Initial Publication Date: June 15, 2018

Element Mapping

John Goodge, University of Minnesota-Duluth

Element map of Mg zoning in garnet. Details

An element map is an image showing the spatial distribution of elements in a sample. Because it is acquired from a polished section, it is a 2D section through the unknown sample. Element maps are extremely useful for displaying element distributions in textural context, particularly for showing compositional zonation.

How it works—Element mapping

One can use either an EDS or WDS system to produce an element map. Either way, the image is produced by progressively rastering the electron beam point by point over an area of interest. Think of an element map as a pixel by pixel (bitmap) image based on chemical elements. Resolution is determined by beam size, and relative response of each element is determined by how long the beam dwells on each point (and of course the actual concentration). Greater distinction can be made by longer analysis, but at the cost of time.

In many cases, adequate element maps can be acquired by EDS systems. This is typically a faster approach, but sacrifices resolution and detection limits. The best element maps are acquired using a WDS system on an electron microprobe, but the trade-off in using the spectrometers is longer acquisition time.

Strengths

Element maps show the spatial distribution of elements in a sample.
Maps of different elements over the same area can help to determine what phases are present.
Element maps give a complete 2D picture of internal chemical zonation within a mineral, which is more informative than a simple line traverse.
Element maps are recorded digitally and can be saved to use as sample maps for locating spot analyses.

Limitations

Because the dwell times are typically shorter than in WDS analysis, elements in low concentration may not give a response.

Results

Element maps showing Ca and Na zonation in plagioclase. Details

Unlike back-scattered electron images (BSE), which only show average Z in a phase, element maps show the true spatial distribution of each element of interest. In these examples, the chemical zonation in plagioclase is clear for the elements Na (left) and Ca (right). Element maps are often displayed in false color, which helps the human eye distinguish subtle variations by expanding the range of visible hues. Here, higher concentrations are yellow and orange, whereas lower concentrations are magenta to blue.

Unlike back-scattered electron images (BSE), different element maps can be made of the same area for comparison. For example, the set of individual element maps shown below was acquired from a pelitic schist at the margin of a large garnet porphyroblast. In contrast to BSE images, where two minerals can have similar average Z and thus look similar, element maps can be compared to better evaluate the minerals present. In this set of element maps, garnet shows high Fe and Mg, kyanite shows high Al, muscovite shows high K, and plagioclase is indicated by matching Na and Ca. Conversely, you can see that garnet contains Si, Al, Fe, Mg, and Ca, but is richest in the Fe component of the solid solution.