14” that was drilled just 800 m north of the DSF. The drilling
penetrated over 3,220 m of lacustrine sediments and two thin diabase
intrusions were intersected at 2880 and 2896 m depth. These
intrusions are too thin to explain the extreme thermal anomalies in
the area, but the sills are likely part of a bigger intrusive complex
located a few kilometres deeper. (Fig. 7). Microseismicity (Boyle et al.,
2007) showed that local earthquakes cluster in a conduit-like zone
that most likely represents the region of focused fluid flow. These
features extend as deep as 5 km (and possibly up to 10 km).
5.4. Carbon dioxide and methane degassing
The CO2/CH4 flux ratio measured by the closed chamber is similar
to the compositional CO2/CH4 ratio measured in the vents. This is a
remarkable result considering that the error of individual measurements can be significant. This also implies that methane degassing
through the soil is not significantly consumed by methanotrophic
bacteria, except where gas fluxes are relatively low (orders of 10 010 1 mg CH4 m -2 day -1; Fig. 8). This result is similar to observations of
Japanese mud volcanoes (Etiope et al., 2011), but is in contrast with
what was reported by D'Alessandro et al. (2009) from geothermal
areas, who claimed that the methanotrophic effect is substantial
even in high flux degassing zones (N10 2 mg CH4 m -2 day -1). We
believe that the major controlling factors are the soil characteristics
(wetness, organic matter) and the availability of methanotrophs, and
that the effects of methanotrophic activity in soils increase as
residence times of gas in the soil increase due to lower gas migration
rates. Our conservative estimate reveals that at least 3,400 t/y of CO2
and 16 t/y of CH4 are released throughout the monitored field
(0.05 km 2). The methane emission factor (i.e. the total output
divided by the area investigated), which is a fundamental parameter
for regional and global emission estimates of gases from natural
sources (Etiope et al., 2007a, 2011), is then 320 t km -2 y -1: this is at
least one order of magnitude lower than typical emission factors of
CH4-rich seeps and mud volcanoes in sedimentary basins (Etiope
et al., 2011) but up to 2 orders of magnitude higher than those of
non-sedimentary geothermal and volcanic manifestations (Etiope
et al., 2007a). Sediment-hosted hydrothermal systems may thus
represent an intermediate class of geologic methane sources, and on
global scale they may significantly contribute to the atmospheric
methane budget.
A. Mazzini et al. / Journal of Volcanology and Geothermal Research 205 (2011) 67–83
A
Davis-Schrimpf
Field
Red Hill
intrusion
0 (km)
0.5
1.0
1.5
2.0
79
State 2-14
B
3.0
2.5
CO2 reservoir
70 °C
Oil
Thermal anomaly/Fluid flow
Low TDS fluids
High TDS fluids
Thermogenic CH4?
?
355 °C
Decarbonation CO2?
260 °C
1
305 °C
2
Sill intrusions
355 °C
3
Depth (km)
300 °C
4
CH
4
abiogenic,
5
post or late magmatic
?
Intrusions/Mantle Rocks
CO2 He
10-15?
Fig. 7. Schematic cross section (not to scale) through points A-B (see Fig. 1B) of the geothermal system crossing the DSF. Temperatures from the State 2–14 borehole are from (Sass et al.,
1988), and the temperatures at 914 meters depth at various distances from the Red Hill intrusion are taken from Elders and Sass (1988). The perturbation of the gradient below the seep
field is based on the assumption that the surface perturbation (70 °C in gryphons; Svensen et al., 2009) can be extrapolated. Also shown are the sources of the seep gases, and the source of
petroleum (Svensen et al., 2007) which is assumed to be shallow since it is only locally present in the seep field. Shallow meteoric fluids represent the main source of water for the system
although a deep origin cannot be excluded. The Imperial CO2 gas field is present in the upper part of the section. Mafic intrusive (?) rocks are inferred at 10–15 km depth based on seismic
interpretations (Fuis et al., 1982).
More than 75% of emission is from the invisible soil degassing.
Indirect estimates by Kerrick et al. (1995), based on convective heat
flow, temperatures and CO2 concentrations of reservoir fluids,
indicated a total emission of 44,000 t/y of CO2 for the whole Salton
CH4 flux (mg m-2 d-1)
1000000
100000
10000
R2 = 0.9601
1000
100
CH 4 flux detection limit
10
Methanotrophic consumption?
1
1
10
100
CO2flux (g
1000
m-2
10000
100000
d-1)
Fig. 8. Correlation between CO2 and CH4 fluxes. CH4 values below detection limit are set at
5 mg m-2 d-1 only for graphical purpose. CH4 consumption by methanotrophic bacteria
may be significant only at low gas flux soil conditions.
Trough and SSGS (about 200 km 2). This is equivalent to an average
diffuse CO2 flux of only 0.6 g m -2 d -1, that is virtually indistinguishable from the background biologic CO2 flux of soil respiration
(typically in the order of units to tens of g m -2 d -1), and four orders
of magnitude lower than the mean CO2 flux of the DSF as reported
here (133 g m -2 d -1). This suggests that the DSF is an anomalously
high degassing zone within the SSGS. However active venting has
been witnessed periodically at various locations north west of the
seep field, along the shore of the Salton Sea and offshore in the Salton
Sea (e.g. permanent mud pots fields and numerous ephemeral seeps
and pots, see Lynch and Hudnut, 2008 for a detailed list). It is
therefore likely that the regional assessment by Kerrick et al. (1995)
might underestimate the actual volume of gas being released to the
atmosphere within the SSGS.
Further flux measurements should be carried out to understand
possible temporal variations of the degassing. We observed that the
gas flux was higher in the soil around active vents and in the areas of
dry and more fractured ground; this would suggest that a survey
conducted in a dry season with less compacted soil, would most likely
record higher fluxes. It is known, however, that gas seepage may
increase after rainy periods and it also strongly depends on
barometric pressure and groundwater level (Koch and Heinicke,
2007; Spulber et al., 2010).
Overall a NW-SE alignment of the vents and the soil degassing
pattern can be inferred. This direction coincides with the direction of
the Calipatria fault suggested by Lynch and Hudnut (2008). However
80
A. Mazzini et al. / Journal of Volcanology and Geothermal Research 205 (2011) 67–83
1,8
the data presented in this article are not sufficient to confirm the
presence of this fault and a flux survey over a broader region should be
completed to highlight if the orientation of the main soil degassing
has a clear NW-SE trend.
A
1,7
1,6
6. Conclusions
• The composition of the gas released is consistent throughout the
field (~ 98% of CO2, 1.5% of CH4 and b0.5% of C2+) indicating the
presence of the same type of gas mixture in the whole plumbing
system.
• The CO2 stable carbon and helium isotopic composition (R/Ra N 6)
suggest a considerable Sub-Continental Mantle source, whereas the
CH4 is likely to be a mixture of high temperature pyrolitic and
abiogenic gas. These results define a deep rooted plumbing system
that allows rapid release of fluids to the surface.
• Water analyses reveal a strong component of meteoric input both in
gryphons and pools, with water geochemistry affected by in situ
evaporation.
• The concentrations of major elements correlate with Cl, except for
Br and Ba. Pool water in apparent equilibrium with halite crusts
show a Cl concentration of 178,000 ppm Cl and an extremely high
Cl/Br of 35,600, in support of halite dissolution buffering the Cl and
Br concentrations.
• The sum of gas released by invisible, diffuse soil degassing (7,310 kg/
day of CO2 and 33 kg/day of CH4) and by vents (2,108 kg/day of CO2
and 11.5 kg/day of CH4) results in a significant emission of CO2,
3,400 t/y, over a restricted area. An even larger amount of gas is likely
to be released all along the faulted area in the Salton Through. Soil
degassing represents the dominant (~75%) component of gas released
over the monitored field.
• Sediment-hosted hydrothermal systems may represent an intermediate class of geologic methane sources for the atmosphere, with
methane emissions lower than those of sedimentary seepage in
petroleum basins but higher than those in traditional geothermalvolcanic systems; on global scale they may significantly contribute to
the atmospheric methane budget.
1,4
1,3
1,2
1,1
1
5,2
5
5,4
5,6
5,8
6
6,2
6,4
6,6
6,8
pH
1,8
B
1,7
1,6
1,5
g/cm3
This study in the Davis-Shrimp field represents one of the most
complete surveys over a sediment-hosted hydrothermal field. Our
approach was to combine geochemical and temperature monitoring
with detailed measurements of gas released by soil degassing and
focused venting. The results can be summarized as follow:
g/cm3
1,5
1,4
2003 pool
1,3
2003 gryphon
1,2
2006 gryphon
2006 pool
1,1
water river, S. Sea,
ditch
1
10
30
temp ( C)
˚
50
70
50
70
6,6
C
6,4
6,2
6
The editor G. Chiodini, A. Sturz, and J. Fiebig are thanked for their
constructive reviews. We gratefully acknowledge support from the
Norwegian Research Council (PETROMAKS grant NFR-163469) to
H. Svensen. S. Polteau, O. Galland, Ø. Hammer, A. Nermoen, K. Fristad
and S. Planke are thanked for their support during field campaigns and
for the fruitful discussions. Thanks are due to D. Hilton and D. Tedesco
for useful discussions.
pH
Acknowledgements
5,8
5,6
5,4
5,2
5
Appendix A
10
30
temp ( C)
˚
Table A1
Summary of main characteristics of Davis-Schrimpf-field seeps (average values from
2003-2007).
Structure
Size (m)
Gryphon
0.5-4
Oil pool
2-4
Water pool 0.5-6
Height (m)
pH*
0.4-2.5
-0.3
-0.5
5.98 Up to 70
5.89 Up to 40
6.10 20-30
Temp. (°C)
Density
Salinity (TDS)*
N 1.4
b 1.4
b 1.4
36.4
2.7
44.0
Fig. A1. (A) pH vs density reveals that the gryphons have density higher than 1.4 g/m3;
(B) density vs temperature shows that many denser gryphons have temperatures
significantly higher than the more viscous pools; (C) pH vs temperature shows that
gryphons with higher temperature also have higher pH.
A. Mazzini et al. / Journal of Volcanology and Geothermal Research 205 (2011) 67–83
81
Trench
A
B
Reference
A
1
B
Trench
C
2
3
D
C
D
Fig. A2. (A) Outline of the sulphate-covered field showing the trench that was dug for stratigraphic purposes. (B) The surface mineralization at the centre of the field is characterized
by the presence of white and yellow sulphate minerals (Tamarugite and Blodite) in addition to halite. (C) Close-up of the trench where distinct layers are labelled A-D (Fig. A3).
(D) A reference trench was dug at ten meters distance from the sulphate-covered field, showing the shallow stratigraphy dominated by two brown units and deeper dark grey units.
Note the differences between the layers in the two trenches.
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Authigenic minerals
0
1
NaCl, NaAl(SO4)2*6H2O (Tamarugite, white),
Na2Mg(SO2)4*4H2O (Blodite, yellow),
KFeO2 (Potassium ferrite, reddish brown)
A
2
B
10
Depth (cm)
3
C
Gypsum
20
4
D
Gypsum
E
Gypsum, Calcite
30
Mg-Calcite
F
G
Mg-Calcite, Ankerite
40
Fig. A3. Lithological logs of the reference trench and the thermal anomaly trench in the
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35
45
40
Air Temperature (°C)
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Sulfur
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33
25
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Air
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15
10
31
5
Sulfur zone Temperature (°C)
R
A
ef
no
er
en
m
al
y
ce
82
0
24.12.2007
13.01.2008
02.02.2008
22.02.2008
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02.04.2008
Date
Fig. A4. Temperature Time series from December 2007 to April 2008 from the underground station at the sulphate field thermal anomaly and air temperature.
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