Lunar and Planetary Science XXXIII (2002)
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SPECTROSCOPIC METHODS FOR ANALYZING ORGANIC COMPOUNDS IN FLUID INCLUSIONS
DURING PLANETARY EXPLORATION. A. Mazzini 1, R. Li 2, and J. Parnell 1, 1Department of Geology and
Petroleum Geology, University of Aberdeen, Meston Building, King's College, Aberdeen AB24 3UE, Scotland, UK,
2
Open Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang
550002, China.
Introduction: Fluid inclusions are valuable sources of
information on the conditions under which their host minerals were precipitated. In the context of planetary exploration,
information about fluid chemistry is particularly important,
as it could provide constraints on the suitability of the environment to support of life, and could even contain direct
biomolecular signatures of life. Accessing the fluid in inclusions trapped at the surface/near-surface of a planet such as
Mars will therefore be an objective of high priority.
In this report we show examples of two case studies using spectroscopic techniques which provide data on organic
compounds in fluid inclusions. The techniques used are Raman spectroscopy and Time of Flight-Secondary Ion Mass
Spectrometry (TOF-SIMS), both of which are being miniaturized for future space exploration missions. Both of these
techniques can provide data on both organic and inorganic
compounds in the same analysis, so offer a versatility that is
suitable for fluids that are mixed or of uncertain nature. Raman spectroscopy was undertaken to demonstrate the organic
content of inclusions entrapped in chemosynthetic carbonate
at a modern cold seep site. Cold seeps have been identified
as targets for astrobiological exploration [1], so this demonstration is a valuable indicator that we may be able to identify organic fluids within them. TOF-SIMS was undertaken
on a sample containing two populations of oil inclusions, to
demonstrate that they can be distinguished.
Raman Study: Raman spectroscopy relies on the scattering of monochromatic radiation due to interactions with
molecules in solids and fluids. Spectra show shifts in the
energy of the scattered radiation to frequencies with a wavelength distribution characteristic for a given compound.
The technique was demonstrated using a sample from the
seafloor on the Vøring Plateau, offshore Norway. Methane
seepages have been recorded on the sea floor, around which
there has been precipitation of carbonate crusts containing a
shelly community. The carbonates are assumed to be chemosynthetic. Fluid inclusions have been identified in aragonite
cements within the crusts. Measurements were made using a
Renishaw Raman microscope at Kingston University, under
the guidance of Dr. P. Murphy.
Figure 1 shows a Raman spectrum from a 2 micron inclusion, in which two components are evident; the aragonite
host, and hydrocarbons within the inclusion. We are unable
to resolve the hydrocarbons, suggesting that they are not
simply methane. Larger inclusions would allow better resolution.
TOF-SIMS Study: Secondary Ion Mass Spectrometry
(SIMS) has been applied to fluid inclusions by several workers [e.g. 2]. However the particular technique of TOF-SIMS
has previously found only limited use in geological studies
[3]. TOF-SIMS instruments are especially suitable for studies where limited quantity of sample is available, as they
allow increased transmission of material and simultaneous
detection of all masses leading to very high sensitivity. They
also have the advantage of measurement of trace quantities of
both organic and inorganic components.
Organic fluid inclusions were examined from the Eocene
Shahejie Formation in the Bohai Basin, Eastern China. The
organic inclusions are assumed to represent entrapped oil.
Two populations of organic inclusions are distinguished in
limestone. These are primary (syn-cementation) and secondary (in cross-cutting trails). Microthermometry and fluorescence characteristics [4] show that these two populations
have a different composition, and therefore constitute a test
for the sensitivity of the TOF-SIMS techniques. A TOFSIMS 2000 produced by φ Physics Electronics Phi Evans
was used to analyse the organic composition of the inclusions, using the methods of Diamond et al [2].
TOF-SIMS Spectra for the organic inclusions are shown
in Figure 2. The inorganic fragment ions of both the primary
and secondary organic inclusions are mainly Na+ (23), Mg2+
(24), Si4+ (27), K+ (39) and Ca2+ (40). Ca2+(40) is the strongest peak, because the host mineral of the organic inclusions
is calcite (CaCO3). In the organic fragment ion peaks, the
organic structures of the secondary organic inclusion are
mainly aliphatic hydrocarbon (with mass number of
29,43,57,71… with formula CnH2n+1), unsaturated olefinic
hydrocarbon (with mass number of 41,55,69,83 … with formula CnH2n-1), but relatively little aromatic hydrocarbon
(with mass number of 65,77,79…). The inorganic ions of the
primary organic inclusion are similar to that of secondary
organic inclusions, but we can also detect H2O+(18), and
higher Na+ (23) and K+ (39) contents. This suggests that the
primary organic inclusions contain a mixture of organic material with saline water. Compared with secondary organic
inclusions, the primary organic inclusion contains proportionally more olefinic hydrocarbon, with lesser quantities of
saturated aliphatic hydrocarbon, and negligible aromatic
hydrocarbon. The differences in organic compounds indicate
that distinct compositions of oil were present during the two
stages of fluid entrapment.
Application to Exploration on Mars: The two techniques discussed both have the advantage of being used
Lunar and Planetary Science XXXIII (2002)
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SPECTROSCOPIC ANALYSES OF ORGANICS IN INCLUSIONS: A. Mazzini, R. Li, and J. Parnell
mineral (which incorporates inclusions), rather than reducing
the beam size and imaging individual inclusions. This would
mean that the chemistry of inclusion fluids would be diluted
by the chemistry of the host mineral, but where the objective
is to identify organic compounds any ambiguity about precisely where the compounds are located is of secondary importance.
References: [1] Komatsu G. and Ori G. G. (2000) Planet
Space Sci., 48, 1043-1052. [2] Diamond L. W. et. al. (1990)
Geochim. Cosmochim. Acta, 54, 545-552. [3] Hou, X. et. al.
(1995) Int. Jour. Coal Geol., 27, 23-32. [4] Li R. (1998)
Unpublished PhD thesis. [5] Steele A. et. al. (2001) Astrobiology, 1, 214. [6] Brinckerhoff W. B. and Cornish T. J.
(2000) Concepts and Approaches for Mars Exploration,
6027. [7] Brinckerhoff W. B. et. al. (2000) LPS XXXI, 18171818. [8] Wynn-Williams D. D. and Edwards H. G. M.
(2000) Icarus, 144, 486-503. [9] Edwards, H. G. M. et. al.
(1999) Planet. Space Sci., 47, 353-362. [10] Dickensheets,
D. L. et. al. (2000) Jour. Ram
without the need for prior extraction or derivatization, and
are non-destructive. They are therefore regarded as suitable
for in situ analysis on Mars. It has been suggested that TOFSIMS can detect various biomolecules in fossilized bacterial
biofilms, and consequently the technique has been advocated
for non-destructive analysis of returned samples [5]. There is
also potential to use TOF-SIMS for in situ analyses [6, 7].
The application of Raman spectroscopy to astrobiological
exploration is being developed by the identification of organic compounds that might be expected in stressed microbial communities [8], demonstration studies on meteorites
[9], and miniaturization for Martian missions [10].
Our specific application of these techniques to fluid inclusions required fine laser focussing to target the inclusions.
Although the techniques can be directed by imaging on a
scale of microns/tens of microns, we are not yet at the stage
where we can apply them to such specific targets during a
remotely conducted analysis. It is likely that successful remote targeting of the contents of fluid inclusions will be
made by increasing the sensitivity of analysis for a volume of
14000
12000
Aragonite
Hydrocarbons
Intensity (a.u.)
10000
8000
6000
4000
2000
2850
2950
3458
3253
3048
0
2843
2638
2433
2228
2023
1818
1613
1408
1203
998
-1
Wavenumber (cm )
Figure 1. Raman spectrum for inclusion in chemosynthetic aragonite, Vøring Plateau.
1400
1200
1000
800
600
400
200
0
23
Primary
41
27
43
29
15
55
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40
Secondary
15000
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23
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0
15
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27
43
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69
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81
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Figure 2. TOF-SIMS spectra (in mass units) for primary and secondary organic inclusions, Bohai Basin.