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Mantle Petrology and the Source of Diamonds

Shown here is a SEM-XRF element map of a mantle garnet-spinel peridotite from the diamond-bearing Newlands kimberlite (South Africa, Kaapvaal Craton). The intensity of the various elements indicates certain minerals that are present in the sample; for example Ca (green) – clinopyroxene; Cr (blue) – chromite; Al (yellow) – garnet, and K (orange) for metasomatism. The thin section was analysed using the MicroXRF on SEM. The sample is approximately 3 cm x 2 cm and was analysed as a single frame. A full spectrum for each pixel allows further offline processing such as adding elements, spectra extraction out of the map, quantification of the map or auto phase. 

The micro-XRF data has the advantage of identifying high energy X-ray lines as well as trace elements within the sample. The micro-XRF is a small spot analysis (around 35 micrometers), albeit larger than an electron beam. The interaction volume is much greater than that of an e-beam, and is element and sample matrix dependent. Consequently, the 2D elemental map may yield differences between the e-beam and the X-ray generated maps. The lower spectral background allows the detection of trace elements that is not possible with electron beam. Sample preparation requirements are different; for example there are no charging effects and accordingly no sample coating. In addition, because of the information depth there is no need for high-quality polish. Furthermore, a rough sample can be analysed as long as it has a planar surface. The quantification can be standardless or standard-based.

Large area X-ray map (30 mm x 20 mm) of a diamond-bearing eclogite from Newlands kimberlite acquired with XTrace (Analytical parameters: tube voltage Rh at 50 kV, anode current 600 µA, pixel spacing 25 µm). Mixed elemental intensity map (Ca – green, Fe – sky blue, K - orange, Ti - purple).
Large area X-ray map (30 mm x 20 mm) of an diamond-bearing eclogite from Newlands kimberlite acquired with XTrace (Analytical parameters: tube voltage Rh at 50 kV, anode current 600 µA, pixel spacing 25 µm). Top row from left to right: Mn (red), Ca (green), Cr (blue); middle row from left to right: Fe (sky blue), Si (purple), Al (yellow); bottom row from left to right: K (orange), Mn (light green), Ti (dark purple).
Large area X-ray map (30 mm x 20 mm) of an diamond-bearing eclogite from Newlands kimberlite acquired with XTrace (Analytical parameters: tube voltage Rh at 50 kV, anode current 600 µA, pixel spacing 25 µm). Top row from left to right: F1 Total X-ray intensity map (grey), Ni (green); bottom row from left to right: Cu (blue), S (yellow).