ASTROBIOLOGY
Volume 2, Number 1, 2002
© Mary Ann Liebert, Inc.
Research Paper
Fluid Inclusion Studies of Chemosynthetic Carbonates:
Strategy for Seeking Life on Mars
JOHN PARNELL, ADRIANO MAZZINI, and CHEN HONGHAN
ABSTRACT
Fluid inclusions in minerals hold the potential to provide important data on the chemistry of
the ambient fluids during mineral precipitation. Especially interesting to astrobiologists are
inclusions in low-temperature minerals that may have been precipitated in the presence of
microorganisms. We demonstrate that it is possible to obtain data from inclusions in
chemosynthetic carbonates that precipitated by the oxidation of organic carbon around
methane-bearing seepages. Chemosynthetic carbonates have been identified as a target rock
for astrobiological exploration. Other surficial rock types identified as targets for astrobiological exploration include hydrothermal deposits, speleothems, stromatolites, tufas, and
evaporites, each of which can contain fluid inclusions. Fracture systems below impact craters
would also contain precipitates of minerals with fluid inclusions. As fluid inclusions are
sealed microchambers, they preserve fluids in regions where water is now absent, such as regions of the martian surface. Although most inclusions are 5 m, the possibility to obtain
data from the fluids, including biosignatures and physical remains of life, underscores the
advantages of technological advances in the study of fluid inclusions. The crushing of bulk
samples could release inclusion waters for analysis, which could be undertaken in situ on
Mars. Key Words: Fluid inclusions—Chemosynthesis—Mars—Carbonates—Planetary exploration strategy. Astrobiology 2, xxx–xxx.
log deposits (e.g., Walter and Des Marais, 1993;
Cady and Farmer, 1996; Russell et al., 1999;
Farmer and Des Marais, 1999; Horneck, 2000).
Carbonate rocks, in particular, serve as important paleobiological repositories (McKay and
Nedell, 1988; Kempe and Kazmierczak, 1997).
Though there is debate about whether sedimentary carbonates should be present at the martian
surface (McKay and Nedell, 1988; Blaney and McCord, 1989; Schaefer, 1990; Calvin et al., 1994; Komatsu and Ori, 2000), it is plausible that hy-
INTRODUCTION
P
RIME TARGETS in the search for sedimentary de-
posits on the surface of Mars are analogs of
those known on Earth to preserve a fossil record
(Walter and Des Marais, 1993; Rothschild, 1995;
Westall, 1999; Farmer, 2000; Westall et al., 2000b).
Chemically precipitated sedimentary rocks are
viewed as prime targets in the search for a martian fossil record (Farmer, 1998), and recent efforts have focused on describing terrestrial ana-
Department of Geology and Petroleum Geology, University of Aberdeen, King’s College, Aberdeen, U.K.
43
44
drothermal carbonates precipitated within hydrothermal systems that vented at the surface
(Griffith and Shock, 1995). Several lines of evidence, including the prevalence of extrusive volcanic rocks, suggest that hot spring activity has
been important throughout Mars’ history (Walter and Des Marais, 1993; Farmer, 1996; Jakosky,
1996).
On Earth nonbiologically and biologically precipitated carbonates can preserve microbial remains, morphological biogenic features from
megascopic to microscopic scales, and chemofossils such as biogenic isotopic signatures (Westall,
1999; Riding, 2000; Wynn-Williams and Edwards,
2000). Proponents for the exploration of carbonates on Mars suggest that we should seek morphological features at megascopic and microscopic scales that are comparable with those
found in terrestrial microbial deposits. Possible
depositional environments for carbonate deposits
that display a wide range of morphological biogenic features include subaerial or subaqueous
springs and shallow depths in standing water.
In addition to morphological evidence, a potential source of geochemical information may be
obtained from fluid inclusions entrapped in carbonate rocks. Fluid inclusions form as microscale
volumes of fluid are entrapped during mineral
growth. Although fluid inclusion studies of carbonates in sedimentary basins are routinely undertaken by petroleum geologists to understand
diagenetic modifications of the deposit during
burial, such studies are not routinely applied to
surficial samples because most primary precipitates are not conducive to the preparation of the
doubly polished thin wafers required. Also, fluid
inclusion studies generally involve measurements of aqueous fluid homogenization temperatures that are often not possible below about a
60°C entrapment temperature.
The results of two case studies presented here
demonstrate that shallow sea floor carbonates associated with methane-rich seepages contain inclusions that can be successfully studied to reveal
the nature of their enclosed fluids, and thus a
record of the processes that formed them. We propose that similar studies can be conducted on any
carbonates collected from former fluid seepage
sites encountered on Mars.
Geomorphological features on Mars, such as
terraced pools where hydrothermal fluids may
have been discharged (Brackenridge et al., 1985;
PARNELL ET AL.
Farmer, 1996; Gulick, 1998; Newsom et al., 2001),
are likely settings for chemically precipitated carbonates that contain entrapped fluid inclusions.
Subsurface hydrothermal activity has probably
occurred throughout martian history (Jakosky,
1998), and geochemical models predict hydrothermal carbonate precipitation (Griffith and
Shock, 1995). There may still be widespread
groundwater systems in the martian subsurface
(Clifford, 1993; Carr, 1996) that could source surface seepage sites. Landforms that show seepage
into valleys and craters (Malin and Edgett, 2000)
indicate groundwater on Mars must have been at
relatively shallow depths in recent times.
A constraint on the distribution of recent life
on Mars is the extreme hostility of the surface due
to oxidation, exposure to ultraviolet radiation,
and low temperature, all of which suggest that
subsurface environments would be more favorable to life if it ever emerged on Mars (Mazur et
al., 1978; Klein, 1998). Since subsurface existence
would preclude photosynthesis, chemosynthesis
would likely be the preferred metabolic strategy
for a biota in such an environment (Rothschild,
1990; Boston et al., 1992).
Chemosynthesis is a normal part of the upper
crustal fluid plumbing system. On the sea floor
chemosynthetic ecosystems occur both at hydrothermal vents (e.g., Jannasch, 1985; Little et al.,
1998) and at cold seepage sites where they may
be associated with gas hydrates (Roberts and Carney, 1997; Sassen et al., 1999). Fossil evidence of
chemosynthetic communities has also been identified in the geological record (e.g., Kelly et al.,
1995). Chemosynthesis focuses biomass: Bacteria
use geochemical energy from reactions involving
simple compounds including methane, carbon
dioxide, and sulphate ions, and higher organisms
either browse microbial communities or are in
symbiotic relationships with them. Biologically
induced oxidation of methane leaves an isotopic
signature in the fossil record as recorded in carbonate shells, secretions, and microbial deposits
(e.g., Paull et al., 1992).
Since chemosynthetic carbonates precipitated
at cold seepage sites have been specifically suggested as possible analog sites on Mars (Komatsu
and Ori, 2000), we have therefore sought to show
that carbonates deposited at such sites yield fluid
inclusions that can provide insight regarding the
fluid geochemistry of the system in near surface
environments.
FLUID INCLUSIONS AND MARTIAN EXPLORATION
FLUID INCLUSIONS
In sedimentary rocks a fundamental distinction
is made between primary and secondary inclusions. Primary inclusions, which form at the time
mineral cements are precipitated in open pore
spaces on existing sediment grains, are identified
by their spatial relationship or during crystallization to growth zones in the host mineral. After mineral precipitation occurs, deformation
may cause microfractures to form in the minerals that make up the cement matrix, a process that
can release and reentrap migrating fluids as the
microfractures heal. Secondary inclusions generally cut across primary growth zones and can
therefore be distinguished from primary inclusions in doubly polished wafers (i.e., thick sections,
100–200 m) examined using optical microscopy
in transmitted light. Although secondary inclusions contain a fluid that postdates precipitation
of the primary cement, they are nevertheless a
valuable record of the fluid flow history of the
rock.
Since most inclusions are entrapped during cementation or during healing of microfractures
during sediment burial, their fluid chemistry is
thus representative of the ambient fluid present
at depth when the inclusions formed. Reconstruction of the initial temperature (and sometimes pressure) conditions of fluid entrapment
can be obtained using well-established microthermometric techniques: observing the behavior of fluids in inclusions viewed in thick sections as a function of temperature. Since fluid
inclusions function as sealed vessels of constant
volume, their contents adjust to the changing
pressure and temperature conditions experienced
upon heating or cooling. Upon heating, the temperature of formation of a fluid inclusion can be
determined. Fluid inclusions that consist at room
temperature of two phases, fluid and a vapor
bubble, usually formed at higher temperatures
and pressures when the vapor phase was not stable. When a two-phase inclusion is heated to its
formation temperature, the vapor bubble is eliminated, and the fluid homogenizes back to a single phase. This homogenization temperature indicates a minimum trapping temperature for the
fluid during mineral precipitation. Monophase
inclusions, which have no vapor bubble, indicate
entrapment at low temperatures (i.e., usually
50°C). The long-term sealing of fluid inclusions
45
means that even Archean (2.5–3.8 billion years
old) fluids are preserved in them (Channer and
Spooner, 1994; de Ronde et al., 1997a,b).
Upon cooling, the freezing behavior of fluid inclusions reflects their chemistry. Aqueous fluids
with increasing concentrations of dissolved salts
freeze at decreasing temperatures. The behavior
of aqueous fluids that contain dissolved salts is
well understood. Fluids dominated by methane,
carbon dioxide, or both gases can also be recognized by their response during laboratory heating and cooling. Of particular relevance to this
study are methane-rich inclusions that commonly
do not freeze completely below 100°C.
The data reported here are typical of petrographic and microthermometric analyses undertaken during a routine fluid inclusion study of
authigenic mineral phases in sedimentary rocks.
For further details of this methodology of fluid
inclusion study the reader is referred to texts by
Roedder (1984a), Shepherd et al. (1985), and Goldstein and Reynolds (1994).
More advanced analytical tools used to study
the contents of individual fluid inclusions in situ
include laser Raman analysis, proton microprobe
analysis (e.g., Ryan et al., 1993), secondary ion
mass spectrometry (e.g., Diamond et al., 1990),
Fourier transform-infrared spectroscopy (e.g.,
Pironon and Barres, 1992), confocal scanning
laser microscopy (e.g., Pironon et al., 1998), and
laser micropyrolysis (e.g., Wilkinson et al., 1994;
Anderson et al., 1995). These measurements allow
collection of detailed qualitative and quantitative
geochemical data, including metal and organic
speciation, bond types, and isotope analysis.
At the present time, most techniques for examining individual inclusions require them to be
10 m in diameter, a constraint that limits the
applications of such techniques to the study of
fluid inclusions within sedimentary rocks. Fluid
inclusions in sedimentary rocks tend to be 5 m
in diameter, with fluid volumes 6.5 1014 L.
However, as analytical resolution inevitably improves, the utility of using fluid inclusion studies in sedimentary rocks will become more routine.
An important topic for future fluid inclusion
studies in sedimentary rocks stems from the potential for microbial remains to be preserved in
fluid inclusions. Biological remains preserved
within fluid inclusions are protected from degradation in the same way that hydrocarbon fluid
46
PARNELL ET AL.
inclusions in oil reservoirs survive in a fresh state
even though the oil in adjacent pores may be experiencing thermal, chemical, or biochemical alteration (Parnell et al., 2001). At the micron scale
microbial remains could be detected with the use
of techniques now utilized to label microbial biomarker molecules with chromogenic or electrondense markers (Wainwright et al., 2001).
METHODS
T1
For analysis in this study, samples of rock were
prepared as doubly polished wafers and examined using a Linkam THM600 heating-freezing
stage attached to an Olympus BH-2 petrographic
microscope. Organic chemicals of known melting
point were used as standards (some standards
listed in Table 1; others summarized in Shepherd
et al., 1985). Autofluorescence of the fluid inclusions under ultraviolet light (illumination source
435 nm) was determined using a Nikon Eclipse
600 microscope with a UV-2A filter block (excitation 330–380 nm, barrier filter 420 nm, dichroic
mirror 400 nm).
RESULTS
F1
Recent and fossil chemosynthetic carbonates
were collected from outcrop on the Vøring
Plateau (offshore Norway) and Barbados, respectively (Fig. 1). Both sets of samples yield
monophase and two-phase inclusions that range
TABLE 1.
3
Vøring
1
2
3
OF
Vøring Plateau chemosynthetic carbonate
The Vøring Plateau was sampled on a smooth
and gently south-deepening slope characterized
by depressions and boulders on the sea floor. The
Storegga Slide provides evidence for recent deformation of the slope. An underwater televised
survey along an active fluid escape structure, discovered on the sea floor in 1998 using a 100-kHz
OREtech Side Scan Sonar (Kenyon et al., 1999),
showed a relatively flat morphology characterized by a laterally extensive carbonate crust. Slabs
of the crust are locally interrupted by edifices elevated above the sea floor through which the
venting of fluids has been observed (Kenyon et
al., 2001).
A sample consisting of skeletal bivalve remains, worm tubes, carbonate clasts and cement,
and siliciclastic sediment was obtained using a
televised grabbing device. Thick sections revealed the nature of the inclusions. Acicular aragonite cement is nucleated on the shells and on carbonate crystals in the detrital sediment. Primary
fluid inclusions occur in intracrystalline and intercrystalline sites in the aragonite cement (Mazzini et al., 2001). Most inclusions in the aragonite
cement are monophase with a probable trapping
temperature of 50°C. Two-phase inclusions
were also identified in the same acicular arago-
PRIMARY FLUID INCLUSION POPULATIONS
IN
CASE STUDIES
Fluida
Host
Size
(m)
Fill
(%)b
Th(°C)c
Tf (°C)d
2-Phase
2-Phase
Oil
Aqueous
Calcite
Calcite
13
6
Aqueous
Calcite
6
—
121–137 (L)
150–164(V)
(50)
—
100
Monophase
90
90
80
100
—
10
2-Phase
Water HC
Aragonite
6
9
Water HC
Water HC
Aragonite
Aragonite
6
8
27–30 (V)
27–50 (L)
65–75000
0–2000
100
2-Phase
Monophase
70
70–80
60
100
100
—
15
6
Inclusion type
Barbados
1
2
SUMMARY
in size from 1 to 5 m. Carbon isotope data confirm the chemosynthetic origin of the carbonate,
with values at about 51‰ in the Vøring Plateau
carbonate and values as light as 55‰ in the Barbados deposits (Torrini et al., 1990).
hydrocarbon.
proportion of liquid in fluid.
cT homogenization temperature; L homogenization to liquid; V homogenization to vapor.
h
dT freezing temperature.
f
en number of measurements.
aHC
bFill
ne
15
15
FLUID INCLUSIONS AND MARTIAN EXPLORATION
47
FIG. 1. Location of sample sites for chemosynthetic carbonates from (A) the Vøring Plateau, offshore Norway
and (B) Bath, Barbados.
F2
nite cement. Microthermometry on these twophase inclusions (Fig. 2) shows homogenization
to liquid occurs at around 65–75°C (Table 1). The
freezing behavior of the two-phase inclusions is
consistent with the presence of methane in the
fluids and suggests that the cements were precipitated as a result of oxidation of methane.
Barbados chemosynthetic carbonate
F3
The Barbados site contains a mixture of diagenetic carbonate and bitumen from the Bath Fault
Zone (Torrini et al., 1985). This locality consists of
a number of fault-slices, within which there are
frameworks of coalesced calcite growths with a
matrix of bitumen, precipitated at sites where hydrocarbons emerged from fractures onto the contemporary (Tertiary) sea floor. Bitumen-filled
fractures also cross-cut the calcite. The carbonate
contains molluscan fossils and worm tubes,
which suggests a sea floor vent fauna (Speed,
1990).
There are three types of primary fluid inclusion in the calcite: monophase aqueous inclusions, two-phase inclusions that homogenize to
liquid between 121 and 137°C (Fig. 3), and twophase inclusions that homogenize to vapor between 150 and 164°C (Table 1). The freezing behavior of the two-phase inclusions is consistent
with the presence of methane-rich fluid (Goldstein and Reynolds, 1994). The coexistence of
primary fluids that homogenize into either liquid or vapor suggests heterogeneous entrapment of water and methane. The high temperatures determined reflect the complex mixture of
the inclusion fluids and do not represent true
temperatures of fluid entrapment. The low temperatures indicated by the monophase inclusions are more significant in this respect. Some
samples also contain inclusions of liquid oil, as
the chemosynthetic carbonates were precipitated at the site of a sea floor oil seepage (Parnell et al., 1994). Inclusions of higher hydrocarbons (not methane) autofluoresce under
ultraviolet light and were readily detectable in
the thick sections studied here (Fig. 4).
FIG. 2. Two-phase (liquid, vapor bubble) aqueous
fluid inclusions (encircled) entrapped in anhydrite cement, Vøring Plateau, offshore Norway.
F4
48
PARNELL ET AL.
FIG. 3. Two-phase (liquid, vapor bubble) aqueous
fluid inclusions (encircled) entrapped in calcite cement,
Bath, Barbados.
DISCUSSION
The observations made on fluid inclusions in
chemosynthetic carbonates demonstrate that
fluid inclusions in a rock type that has been identified as a target for astrobiological exploration
on Mars can be successfully identified. Samples
of this type returned from Mars should therefore
allow us access to martian fluids. Despite their
small size, it has been possible to determine that
the terrestrial samples contain methane-rich fluids, and it should be possible to undertake more
sophisticated analyses, such as Raman spectroscopy, to assess further the inclusion chemistry. A potential approach to samples in which
inclusions are small and sparse is to crush the
samples and liberate the inclusion fluids for
analysis. However, in the case of methane-rich
fluids, care would be needed to avoid volatilization of the contents. It is also important to consider that in both case studies, multiple inclusion
populations were identified, so any analysis from
bulk crushing would inevitably be a hybrid from
all the inclusions.
Since the potential exists for fluid inclusions to
form at all depths encountered in a typical sedimentary basin, the range of circumstances under
which microbes could become entrapped in fluid
inclusions during mineral growth is broad. There
is increasing evidence for the preservation of bacteria at deep levels in oil reservoirs and in other
rocks down to a depth of several kilometers
(Ghiorse and Wilson, 1988; Onstott et al., 1999;
Stetter and Huber, 2000). Though the upper temperature limit for bacteria is 113°C (Blöchl et al.,
1997), it has been theorized that life can survive
temperatures as high as 150°C (Parkes et al., 1994),
which equates to a 5 km depth for a typical terrestrial geothermal gradient.
It is worth noting that any chemical precipitate
(usually carbonates and silica) focussed around
fluid seepage at the planetary surface may give
comparable geochemical information. Mineral
precipitation occurs where seepages emerge at
the surface because of the mixing of fluids of different chemistries and, to a lesser extent, because
changes in temperature, pressure, or both can
cause a drop in mineral solubility. Seepages are
sites of fluid movement and biomass concentration, and records of both are entrapped within the
mineral precipitate. We predict that higher-resolution analytical techniques will be able to detect
multiple types of signatures of a biomass within
entrapped fluids.
FIG. 4. Oil inclusions entrapped in calcite cement,
Bath, Barbados, (A) as viewed in plane transmitted light
and (B) fluorescing as viewed in ultraviolet light.
FLUID INCLUSIONS AND MARTIAN EXPLORATION
Relevance of fluid inclusion studies to
astrobiology search strategies
As the search for life is recommended to spatial scales on the order of 0.01 m (Westall et al.,
2000a), fluid inclusions in sedimentary rocks are
clearly a potential target in the search. Most fluid
inclusions in sedimentary rocks are similar in size
to that of many terrestrial bacteria, which typically range from 0.5 m to a few micrometers in
diameter (Nealson, 1997). The theoretical minimum diameter of a cell has been estimated by
Mushegian and Koonin (1996) at 0.14 m. However, ultramicrobacteria have been reported (e.g.,
Button et al., 1993), and some bacteria are known
to shrink substantially in size under stressed conditions (Morita, 1988; Amy et al., 1993). There is
also the possibility of nanobacteria, typically
50–200 nm in size, the origin of which is contentious though at least in some cases may be biological (Kajander and Ciftioglu, 1998). Pertinently, objects with morphologies comparable to
nanobacteria have been reported from terrestrial
hot spring deposits and in meteorites (Folk and
Lynch, 1997, 1998).
Fluid inclusions trapped at temperatures
100°C may yield evidence of microbial life entrapped within the fluid and thus yield a DNA
record. Although it is not known whether nonterrestrial organisms use DNA, the long-term survival of DNA (Cano et al., 1994) offers the possibility that fundamental molecular material can be
preserved. It has recently been proposed that bacteria cultured from Mesozoic salt deposits represent dormant bacteria within fluid inclusions
(Fredrickson et al., 1997) and that bacteria in inclusions within Permian salt were incorporated
during salt deposition (Vreeland et al., 2000).
There are also several older claims for bacterial
preservation in salt deposits (e.g., Reiser and
Tasch, 1960; Dombrowski, 1963; De Ley et al.,
1966; Norton et al., 1993; Denner et al., 1994), although there must be some reservation about any
inclusion record from halite due to its susceptibility to deformation and recrystallization (Hazen
and Roedder, 2001). However, it should be possible to eliminate contamination from the procedure for extracting fluids from inclusions if their
longevity can be proven (Vreeland and Powers,
1999; Vreeland and Rosenzweig, 1999; Rosenzweig et al., 2000). To date, no significant differences have been recorded between the RNA in
49
bacteria extracted from halite and in surface environments (Gemmell et al., 1998; Grant et al.,
1998); thus this evidence does not confirm
longevity of preservation. Manipulation of inclusions in a frozen state may be a useful strategy to
reduce contamination, as bacteria can survive
very low temperatures (Crowe and Crowe, 1992;
Clark, 1998). Indeed, if microbial life ever
emerged on Mars, it would have been at low temperatures if it is discovered in fluid inclusions in
sedimentary rock located at or near the martian
surface.
Meteorites from Mars and elsewhere also contain fluid inclusions in mineral phases (e.g., Bodnar, 1999; Zolensky et al., 1999; Bridges and Gray,
2000; Saylor et al., 2001), confirming that they
could be a valuable source of information in planetary exploration. The preservation of terrestrial
fluid inclusions of Archean age provides encouragement that early rocks on Mars may preserve
samples of inclusion fluids from a time when water was more abundant at the surface.
Potential for other surficial chemical precipitates
to harbor fluid inclusions
Several other types of surficial chemical precipitates proposed as targets for martian exploration are expected to contain fluid inclusions.
Hydrothermal systems emerging at the surface
are attractive candidates for fluid inclusion studies because they imply active water flow, supply
of nutrients in the form of reduced simple organic
molecules, hydrogen, and trace elements, and enhanced temperatures that are considered favorable for the evolution of primitive life (Walter and
Des Marais, 1993; Bock and Goode, 1996, and references therein; Farmer and Des Marais, 1999;
Farmer, 2000; Nisbet and Sleep, 2001). Indeed, it
has been suggested that hyperthermophilic life
may be the normal mode of life on Earth (Jakosky,
1998), although Forterre (1999a,b) has presented
cogent arguments against such models. Travertine hot spring carbonates have attracted particular attention where microbial mats are readily
mineralized by carbonate precipitates (Farmer,
2000). Fracture-filling freshwater banded carbonates have yielded measurable inclusions (Goldstein, 1986). Hydrothermal siliceous sinters have
also been recognized as potential analog targets
on Mars (Walter and Des Marais, 1993; Cady and
Farmer, 1996; Plescia and Johnson, 2001). Fluid
50
T2
inclusions in hydrothermal precipitates located
beneath siliceous sinters at Yellowstone National
Park are reported to contain inclusions of special
interest because of the possible bacteria identified
within them (Bargar et al., 1985; Bargar, 1992). Hydrothermal sinters are also notable for their potential to preserve bacterial microfossils (e.g.,
Schultze-Lam et al., 1995; Cady and Farmer, 1996).
Caves can also contain substantial developments of biofilms and microbial mats and are particularly interesting targets on Mars given the
hostile environment of the current martian surface (Grin et al., 1998; Boston, 2000; Boston et al.,
2001). Many of these cave deposits include mineralized growths that incorporate microbial life,
including speleothem carbonates. Fluid inclusions analyzed in several carbonate speleothems
have been found to incorporate aqueous fluids
(Goldstein, 1990), atmospheric gases (Goldstein,
1986), and even seeping petroleum (Elmore et al.,
1987, 1989). As an example of the potential for sophisticated chemical analyses in low-temperature
precipitates, measurement of hydrogen isotope
compositions in speleothem inclusions has been
used to assess fluctuations in meteoric precipitation (Harmon et al., 1979).
Other types of microbial carbonates include
stromatolites, calcretes, coated grains, and fluvial
tufas (see review by Riding, 2000). Although the
crystal size and fabric in these deposits may generally preclude inclusion entrapment, in some
cases sparite cements precipitate as large enough
crystals to harbor fluid inclusions.
Evaporites have also been suggested as potential hosts to microbial communities on Mars
(Rothschild, 1990; Horneck, 2000; Westall et al.,
2000a), and some interpretations suggest that significant evaporite basins exist on the planet
(Forsythe and Zimbelman, 1995; Ori et al., 2000).
Occurrences in meteorites also suggest that soils
on Mars may be partially cemented by evaporitic
salts (Bridges et al., 2001). Evaporites tend to have
abundant fluid inclusions (e.g., Roedder, 1984b),
and, as noted previously, given the numerous
records of bacteria in terrestrial salts the potential for such deposits to harbor evidence of life
will undoubtedly be explored further.
The likelihood of entrapment of inclusions has
changed with the evolving history of water on
Mars. Table 2 shows a summary of the potential
for entrapment of inclusions in different settings
through the stages distinguished by McKay
(1997).
PARNELL ET AL.
Exploration strategies for analyzing fluid
inclusions on Mars
The surface and near-surface deposits from
which inclusion data might be obtained could be
detected using spectral data such as the thermal
emission spectrometer on the Mars Global Surveyor (Christensen et al., 1992). Microthermometric and petrographic analyses of samples
would require them to be returned to Earth, but
some bulk compositional data could be obtained
in situ at Mars by robotic crushing of samples and
liberating the inclusion fluids for analysis by established geochemical techniques. Such bulk
volatile analyses are undertaken routinely to detect hydrocarbon-bearing sections in sedimentary
rocks (Hadley et al., 1997; Parnell et al., 2001). The
analyses are largely automated, require only
small samples (1 g), and do not require elaborate cleanup of the samples, all of which suggest
that this technology could be adapted for remote,
robotic analysis. The technique involves passing
the fluid from the crushing process into mass
spectrometers for detection of organic compounds and gases (Hadley et al., 1997).
Existing strategies for in situ analysis on Mars
already incorporate the use of mass spectrometry
(e.g., Sims et al., 1999), including its use for analysis of specific organic molecules (Brack et al., 1999,
2000). The development of microfluidic devices
will allow delivery of reagents to help clean up
and extract specific types of compounds using online microdialysis, which can be connected to mass
spectrometers (Waite et al., 2000). Other promising
developments for in situ organic analysis could be
the detection of amino acids at the sub-parts-perbillion level, using microchip-based capillary electrophoresis (Hutt et al., 1999), and detection of biological species using nanowire nanosensors (Cui
et al., 2001). Thus, the necessary miniaturization of
sample handing, mass spectrometry, and other analytical tools is presently under development.
It is unknown whether living microbial cells
can be preserved in, and liberated from, fluid inclusions. If they can, they could be detected by
techniques currently being established for martian missions (e.g., Sims et al., 1997, 1999). Rapid
advances in microbial enumeration are expected
(Mancinelli, 2000), and the detection of living
cells using a technique to release photons during
reaction with ATP is promising (Obousy et al.,
2000).
Though this discussion has focussed on the po-
3.8–3.1
3.1–1.5
1.5–
present
I
II
III
IV
OF
STAGES
Abundant; rivers and
lakes, but partly
dependent on
geothermal heat flux
Ice-covered lakes;
permafrost; saline
groundwater at 21°C
Liquid water in
porous rocks;
permafrost;
groundwater
No liquid water on
surface; permafrost
and ground-ice;
geothermal
groundwater
P 1 bar
T 0°C
Peak T 0°C
P 100 mb
Peak T 0°C
P 7 mb
T range 110
to 7°C
State of liquid water
in Mars
SEQUENCE
P 2 bar
T 0°C
Volcanoes
Meteorites
Conditionsa
pressure; T, temperature.
4.2–3.8
Epoch
aP,
Period
(Ga)
TABLE 2.
OF
Impact
fracture
systems
WATER
ON
Thermal
vent
precipitates
Methane
vent
precipitates
Surface
crusts
Potential inclusions
MARS (AFTER MCKAY, 1997)
Lacustrine
precipitates
Cements
in
aquifers
FLUID INCLUSIONS AND MARTIAN EXPLORATION
51
52
tential of fluid inclusion studies in surficial deposits, the potential to undertake studies on inclusions entrapped in rocks beneath the surface
that are now exposed at the surface due to erosion also exists. Most porous sedimentary rocks
in the subsurface contain sufficient pore space for
microbes to live. Ghiorse and Wilson (1988) estimate that openings in the range 0.1–1.0 m are
needed to accommodate bacteria, and porosity
measurements in aquifers indicate that this requirement is readily fulfilled. In addition to the
interstitial pores in sedimentary rocks, it is apparent that rocks undergo repeated microfracturing and healing during burial. The fracturing
can occur in incompletely consolidated sediments
that have not been deeply (1 km) buried. A
large proportion of sandstones, when viewed under cathodoluminescence, exhibit extensive networks of silica-cemented microfractures, which
are often in the range of 5–10 m in diameter.
These cemented microfractures contain fluid inclusions that entrapped the same fluids that occur in interstitial pores. In fact, healed microfractures may represent the predominant type of
inclusions in sedimentary rocks, including sandstones, and they deserve more attention. They are
certainly large enough to have entrapped microbial life.
Fracture systems are of particular interest in a
martian context. The surface of Mars exhibits
widespread evidence for impact cratering, which
must have created an extensive fracture network
with high porosity and water storage (Clifford,
1993). Large-scale permeabilities involving these
networks may be much higher than on Earth
(MacKinnon and Tanaka, 1989), allowing good
hydraulic connectivity and fluid flow. The subsurface temperature gradient on Mars would ensure circulation of fluids and their dissolved ions
(Clifford, 1991). Such fracture systems may therefore be good sites for life, and for mineral veining with evidence for life.
The chance of entrapping microbes in inclusions beneath the surface must be limited. In the
terrestrial environment, microbes occur at lower
densities and activities in the subsurface compared with the surface (Ghiorse and Wilson, 1988;
Onstott et al., 1999). However, on Mars this pattern may be reversed owing to the low temperatures at the surface and the greater likelihood of
liquid water in the subsurface. An important consideration is that in the pore spaces of buried
rocks, the high surface area will enhance the oc-
PARNELL ET AL.
currence of microbes as biofilms rather than suspended in the pore fluid (for review of biofilms,
see Costerton et al., 1995). Thus, they are unlikely
to be incorporated into fluid inclusions in rapidly
precipitated cements. On the other hand, biofilms
on mineral surfaces could be ideally placed for
incorporation in inclusions that are entrapped at
grain–cement boundaries. Many inclusions in
sandstones, for example, are entrapped at the
boundaries between detrital sand grains and cements nucleated on those grains. As the inclusions tend to be nucleated at irregularities such
as clay particles on the grain surfaces (Goldstein
and Reynolds, 1994), it is plausible that microbial
concentrations could also help to nucleate inclusions. Unfortunately, our experience in sandstones is that grain surface inclusions in quartz
cements tend to be only a few micrometers in size,
although examples exceeding 10 m are reported
(Goldstein and Reynolds, 1994).
It is important to bear in mind that this brief
discussion relates to quartz-rich sandstones with
quartz cements in a typical sedimentary basin in
a terrestrial setting. The precipitation of minerals
is controlled by temperature-dependent kinetics
and the availability of ions in solution. Mineral
growth in the subsurface of another planet, with
different mineralogy, ambient fluids, ambient
temperature, subsurface thermal gradient, burial
rate, etc., cannot be modeled simply by a terrestrial sandstone. Indeed, as Mars does not have the
large subsiding sedimentary basins found on
Earth, the terrestrial concept of “burial diagenesis” is clearly not applicable. We cannot expect to
predict the value of fluid inclusions from elsewhere until we sample them.
Valuable insights may be gained by the preparation of artificial fluid inclusions in bacteria-rich
environments and subsequent experimentation
to sample them. Indeed this has been attempted
in halite (Norton and Grant, 1988). Experience in
entrapping artificial inclusions in a variety of
minerals exists (e.g., Pironon, 1990) and could be
developed readily.
CONCLUSIONS
Our findings indicate that a type of rock sought
on the surface of Mars can contain identifiable
and interpretable fluid inclusions that would
yield valuable information about fluids in the
near-surface, including the presence of organic
FLUID INCLUSIONS AND MARTIAN EXPLORATION
molecules. Assuming advances in analytical precision will meet the demands of micrometer-scale
fluid inclusion studies, objectives that might be
met upon securing surficial samples with fluid inclusions include determination of:
1. The presence or absence of water
2. The presence or absence of organic molecules,
including methane
3. The salinity of any aqueous fluids. This may
help to address the concept that near-surface
fluids on Mars must be highly concentrated
brines to avoid their freezing (Brass, 1980;
Knauth and Burt, 2001).
4. The stable isotopic compositions of the contents, although a martian frame of reference
(database of known types of sample) would be
needed to interpret the values. The same applies to isotopic compositions of host carbonate rocks, though some insight into both
crustal and mantle stable isotopic compositions has been obtained through analyses of
meteorites (Wright et al., 1992; Romanek et al.,
1994; Jull et al., 1995; Jakosky and Jones, 1997;
Jakosky, 1999).
5. The presence or absence of microbial remains
or microbial biosignatures in the fluid
6. Some degree of reconstruction of temperature
and pressure conditions of entrapment, constrained by our understanding of the controls
and gradients of these parameters at the martian surface
ACKNOWLEDGMENTS
This manuscript benefited considerably from
reviews by P. Boston and M. Russell and from editorial work by S. Cady. We would like to acknowledge the Scientific Party of the TTR-10
cruise for their cooperation during the oceanographic mission offshore Norway, and in particular M. Ivanov for fruitful suggestions and the
UNESCO-MSU Centre for collaboration.
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Address reprint requests to:
Dr. John Parnell
Department of Geology and Petroleum Geology
University of Aberdeen
King’s College
Aberdeen AB24 3UE, U.K.
E-mail: J.Parnell@abdn.ac.uk