What are Fluid Inclusions?
Fluid inclusions are tiny quantities of liquid, vapour, or mixtures of these phases, trapped as impurities within minerals. Their sizes range from submicroscopic up to several hundred micrometers in diameter, and their masses are typically in the order of nanograms to femtograms. Where they are enclosed by minerals that are transparent to visible or infra-red light, fluid inclusions may be observed in a microscope once the host minerals are cut into thin slices and polished. Observations of this kind reveal that rock samples of the Lithosphere commonly contain billions of fluid inclusions. Microscopic study of specific samples permits inclusions that formed during initial mineral growth ("primary inclusions" and "pseudosecondary inclusions") to be discriminated from those that formed some time after mineral growth ("secondary inclusions").
Various sources of evidence suggest that many fluid inclusions preserve the chemical and physical properties of the original parent fluids from which they formed. Fluid inclusions are therefore considered to be direct samples of the volatile phases which circulated through the Lithosphere over the course of the Earth's history, and their chemical analysis provides information on the composition and density of these geologically important phases. Consequently, the results of fluid inclusion research are relevant to our understanding of many natural sub-surface processes in which fluids play a role, such as ore transport and deposition, petroleum generation and migration, explosive volcanism, geothermal energy, earthquake mechanics, petrogenesis of igneous, metamorphic, and diagenetic rocks, contaminant (including radionucleide) transport, and so on.
In addition to exploring applications of fluid inclusion analyses, current research into fluid inclusions is pursuing several directions, including studies of the systematic behaviour of fluid inclusions in rocks that have undergone long and complicated changes in pressure, temperature and deformation rates; improvements to micro-analytical techniques; experimental synthesis and simulation of the behaviour of natural fluid inclusions; experimental determination of the physicochemical properties of fluids found in natural inclusions; and many more. Aspects of all of these fields of research are also being investigated at the University of Leoben.
Details about the study of fluid inclusions can be found in: R
oedder, E. (1984) "Fluid inclusions" Vol. 12. Reviews in Mineralogy, (Ed. Ribbe, P. E.) Mineralogical Society of America, 644 pp.
Sheppard, T.J., Rankin, A.H. and Alderton, D.H. (1985) A practical guide to fluid inclusion studies. Glasgow, Blackie & Son, 239 p.
Goldstein, R.H. and Reynolds T.J. (1994) Systematics of fluid inclusions in diagenetic minerals. Short Course, v. 31, SEPM, 199 p.
De Vivo B. and Frezzotti M. L., editors (1994): Fluid Inclusions in Minerals: Methods and Applications, Short Course of the IMA Working Group "Inclusions in Minerals" (Pontignano - Siena, 1-4 September, 1994), Published by Virginia Tech, USA.
Samson, I., Anderson, A., and Marshall, D. (2003) Fluid Inclusions: Analysis and Interpretation. Short Course, v. 32, Mineralogical Association of Canada, 374 p.
Petrogrpahic examination of fluid inclusions is one of the most important steps in reconstructing the P-V-T-X evolution of paleo-fluids. The aims of petrography are to discriminate the various compositional types and generations of fluid inclusions in the sample of interest, to identify their mechanism of formation (primary, secondary or pseudosecondary) and their ages relative to each other and relative to the mineral parageneses in which they occur. Our laboratory is equipped with several petrographic microscopes which allow fluid inclusions to be observed in transmitted and reflected light with objective lenses up to 100X magnification power.
Several modes of observation are possible:
- Transparent samples can be observed in visible light
- Inclusions containing aromatic hydrocarbons can be distinguished by their fluorescence in UV light, and their spectral characteristics (intensity versus wavelength of emitted light) can be analysed quantitatively using a microspectrometer
- Inclusions in certain nominally "opaque" minerals can be observed in IR light, and their images made visible using a video camera sensitive to 1100 nm wavelength.
Petrographic images can be recorded as follows:
- Direct digitization onto a computer disc via the video camera and frame-grabber
- Direct printing on a thermal printer attached to the video or computer monitors
- Photography with conventional SLR cameras mounted on the microscopes
Next to petrographic examinations, "microthermometry" is the most important analytical technique for characterizing fluid inclusions. It involves measuring the temperatures at which phase-transitions are observed to occur in fluid inclusions. If the inclusions have simple compositions (less than 3 or 4 major components) then the microthermometric measurements allow the bulk composition and density of the inclusions to be calculated. For example, the extent to which the melting point of ice is depressed, provides an indication of the bulk salinity of the inclusions. If the inclusions are more complex, then the phase-transition temperatures provide useful constraints on the bulk composition and density, but additional analytical results must be combined to arrive at an exact solution.
Three types of commercially available microthermometric stages are available in our laboratory, each of which allows phase transitions to be observed in fluid inclusions over the temperature range from -193 to + 600°C:
- 1 Linkam MDS 600 stage controlled from a Pentium III 450 MHz computer with a Nokia445Xpro monitor. The stage is mounted on an Olympus BX 60 microscope (modified and supplied by Fluid Inc.) outfitted for use with reflected- and transmitted-visible light, reflected UV light, and transmitted IR light, using 4x, 10x, 40x and 100x Olympus long-working distance objective lenses for visible light, and 50X and 80X Olympus long-working distance objectives for IR light. Visible-light images are digitally acquired and viewed on the Nokia monitor using a JVC F553 3-chip video camera. IR images are aquired using a black and white CCD (no-name) camera, sensitive to 1100 nm.
- 1 Linkam THMSG 600 stage mounted on an Olympus BX 40 microscope outfitted for use with reflected- and transmitted-visible light, using 4x, 10x, 40x and 100x Olympus long-working distance objective lenses. The same stage is also used to conduct Raman spectroscopic analyses at controlled temperatures.
- 1 Fluid-Inc. modified USGS gas-flow stage, mounted on a Zeiss Universal, reflected- and transmitted-light microscope equipped with Zeiss 10X and 20X objectives, and a Nikon 40X long-working distance lens. Sample viewing is through a Sony 3CCD video camera and Sony monitor.
Synthetic fluid inclusions are used to calibrate the thermocouples in these stages, resulting in measurement accuracies to within +/- 0.2 degrees at temperatures below 100 C, and +/- 0.4 degrees at higher temperatures.
A simple hinge device allows fluid inclusion samples to be cracked under oil while they are being observed through a microscope. The behaviour of gas bubbles that escape from the inclusions provides information on the internal pressure of the gases and on their composition.
Laser Raman Microspectrometry
Laser Raman microspectroscopy allows the covalently-bonded chemical species within fluid inclusions to be identified, and in some cases, quantitatively analysed.
The Dilor LabRAM instrument at Leoben works as follows: a laser beam is focussed through an Olympus BX 40 microscope onto the fluid inclusion of interest. The objective lenses of 50X and 100X magnification, combined with a confocal optical arrangement, enable a spatial resolution in the order of a cubic micrometre. Thus the laser can often be focussed on individual phases within multi-phase inclusions. Our LabRAM has two lasers: a 100 mW frequency-doubled Nd-YAG with 532 nm wavelength (green) and a less powerful He-Ne laser with 613 nm (red). Detection limits are lower using the energetic green laser, but the red laser is preferable when the samples for analysis are fluorescent. The interaction of the incident laser light with the molecular bonds in the target species scatters some of the incident light via the "Raman effect", emitting light with a frequency that is shifted from that of the laser, and that is characteristic of the vibrational mode and energy of the bond. A portion of the scattered light is collected through the microscope and focussed onto a diffraction grating. The grating selects the desired region of the Raman spectrum and reflects this onto a Peltier-cooled, CCD matrix detector. The resulting spectrum (intensity versus Raman-shifted frequency) is displayed on a computer monitor for further processing and interpretation. Measurements can also be performed at controlled temperatures between -190 and 600 degrees C, using the Linkam THMSG 600 heating-cooling stage described above.
Details of applications of this method to fluid inclusions can be found in:
Dubessy et al. (1989): Advances in C-O-H-N-S fluid geochemistry based on micro-Raman spectrometric analysis of fluid inclusions. European Journal of Mineralogy 1: 517-534.
Experimental Capabilities at Leoben
10 externally-heated, cold-seal pressure vessels are available for hydrothermal experiments up to conditions of approximately 800 C and 10 kbar. Argon is used as a pressure medium. Fluid inclusions are synthesized in pre-fractured quartz sealed in gold capsules with fluid components of known concentrations. In this laboratory, fluid inclusions will be manufactured to calibrate the temperatures of phase transitions observed in natural fluid inclusions, and to provide calibration standards for analytical methods.
Details of this method can be found in:
Bodnar R. J. and Sterner S. M. (1987): Synthetic fluid inclusions. In Hydrothermal experimental techniques (ed. G. C. Ulmer and H. L. Barnes), pp. 423-457. J. Wiley & Sons.
The fluid inclusion laboratory is available to all members of the University of Leoben. Guests are also welcome from other research and teaching institutions. Analyses for industry may be made by the laboratory staff on a negotiated contract basis.
Further Information: Dr. Ronald Bakker