NanoSIMS imaging of D/H ratios on FIB sections

International audienceThe D/H ratio imaging of weakly hydrated minerals prepared as Focused Ion Beam (FIB) sections is developed in order to combine isotopic imaging by Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS) of micrometer-sized grains with other nanoscale imaging techniques, such as Transmission Electron Microscopy. In order to maximize the accuracy, sensitivity, precision and reproducibility of D/H ratios at the micrometer-size, while minimizing the surface contamination at the same time, we explored all instrumental parameters known to influence the measurement of D/H ratios in situ. Optimal conditions were found to be obtained with the use of (i) a Cs$^+$ ion source and detection of H$^-$ and D$^-$at low mass resolving power, (ii) a primary beam intensity of 100 pA, and (iii) raster sizes in the range 8-15 $\mu$m. Nominally anhydrous minerals were used to evaluate the detection limits and indicate a surface contamination level of about 200 ppm equivalent H$_2$O in these conditions. With the high primary intensity used here, the dwell time is not a parameter as critical as found in previous studies and a dwell time of 1 ms/px is used to minimize dynamic contamination during analysis. Analysis of FIB sections was found to reduce significantly static contamination due to sample preparation and improved accuracy compared to using polished sections embedded not only in epoxy but in indium as well. On amphiboles, the typical overall uncertainty including reproducibility is about 20 ‰ on bulk FIB sections and about 50 ‰ at the 1.5 $\mu$m scale using image processing (1$\sigma$)