Imaging – UMass Geosciences Electron Microprobe / SEM Facility

Imaging by EPMA and SEM are done by accumulating and displaying positionally-related signal intensities – essentially signal intensity maps of a sample surface. The detected signals include any signal generated by electron beam – specimen interactions and, in this facility, those signals include secondary electrons, backscattered electrons, characteristic X-rays, and light (cathodoluminescence). Examples are shown in the first sidebar.

Characteristic X-ray mapping (compositional mapping)

X-ray maps represent relative elemental concentration between and within phases, and arise from emission of charateristic X-rays during inner shell ionization and subsequent relaxation in target atoms during electron bombardment.

The characteristic X-rays can be selected by either wavelength dispersive spectrometry (WDS, the essence of EPMA), or by lower spectral resolution energy dispersive spectrometry (EDS). X-ray intensities are collected and displayed pixel by pixel, and are qualitative until all elements within the material of interest have been mapped, and pixel intensities are converted to concentration after background subtraction, dead-time correction, and matrix (ZAF) correction. Typically maps are utilized as a qualitative guide. Here is an example…

**Multi-stage coronitic overgrowths on OPX in mafic granulite** Ca map of an orthopyroxene core and its surrounding muti-generation mantle from the 2.6 Ga East Athabaska mylonite triangle located north of Stony Rapids, Saskatchewan. The complex coronitic sequence progresses from the original opx core in the lower right corner to a mantle of cpx+qtz, then to a second generation of opx, outward to a mote of plagioclase, a simplectitic intergrowth of opx+plag+magnetite, then to an outer shell of garnet, and finally into the matrix plagioclase in the upper left cornerA proposed reaction history for this sample involves the prograde growth of a clinopyroxene+garnet+quartz assemblege at the expence of orthopyroxene +plagioclase, and retrograde growth of orthopyroxene+plagioclase+oxide from the peak assemblege. The map presented illustrates the complex distribution of Ca in all of the above phases. The distributuion of Ca in plagioclase is particularly surprizing in this sample, given its optically homogenous and course character. The large matrix plagioclase in the upper left corner is here revealed as an aggregate of albitic cores with anorthitic overgrowths. The plagioclase mote is also zoned from anorthitic in contact with the simplectite to albitic in contact with the second generation opx. Note also the zonation in garnet at the top of the image, suggesting a high grossular core that was truncated by the late stage simplectite. This sample demonstrates the benefit, or even necessity, of compositional mapping when modeling complex reaction histories and interpreting quantitative analyses.

Backscattered Electron Imaging

Backscattered electron imaging originates from beam electrons being elastically scattered in interactions with the strong core field near nuclei in the target material. Often referred to as phase contrast imaging, the signal essentially represents average atomic number as the backscatter efficiency increases with Z. This signal is useful in basic navigation in multi-phase materials. Here are a few examples…

Backscattered electron image of basaltic glass from Mauna Loa Hawaii. Quench crystals are olivine nucleating on plagioclase.

Olivine in NWA 1110. This is a Martian meteorite (shergottite).

Altered basaltic glass, Surtsey, Iceland. Vesiculated fresh glass lower left. Cementing zeolites upper right include analcime and tobermorite.

Secondary Electron Imaging

Secondary electrons are very low energy electrons emitted from the target during exposure to an energetic electron beam. Generally conduction band electrons emitted at a few eV, they are eminate from the outermost few angstroms of the surface, resulting in a detailed, high resolution representation of the surface morphology of a sample. Here are a few examples…

Analcime cementing basaltic glass, Iceland subglacial volcano.

Zeolites in hydrothermally altered basaltic glass, Iceland.

Cathodoluminescence Imaging

Cathodoluminescence originates from inelastic scattering, electron-hole pair production, and recombination that results in the emission of a photon.

Cathodoluminescence emission wavelengths can result from impurites, crystal dislocations and other defects, in many cases revealing internal mineral structures not seen by any other method. Generations of zircon growth are often revealed by CL, which is now a fundamental characterization tool in geochronology (see images above).