Ultrathin section preparation of phyllosilicates by Focused Ion Beam milling for quantitative analysis by TEM-EDX

The composition of the phyllosilicates mica and chlorite is commonly used in thermobarometry to decipher the geological history of rocks. The thermobarometers that have been proposed during the last thirty years rely on the use of electron microprobe (EMP) analyses, with a spatial resolution limited to the micrometer scale. Such a resolution may not be appropriate for the size of clay particles or chemical zoning in very low grade rocks. In this respect, the transmission electron microscope coupled to an X-ray energy dispersive spectrometer (TEM-EDX) provides the proper spatial resolution in analysis. It is combined here with Focused Ion Beam (FIB) milling which makes possible in situ preparation of TEM samples with high spatial control and preservation of mineral microtextures. We test the suitability of FIB-milling for TEM-analysis of clay minerals, considering the effects of intrinsic preparation artifacts like amorphization, beam damage, Ga ions implantation and material redeposition. In addition, we evaluate a TEM-EDX analytical protocol adapted to clay minerals and yielding the FIB-section thickness at each analysis point in a non destructive way. The TEM k factors were calibrated on clay powders. A comparison between TEM-EDX and EMP analyses obtained on the same phyllosilicates in FIB and polished thin sections, respectively, shows excellent agreement, thereby validating the analytical protocol and the use of FIB-sections for the study of clay minerals. The combination of these preparation and analytical procedures opens new avenues for the petrologic study of clay-mineral-bearing assemblages at an unprecedented spatial resolution.