Geology under the microscope

Geological specimens preserving clues to the history of the Earth are examined using temperature controlled microscopy – Ian Pearce tells us how

TEMPERATURE controlled microscopy is being used to investigate geological specimens in industry and academia worldwide. The technology is benefitting the work being routinely performed by the research group of Professor Andrew Rankin of the School of Geography, Geology and the Environment at Kingston University.

Fluid inclusions are small droplets of fluid that have been trapped within crystals either during primary growth from solution or at some later stage, usually as a result of recrystallisation along healed microfractures. Trapped and preserved as microscopic fluid-filled cavities, these fluid inclusions may contain high temperature silicate melts, hydrothermal fluids, and low temperature brines together with various hydrocarbon gases and liquids. They are ubiquitous in both naturally occurring minerals and in laboratory-grown crystals.

To the chemist or materials scientist, these gross defects cause endless obstacles in their quest to grow near perfect crystals. However, to the geologist, they provide a unique fossil record of the various fluids responsible for the formation and evolution of rocks and minerals throughout the history of the Earth.

The formation of a fluid inclusion may be a result of the differential shrinkage of trapped melt or fluid on cooling after entrapment which results in the nucleation of a contractional vapour bubble. Various solids may also precipitate to form multiphase inclusions.

The study of these fluids dictates the use of temperature controlled microscopy, where the temperature of the sample can be manipulated.  This is achieved by attaching heating/freezing stages to diascopic, episcopic and UV-fluorescence research grade microscopes.

The range of sample temperatures is very wide and Professor Rankin’s applied geology laboratory utilises temperature stages covering a range from -196ËšC to 1500ËšC.

For cold temperatures, the laboratory uses the Linkam MDS600 Examina pro, which covers a range of -196ËšC to 600ËšC. It is equipped with a computer controlled x-y stage and programmable heating/cooling rates from 0.1ËšC to 99ËšC per minute. High temperature studies are made using the Linkam TS1500 stage with a top temperature of 1500ËšC.

The heating/freezing stages are mounted on a Nikon Optiphot optical microscope equipped with high quality, long working distance lenses. These allow high resolution observations to be made up to magnifications of x1200. With this set-up, samples between 10μm to 0.5cm may be studied and observations and measurements carried out on both liquids and solids.
     
The heterogenisation process involved in the formation of field inclusions may be reversed and studied in the laboratory by heating the inclusions under the microscope to the point at which the contents become homogenous. The homogenisation temperature, Th, represents the minimum trapping temperature which is a useful parameter as a palaeogeothermometer.

The cold-stage microscope is useful as a means of identifying water and CO2 in the inclusions based on the melting temperature of ice and solid CO2 in frozen inclusions. Depression of these melting temperatures (0oC and -56.6oC respectively) can also be used to determine the salt contents (equiv. wt% NaCl) and the presence of other gases such as N2 and CH4. 

Based on this knowledge, the overall density (or molar volume) of the fluid may be calculated with reference to relevant PVT diagrams for modal systems such as NaCl-H2O. If pressure at the time of trapping can be made independently, the true trapping temperature (Tt) can be estimated. Conversely, if Tt is known, pressure (Pt) and therefore palaeodepth, can be calculated.

Other specific geological applications include the study of geothermal wells, where the temperature to pressure estimates and identification of "boiling zones" in exploration and development are useful in understanding the use of geothermal energy created during the Earth’s formation. This has positive environmental and economic considerations.

Temperature/salinity indicators of the nature and origin of hydrothermal fluids can be used as a guide to mineral exploration. Petroleum exploration and development may benefit from temperature/burial history estimates to establish timing of oil generation and migration.

The Linkam stages used in Professor Rankin’s laboratory have enabled the routine study of geological fluids as well as many other samples. They deliver accurate temperature control, which is vital in any lab-based experiment, yet are simple to set-up and straightforward to operate.  And, the positioning capability of the temperature stages means that the optical microscope systems may be used to visually record sample changes as a function of temperature.

Beyond geological studies, the Kingston University Micro Unit has looked at a broad range of materials. For example, phase transition temperatures in a wide range of materials including polymers, liquid crystals, alloys and micro-crystals of inorganic and organic compounds have been measured. The cold stage has also been used to look at the effect of freeze-thaw cycles of ice and water in biological and cellular materials, and frozen foodstuffs, emphasising the versatility of temperature controlled microscopy.