Wavelength Dispersive Spectrometry – Minor and trace element quantification
Wavelength Dispersive Spectrometry (WDS) on Electron Microprobe Analyzers (EPMA) has been the go-to method for in-situ chemical analysis of geological materials for several decades. While we have shown that Energy Dispersive Spectroscopy (EDS) can match the precision and accuracy of WDS analysis for elements with a concentration of >0.5 wt%, WDS remains an important technique for analysis of minor and trace elements (<0.5 wt%). The JEOL JSM IT800HL Scanning Electron Microscope enables unparalleled control of the beam current used for analysis, ranging from a few pA up to 400 nA. For users interested in trace or minor element concentrations in their materials we first characterize the sample major element chemistry using our Ultim Max 100mm2 EDS detector at ~2nA beam current, before increasing the beam current to >100nA and utilizing the Oxford Instruments Wave WDS detector to measure the concentration of trace components.
So far, our use of the Wave WDS system has focused on 2 approaches, determination of Al concentrations in olivine (0.02 – 0.06 wt%) to estimate magmatic crystallization temperatures. Rigorous analysis of well characterized olivine standards demonstrate that we can determine Al contents with a high degree of accuracy and a relative imprecision of <5% at concentrations of ~0.045 wt%. The image below shows the results of WDS analysis of Al2O3 in 2 olivine standards, including the well-known San Carlos olivine (blue).
We have also used WDS analysis to evaluate the MgO content of plagioclase crystals in cumulate rocks from Hawai’i and the Galapagos. Rim to core transects reveal a huge amount of information in these samples, including the cooling history of intrusive materials and their storage temperature prior to transport to the surface in some carrier melt. The profile below represents an EDS-WDS transect taken across a plagioclase crystal starting at a clinopyroxene-plagioclase grain boundary. Error bars are plotted for the WDS data (red), but these are so small they are almost impossible to spot!
