DIAMAP: NEW APPLICATIONS FOR PROCESSING IR SPECTRA OF FLUID-RICH DIAMONDS AND MAPPING DIAMONDS CONTAINING ISOLATED NITROGEN (TYPE IB) AND BORON (TYPE IIB)

Since infrared (IR) spectroscopy was first used to characterise diamonds in the 1930’s, it has become a commonly used, non-destructive tool in diamond research. Not only does it provide information on various substitutional defects (including boron and nitrogen), it can also be used to characterize fluid micro-inclusions in diamonds. As a result, diamonds are integral to our study of elemental cycling in the Earth’s deep interior (including carbon, nitrogen, boron and hydrogen), as well as providing key information on different C-O-H-bearing mantle metasomatic fluid types.

IR studies of diamonds are typically limited to single point analyses, while linear transects of multiple points have also been used to show core to rim variations. Technological developments over the past 15 years have made the application of IR-mapping feasible. DiaMap freeware was developed to take advantage of these new applications, allowing automated and easy processing of the abundant IR spectra obtained from the IR-mapping of a single diamond. Here we report new applications of the DiaMap software to the automated mapping of single subtitutional nitrogen (DiaMap_Ib) and boron (DiaMap_IIb). Results show that boron is partitioned in the same way as nitrogen, into the octahedral growth sectors of diamond.

In addition, a semi-automated methodology has been developed, allowing easy and quantitative determination of the mineralogy and type of fluids in micro-inclusion-bearing diamonds. DiaMap_Fluid has been applied to 38 fluid inclusion-rich diamonds from worldwide localities that have already been quantitatively analysed. This allows the FTIR data on the water (OH), carbonate and silicate components of the microinclusions to be compared with their bulk chemistry. The results provide the phase concentrations (in ppm by weight) in the diamonds, and show that on a ternary plot of carbonate – water – silicates + apatite, the four mantle fluid end-members fall in fairly distinct groupings. This makes FTIR analyses using the DiaMap_Fluid software a very simple method for determining phase concentations and broadly discerning the chemistry of metasomatic fluids in diamonds.