9.5 REE Diagrams

The lanthanide elements (atomic numbers 57-71), also called Rare Earth Elements, are trace elements of particular interest to petrologists because their distribution coefficients can be different by orders of magnitude in different minerals.
Chondrite REE Diagram

Figure 9.03. C1 Chondrite REE Diagram. Concentrations by weight in parts per million (ppm) for the 14 Rare Earth Elements are shown for a C1 Chondrite (McDonough and Sun, 1995). Notice that the values alternate in magnitude, with the even numbered elements more abundant than the odd numbered elements.

Figure 9.03 shows on a logarithmic scale the abundances of the REE in a C1 chondritic meteorite (McDonough and Sun, 1995), which is a good proxy for the composition of the earth. Notice that the values alternate in magnitude, with the even atomic number elements more abundant than the odd atomic number elements. To remove this systematic pattern and help reveal other patterns, REE abundances are nomalized by dividing them by chondrite REE abundances or REE abundances for another rock type.

REE Diagram of Rocky Type averages

Figure 9.04. REE Diagram for Several Rock Types . Chondrite normalized REE values for average rock compositions are shown here with lines connnecting the points to display a REE pattern. The rock types are average granite (red, Li, 2000), continental crust (orange, Rudnick and Gao, 2003), average andesite (blue, Li, 2000), average basalt (black, Li, 2000), and MORB (green, Gale et al., 2013).

In Figure 9.04, REE values normalized by chondrite REE values are shown for several average rock composition types: granite, continental crust, anorthosite, basalt, and MORB. You can see that several rock types have differences in their values for the light REE (LREE) relative to the heavy REE (HREE). For example, the average granite (red line) is clearly enriched in the LREE relative to the HREE, whereas the MORB (green line) is slightly depleated in the LREE relative to the HREE. Possible causes for these differences can be identified using REE distribution coefficients and models for fractional crystallization or partial melting. A REE fractionation tool on the next page will help you explore these possibilities. If you wish to plot REE data for your own samples use this tool.