•
Chapter 6
GEOCHEMICAL FEATURES OF MUD
VOLCANOES
GEOCHEMICAL MODEL OF MUD VOLCANOES
FROM REVIEWED WORLDWIDE DATA
Short review
Giovanni Martinelli' Andrea Dadomo2
I ARPA Environmental Protection Agency of Emilia Romagna Region, Via Amendola 2,
42100 Reggio Emilia - Jtaly, e-mail: giovanni.martinellil5@tin.it
2 Dept. of Geologica/ Sciences and Geotechnology, University of Milan, Piazza delle Scienze
4, 20126 Milan, Jtaly
Abstract:
Geochemical data from mud volcanic fluids obtained from various geologica)
environments have been reviewed and reprocessed. The chemical and isotopie
components of the liquid and gas phases have been studied. Notwithstanding
the geographical distance between the mud volcanic areas and the differences
between the geologica) environments, a common originating fluid derived
from seawater has been recognized. Diagenetic processes due to sediment
compaction can be considered to be responsible for the evolutionary pattems
observed in the liquid phase and in the associated gas-emissions. Geochemical
data fit the currently available physical models.
Key words:
mud volcanoes, geochemical modelling
1. INTRODUCTION
Accretionary prisms undergoing deformative stresses due to subductive
processes host sediments that are subjected to compaction and minerai
dehydration phenomena. In particular, subduction zones host fluids that are
often expelled from thick sedimentary layers accumulating over millions
of years (Carson and Screaton, 1998). These pore waters still show the
geochemical signature of seawater as the starting materiai and are often
enriched with hydrocarbons. The flow-rate at which pore waters are expelled
may be affected by local permeability coefficients and pressure gradients
strongly linked to the tectonic setting. A significant part of the localized fluid
flow is represented by mud volcanoes (Kopf, 2002).
The available geochemical data obtained in the mud volcanic areas of
Alaska, Azerbaijan, Barbados, ltaly, Russia, Taiwan and Trinidad were
211
G. Martinelli and B. Panahi (eds.), Mud iiJlcanoes, Geodynamics and Seismicity, 211-220
© 2005 Springer. Printed in the Netherlands.
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212
Geochemical model of mud volcanoes
compared. Data inter-comparison can be of help in better understanding thy
genetic features of fluids expelled by mud volcanic areas in question. The
present review tracks the main cations, anions, stable isotopes of waters, Sr
isotopes and the main gases sampled at the considered sites.
1.1 Water origin and classification
All the analyzed fluids show some kind of relation with the meteoric
waters. The geologica! setting and the chemical composition indicate that
sea water and clay minerals are the main starting materials of the sampled
fluids. The strong prevalence of Cl-Na water geochemical/acies allow usto
consider the sampled fluids as a sort of oilfield waters (Fig. 1, Fig. 2, Fig. 3,
Fig. 4 and Fig. 5)
:; Alallka. Mctylraetel.. 1988
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Figure I . Langelier-Ludwig diagram of considered mud volcanic waters
ａ
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Figure 2. Oil field character ofmud volcanic waters sampled in Italy
•
Giovanni Martinelli, Andrea Dadomo
213
aAlask<l. Mot)'ka 11t ai.,
Ｑｾ･＠
e A2etbal}an. AAye-v •t at, 2002
oBarbados.Olaetal.,1995
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• Telwiltl, GlfJskos et al., 19&2
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Figure 3. Na/Mg ratio in considered mud volcanic waters
a Afaska,Mol)t:aatal_19M
e AzefbaljM. A11yo.,.a1.a1 .• 2002
.
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oTafwan, Yahetal.ttt!Jbook.
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Figure 4. (Ca+ Mg) I (Na + K) ratio in considered mud volcanic waters
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etFigure 5. Cl/Na ratio in considered mud volcanic waters
•
Geochemical model of mud volcanoes
214
Most of the geochemical alterations observed in the samples is due to
compaction and dehydration phenomena of the marine sediments. Oxygen
and hydrogen isotope ratios confirm the marine origin of the fluids under
examination (Fig. 6).
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Figure 6. 818 O I 8 D ratio in considered mud volcanic waters. Isotopie data are available
only for some localities. Global Meteoric Water Line is also indicated.
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Figure 7. Cl I 81 8 O ratio in mud volcanic waters.
Giovanni Martinelli, Andrea Dadomo
215
Barbados and Trinidad confirm the common marine ongm of the
considered fluids (Fig. 7). In spite of their well-constrained marine origins
some chemical differences relative to modem sea water have been observed.
Clay behaviour under compaction is subjected to a variety of diagenetic
processes, as they can release water enriched or depleted in electrolytes as
compared with seawater (Bredehoeft et al, 1963). When water and solutes
are driven by hydraulic head gradients across semi-permeable membranes,
the ionie solute passage through the membranes is restricted in relation to
the water. The solute concentration on the input side of the membrane thus
increases in regard to the output concentrations. The ion-exclusion effects are
described as salt-filtering or ultrafiltration processes (Kharaka and Smalley
(1976). Water that accrues from possible gas hydrate dissociation in the clay
layers can be considered responsible for significant dilution phenomena
(Martin et al., 1996). Further amounts of water can be released during
possible conversion from smectite to illite (Bekins et al., 1994).
1.2 The evolutionary trend of water
Fluids sampled in the Barbados area have been interpreted by Martin et
al. (1996) as the result of the mixing of two end-member fluids, seawater and
water derived from gas hydrate dissociation. Fluids sampled in Trinidad have
been interpreted by Dia et al. (1995) and Dia et al. (1999) as the result of
diagenetic processes affecting the originai sea water. Similar low- temperature
seawater diagenetic processes have been proposed by Conti et al.(2000),
Minissale et al., (2000) and Capozzi and Picotti (2002), to explain the origin
of fluids sampled in the mud volcanic areas ofltaly. Fluids sampled in Taiwan
have been interpreted by Gieskes et al.(1992) and byYeh et al., (this volume)
as the result of diagenetic processes affecting seawater. Aliyev et al. (2002)
have considered mud volcanic fluids as oilfield waters syngenetic to extruded
clay sedirnents. Over the past ten years geochemists have paid particular
attention to Strontium isotopes as indicators of diagenetic processes. Indeed,
Dia et al.(1995), Martin et al. (1996), Dia et al. (1999), Conti et al. (2000)
have analyzed Sr86/Sr87 ratio in fluids expelled by mud volcanoes located
in Barbados, Trinidad and Italy. Analyses were carried out on the basis of
findings described by Burke et al. (1982 and references therein) and Dia et al.
( 1992 and references therein) as to the sensitivity of the Sr86/87 ratio to global
clirnatic changes during the geologica! eras. A close correlation between Sr
86/87 and 016/18 was found in the geological samples by Dia et al. (1992)
through the past 300 kyr. Figure 8 shows the available strontium and oxygen
isotopie analytical data accruing from the mud volcano fluids.
•
Geochemical model of mud volcanoes
216
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Figure 8. Relation 818 O vs.87 Sr I 86 Sr. A slight seatter affeets Strontium isotopie data,
while a relatively wide interval affeets Oxygen isotopie ratio.
The available data show that seawater is a sort of common building
material for all the sampled mud volcanic fluids. A slight scattering in 87Sr/
86Sr confirrns the conservative character of the Strontium isotopes. The
wider scattering observed in 18 0/16 O confirms that diagenetic processes
may be responsible for the evolutionary geochemical water trends observed,
but seawater is confirmed as the fundamental primary matter. Paroxystic
eruptive periods have at times caused slight fluctuations in water chemical
composition and have been detected in Italy (Martinelli et al., 1995 and
references therein; Capozzi and Picotti, 2002) and Azerbaijan (Aliyev et al.,
2002 and references therein). The absence of significant thermal anomalies
in the waters sampled in Italy during the paroxystic period indicate that the
ejected liquid phase carne from depths at which thermal re-equilibration
processes had occurred (<50 m.). Slight chemical fluctuations observed
during the paroxystic periods are probably due to the feeding of the main
conduit by the different sub-reservoirs characterized by a different clay-water
suspension ratio and a different water-rock interaction time.
The low magnitude ofthe observed geochemical fluctuations is consistent
with the existence of a main reservoir and the possible existence of some
secondary reservoirs within the mud volcano structure. Significant thermal
anomalies over time were recorded in Italy during the l 9th and 20th centuries
in the course of exceptionally strong eruptive events. Recorded thermal
anomalies fit with the waters coming from layers located at depths of 500600m (Fig. 9).
•
217
Giovanni Martinelli, Andrea Dadomo
REGNANO MUD VOLCANO SKETCH
(NORTHERN ITALY)
Maln vent
500 m
Ｍｾ＠
sea level - - - - - - - - ;
3000m - - - c
o
hi gh permeability layers
6000 m - - - - - - - - ) \ CH 4
Figure 9. Sketch of the Regnano mud volcano (North Italy). After Caneva (I 958), Martinelli
and Ferrari ( 1991 ), Capozzi and Picotti (2002) modified.
1.3 Gas phases
Methane is the most common gas released in virtually all of the mud
volcanic areas listed by Kopf (2002). The data available from the geochemically
surveyed areas come from Barbados, Trinidad, Italy, Azerbaijan and Taiwan.
The isotopie methane composition can be used to distinguish between
biogenic or thermogenic sources (Schoell, 1980) enabling the identification of
the approximate depth at which the methane had formed. Methane sampled in
the Barbados mud volcanoes ranges between -60 and -65 delta 13 C and can
be considered a mix of biogenie and thermogenic gases. Little is known about
the isotopie data, although Dia et al. (1999) report that the methane sampled
in Trinidad should be considered as a deep originating gas. Fluids sampled in
Taiwan show that a thermogenic methane source is mixed with a biogenic one ·
(Gieskes et al., 1992; Yeh et al., this book, and references therein). Methane
analyzed in the Italian mud volcanoes show carbon isotopie data typical of
mixed origin (-40 >delta 13C>-50). Slight fluctuations in carbon isotopes
were observed in a Northem Italian mud volcano during the eruptive phases,
showing the existence of at least two different gas sources (Capozzi and
Picotti, 2002). Similar phenomena were observed in Azerbaijan by Aliyev et
al.(2002). Methane sampled·in Azerbaijan mud volcanoes is characterized by
-40>delta 13C> -50 and a multi-layered gas source has been hypothesized
by Guliev and Feizullaev (1996). The small magnitude of fluctuations over
•
218
Geochemical model of mud volcanoes
time in the methane carbon isotopes recorded in Azerbaijan and Italy lead us
to confirm that the main reservoir is significantly larger than the secondary
ones. Carbon isotope values obtained in methane and upper hydrocarbons in
the mud volcanoes of Azerbaijan and Italy confirm that the gases originated
at about 6-8 Km in depth (Guliyev et al., 2001., Capozzi and Picotti, 2002).
Helium isotopes analyzed in the gases sampled in the mud volcanic areas
of North Italy, Sicily and Azerbaijan show data in the range of the crustal
values. Some relatively anomalous data found in the Helium isotopes show
that mantle- contaminated deep fluids can reach shallow rock layers through
enhanced crustal permeability rock volumes (Aliyev et al., 2002; Etiope et
al., 2003; Martinelli and Judd., 2004 and references therein). Despite the link
with the deep-mantle connected fluids, no chemical or physical differences
were observed in mantle-contaminated mud volcanoes as compared with the
pure crustal mud volcanic structures.
1.4 Transient fluid geochemical phenomena
Transient geochemical phenomena were detected in Italy and Azerbaijan.
Indeed, radon anomalies were detected in the Northem Apennine mud
volcanoes in the liquid phase in the period 1986-1987 by Martinelli et al., (1995
and references therein). Further geochemical fluctuations were detected in the
major stable water isotopes, and in the major cations and anions analyzed in
a Northern Italian mud volcano. Other geochemical fluctuations affected the
gas phase of a mud volcano in Northem Italy in 1988-1992 and in the period
1998-1999. These were revealed by means of radon monitoring (Albarello et
al., 2003 and references therein) and by carbon isotope methane monitoring
(Capozzi and Picotti, 2002). The observed geochemical fluctuations
revealed the sensitivity of mud volcanoes to possible crustal deformative
processes and to local climatological conditions. In actual fact, geochemical
anomalies detected in the liquid phase in the 1986-1987 period tumed out
to be principally related to geodynamic factors, whilst anomalies detected
in the period 1998-1999 followed an extraordinarily warm climatic phase
that dried the ejected mud and sealed the main vents. The overpressure due
to vent-sealing generated a paroxystic eruption and followed the fluid flow
anomalies recognizable within the chemical parameter fluctuations. Similar
anomalies have been observed in Azerbaijan and Romania (Baciu and Etiope,
this book) without reaching definitive conclusions, although many paroxystic
eruptive phenomena in Azerbaijan have been found to be triggered by local
earthquakes. The observed behaviour have confirmed that confined fluids in
the crust-like mud volcanoes can act as natural strain-meters.
•
Giovanni Martinelli, Andrea Dadomo
219
CONCLUSIONS
Mud volcanic fluids are mostly syngenetic to extruded sediments. Ocean
water is the common raw materiai utilized by the "subduction factory" to build
up mud volcanoes. Clay compaction and organic matter diagenesis originates
the liquid and gas phases observed in the vents. Diagenetic processes and
possible mixing phenomena with seawater can account for the observed datascattering. The gases collected in the mud volcanic areas are consistent with
a rnixed syngenetic origin with hosting sediments contaminated by deeper
originating gases. This phenomenon is probably due to fact that gases can
migrate more easily than water in geological environments. Geochemical data
confirm that mud volcanoes are not linked to meteorological environment
or to very deep geological strata. Fluid circulation tracked by geochemical
parameters is characterized by a "mainstream fluid current'', fed by a main
reservoir. Slight data scattering accounts for near-surface fractionation
processes affecting fluid fluxes and chernical parameters. Mud extrusion is
confinned to occur chiefly as a result of the compaction phenomena generated
by the lithostatic load. The peculiar confined fluid nature of mud volcanism
can be considered responsible for virtually all the observed tectonicallyrelated phenomena.
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Andrea Dadomo