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APPLICATIONS OF THE MOLE RAMAN MICROPROBE TO THE STUDY OF FLUID
INCLUSIONS IN MINERALS
J. Dubessy, C. Beny, N. Guilhaumou, P. Dhamelincourt, B. Poty
To cite this version:
J. Dubessy, C. Beny, N. Guilhaumou, P. Dhamelincourt, B. Poty. APPLICATIONS OF THE MOLE
RAMAN MICROPROBE TO THE STUDY OF FLUID INCLUSIONS IN MINERALS. Journal de
Physique Colloques, 1984, 45 (C2), pp.C2-811-C2-814. �10.1051/jphyscol:19842186�. �jpa-00223861�
APPLICATIONS OF THE MOLE RAMAN MICROPROBE TO THE STUDY OF FLUID INCLUSIONS IN MINERALS
J. Dubessy, C. Beny*, N. Guilhaumou**, P.
r ham el in court***
and B. Poty CREGU, B.P. 23, 54501 Vandoeuvre-22s-Naney Cedex, F r a n c e*CRSCM, r u e d e Za F g r o Z Z e r i e , 45045 OrZdans Cedex, F r a m e
**
ER-224-CNRS, Lab. GdoZogie, ENS, 46 r u e dlUZm, 75230 P a r i s Cedex 05, France***
LASIR-LP-2641-CNRS, USTL, B.P. 36, 59650 V<liZZeneuve d l A s c q , FranceRESUME
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Dans les inclusions fluides aqueuses, la microspectrometrie Raman permet d'analyser l'ion SO4 et d'identifier indirectement les ions monoatomiques par les hydrates de sels nuclGBs au cours du refroidissement. Deux exemples d'identification de gaz (C02-H2S) et (H2-02) dans les inclusions fluides sont prGsentGs.ABSTRACT
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In aqueous fluid inclusions, micro-Raman spectrometry allows to analyse SO4 ion and to identify indirectly monoatomic ions by the salt hydrates nucleated during cooling. Two examples of gas identification (C02-H2S) and (Hz-02)in fluid inclusions are given.I
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INTRODUCTIONA rock is the product of chemical reactions between solid phases, the minerals, and fluids which for the most part have disappeared. Some relics of these fluids, however, remain in small intracrystalline cavities, 10 to 100 pm in size.
Knowledge of the composition of the fluids in these inclusions is essential to re- construct the mineral
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fluid chemical equilibria. A single mineral may contain fluid inclusions of various age, origin and consequently composition. Only microsco- pic analytical techniques, therefore, can give fruitful data. Micro-Raman spectrome- try is one of these techniques (1). The machine we used is the MOLE type (Jobin- Yvon) described elsewhere (2).I1
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IONS DISSOLVED IN AQUEOUS SOLUTIONS OF FLUID INCLUSIONSIonic sulphate is the only polyatomic ion which has been successfully identified in fluid inclusions (
>
200 ppm concentration) (3). The ratio of the intensity of the Raman signal of the symmetric stretching vibration of SO4 (vl = 980 cm-l) and of the bending mode of liquid water (1500-1800 cm-')(~i~. 1) has been calibrated as a function of bulk SO4 concentration in solution saturated in halite.Using this calibration curve, the SO4 concentration inside primary fluid inclusions of halite crystals sampled in a present day salt pan has been proved to be the same as that of the parent brine of these host crystals. The same methodology applied to Keuper evaporites (-200 millions years) have shown that the parent brine had a com- position incompatible with the evaporation of present day sea-water (Fig. 2).
As one cannot detect monoatomic ions by Raman spectra, it is necessary to use an indirect method for their determination. During cooling an aqueous solution of the H20-NaC1-CaC12-KC1-MgC12 system, various salt hydrates may nucleate depending on the composition of the solution. The fundamental internal modes of the water molecules of the main salt hydrates (NaC1.2H20, CaC12.6H20, MgC12.6H20, MgC12.12H20, KC1.MgC12.6H20) have different frequencies. Raman spectra of these crystals have been recorded at -190°C (3). These references spectra were subsequently used to identify the salt hydrates which have nucleated during cooling the host mineral of the fluid inclusion with the Chaix-Meca (5) freezing stage. In figure 3, the Raman spectrum of NaC1.2H20 in a fluid inclusion is compared with that of synthetic NaC1- .2H20. The lower resolution of the Raman lines of the hydrate from the inclusion results from a greater supply of heat relative to the reference due to the objective being (LEITZ, Plx160) very close to the mineral plate.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842186
JOURNAL DE PHYSIQUE
Fig. 1
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Raman spectra of sulphate(vial)
and water (v2al) from A) prepared solutions and B) the aqueous phase of a natural inclusionFig. 2
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Diagram SO concentra- tion (molefkg H20) in brines of modern saltworks.Keuper brine with Br- esti- mated from the Br- concen- tration of halite host crystal.
Fig. 3
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Raman spectra of hydro-halite (NaC1.2H20) from A) prepared solutions and B) a natural inclusion recorded at -190°C.The C-O-H-N-S system is the basic system of many geological fluids. C02- CH4-C2H6-N2-H2S-CO-SO2-H2-O2 are the common molecular species identified by micro- Raman analysis (1,6,7,8,9,10). The measurement of the intensity of the Raman signal of each gaseous species and the corresponding cross-section of Raman scattering a (11,12) allows one to determine the mole fraction of each gas. For example, C02-H2S fluids have been detected in native sulphur bearing fluid inclusions, figure 4 (7).
The (C02/H2S) in gas / (C02/H2S) in water ratio measured is 3.3 and is consistent with the theoretical ratio. This indicates that (oH2S/uC02) in water is not very different from (aH2S/uC02) in gas.
solid phase
Fig. 4
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Raman spectra in a single natural inclusion of A) native crystalline sulphur,B) C02 and H2S in gaseous, liquid and aqueous phase.
Free molecular hydrogen and oxygen have been identified inside fluid in- clusions from the Rabbit Lake uranium deposit, Fig. 5. At room temperature, the partial pressure of O2 is around 50 atm. and that of H2 around one atm. This compo- sition is obviously far from equilibrium and must result from radiolysis of pore water by a particles originating from the natural radioactive decay of uranium mine- rals. Numerical simulation shows that one mole % of free O2 and H2 can be yielded by such process in 10 000 years.
Fig. 3
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Raman spectra of H2, O2 and N2 in a natural inclusion from the Rabbit Lake uranium on deposit.JOURNAL DE PHYSIQUE
IV
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CONCLUSIONThe Raman microprobe is now for fluid geochemistry what is the electronic microprobe for mineral geochemistry. The detailed characterization of fluid chemistry through the continental crust is now possible.
REFERENCES
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