University of Rochester, Department of Earth and Environmental Sciences
Archaean age (>2.5 billion years) rocks from the Nuvvuagittuq Supracrustal Belt (NSB) in Northern Quebec in Northern Quebec were analysed under the SEM to check for distribution patterns of redox sensitive elements (Cr, V, Mo and U). These elements, if redistributed by dissolution in an incoming hydrothermal fluid, will undergo an increase in valence state when in solution (Cr(III) increases to Cr(VI)). Upon cooling down of the fluid, hydrous minerals like fuchsite (Cr-mica) are deposited which includes the redox sensitive elements dissolved from the host rock. Upon redeposition, the valence state reduces (Cr(VI) goes back to Cr(III)). If there was dissolution, one should see a decreasing concentration gradient of a redox sensitive element from the relict portions of the partly dissolved grain outward into the surrounding mica.
The optical microscope may be used to identify areas of interest on the sections to be viewed under the SEM. The SEM aids in identifying the composition of certain phases that may remain ambiguous after optical microscopy. The SEM is also useful for creating elemental distribution maps.
2. Samples and general descriptions
Two thin sections were viewed under the microscopes. Both of them were gneisses; i.e. high grade metamorphic rocks. The major phases in the sections are quartz and feldspar making up the felsic (High silica) portion. The mafic (low silica) portions are defined by biotite. These rocks have been metasomatised (altered by incoming hydrothermal fluids) and evidences for the metasomatism are the fuchsite layers which isn't a part of the host rock. This is clear from the way the fuchsite layers weave around some of the remanent post-dissolution crystals. These remanent crystals are apatite (a calcium phosphate phase), chromite (an iron-chromium phase), pyrite (iron sulfide) and chalcopyrite (iron-copper-sulfide).
When the fuchsite layers and the possible post-dissolution phases were viewed under the SEM, some of the textures that these phases show makes it hard to deny that dissolution took place. The strongest evidence is that of the thatched texture shown by a chromite grain. The relict portions of the chromite look as if they have been eaten away and the crystals look wormy. If it was an exsolution texture, we wouldn't have seen the areas in between the wormy portions showing a spectrum for mica. It would've been a solid crystal of chromite. These lines are also not excavation artefacts created during section making because then, the solid portions not showing the textures wouldn't exist without the texture.
3. Sample Preparation
Prior to any electron imaging, the thin sections were coated with a thin layer of carbon a few nanometers thick so as to make the entire sample conducting. The layer was deposited by heating and evaporating a rod of graphite and sputtering the sample with the carbon. A strip of graphite paint was applied connecting the sample to the basal stub. This provides a grounding to the sample and prevents charge accumulation on the sample.
4. Scanning Electron Microscopy
A Scanning Electron Microscope uses a beam of electrons that is being shot at a target to create an image. The electrons interact with the target to create secondary elecrons and back-scattered electrons as well as X-rays. The secondary electrons are inner shell electrons knocked out of their orbits by the energetic electron beam. The back-scattered electrons are particles of the electron beam that were reflected by the elements making up the target. Elements with a larger atomic number scatters more electrons and thus appears brighter. As for the X-rays, they are created when electrons are knocked out of orbit and the space created is occupied by a higher orbit electron by transitioning to a lower orbit. This releases the excess energy in the form of X-rays. All images were collected on a Zeiss Auriga CrossBeam SEM-FIB housed in the Department of Optics, University of Rochester.
Images were collected using the SE2 detector that collects secondary electrons and the BSD detector that collects back-scattered electrons. The EDAX detector that collects X-rays was used to figure out the composition of phases as well as to create elemental distribution maps.
5. X-ray spectrum and elemental map collection
The X-ray spectrum collection was necessary to identify various minor phases too small to be unambiguously indentified by the optical microscope. X-ray spectra may be collected over a single spot, the entire field of view or a reduced area within the field of view. The size of the phase and the magnification in use dictates the mode of detection.
As for the X-ray map, X-rays are collected over the entire field of view and maps are created of all the elements detected. The maps are drawn using pixels where each pixel represents a presence of the element of interest in the vicinity of that pixel. The intensity of the colour of the pixel represets the concentration of that element. The intensity of each pixel is decided upon by the software after the detector spends a specified amount of time collecting X-rays from an area that is represented by a single pixel. Also, the image quality of the map depends on the preset resolution of the map. The greater the resolution, more is the time taken to draw the complete map.
For the maps shown here, the resolution used was 512 x 400 and the time spent collecting a spectrum over a unit area was 300 microseconds. Maps were created for the elements K, Al, Cr, Fe, V, Si, O and Mg. K, Al, Si and O are the major elements comprising the mica with minor concentrations of Cr, Fe and Mg. As for the chromite, the major components are Fe, Cr and O while the minor elements are Mg, Al and V.
6. Optical Microscopy
The main point of using Optical microscopy was to look at the true colour of the phases making up the sample and understand the overall petrology of the samples. The problem with optical microscopes is the low magnification (400X compared to 10000X on the SEM) compared to the SEM as well as the inability to return the composition of phases. Also, the BSD on the SEM is very useful in depicting heavy, minor phases that are easily overlooked under an optical microscope.
The petrographic microscope uses visible light in two ways; one as plane polarised light by passing visible light through a polariser and as crossed polarised light by passing the already polarised light through an orthogonally oriented polariser. the optical properties of various phases under both these modes help in identifying a mineral.
7. Pixel Area Analysis
The main objective of the study is to attempt to prove a dissolution gradient from the body of the chromite into the main body of the mica. However, the resolution of the maps were not as high as required to make a gradient apparent. So, to further try and justify a gradient, the elemental map of chromium was analysed to check for a possible decrease in chromium detected from the grain boundary to the interior of the surrounding mica. The map is a pixellated image and each pixel is a representation of the concentration of chromium in the vicinity of that pixel. Keeping this in mind, the total area covered by chromium represenatative pixels per unit area should decrease from near the grain boundary to away from it. To try and prove this hypothesis, an area was cropped out from the elemental map and divided into equal area rectangles. Each of these rectangles were loaded on to the ImageJ program that counts closed area shapes as particles and reports the total number of particles as well as the total area of these particles. Five rectangles were analysed in all and upon a particle analysis, the total particle areas were plotted on a graph. The resultant graph does indeed show a decreasing pixel area from the grain boundary to the interior of the surrounding mica. Also, as one transitions from the grain boundary, outward, a gradual decrease in the pixel area is clear instead of a sharp drop that would have been seen had no dissolution taken place.
The study of the thin sections under the SEM revealed a lot of minor phases that is important for barometry and thermometry studies. Most importantly, one is more confident about redistribution by dissolution after analysing the elemental maps. However, due to the limitations of the SEM, other redox sensitive elements such as U, Mo and V were not detected because of their low concentrations and so couldn't be analysed. The occurence of dissolution maybe conclusively proved by piston cylinder experiments where grains are dissolved under high pressure in a fluid of the experimentalist's choosing and then observing the grain post-dissolution. Understanding the chemical and physical properties of the dissolving fluid as well as the exact nature of the dissolution itself is important for both industrial as well as academic purposes. Dissolving elements of interest and depositing them in a layer would create an economically viable deposit. As for academic purposes, successfully recreating the oxygen fugacity of the environment of dissolution gives us a better idea of the dissolving fluid.
I would like to thank Prof. Dustin Trail for braving the wilderness and animal attacks to collect the samples from the Canadian Arctic and Gerry Kloc for creating the thin sections.
I would also like to thank Brian McIntyre for letting me annoy him and proving to him that stupid questions do exist; my TA, Caleb, for helping me with the SEM labs. Also, Thank you Sarah, for being the best second set of eyes one could ask for and for helping me out when I evidently was pathetic at correcting stigmation. Not to mention your back seat driving when away from the microscope. It sort of helped.
THIS WAS FUN!