Spatial distribution of nitrogen and hydrogen in natural lithospheric diamonds by in situ synchrotron infrared microspectroscopy

Natural diamonds are exceptional carriers of mineralogical and chemical information from inaccessible depths of our planet. During their crystallization, they can host light elements such as H and N preserving a natural archive of mantle chemistry that, in turn, allows a better understanding of the chemical composition of the growth media, mechanisms of their formation and residence temperatures in the interior of Earth. However, how N and H distribute near entrapped minerals is still unknown. In this study we investigated the effect of chromite mineral inclusions on the spatial distribution of nitrogen and hydrogen in two natural diamonds of peridotitic origin using in situ synchrotron-based Fourier transform infrared microspectroscopy. From the acquisition and optimization of high-resolution maps, we determined the distribution of nitrogen, hydrogen, and nitrogen aggregation state. Our results reveal a dependence between the absorption of H-related peaks with the incorporation of pairs of nitrogen atoms (NA-centers). We explain it as indication that chemical interactions between chromite and H might be masked by variations in the N aggregation state. We also conclude that synchrotron micro-FTIR is an advanced technique to assist the synthesis of N- (and H-) doped diamonds for industrial applications.