General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/about/ebr-terms
FTIR thermochronometry of natural diamonds: a closer lookSimon C. Kohn, Laura Speich, Christopher B. Smith and Galina P. Bulanova School of Earth Sciences, University of Bristol, Queens Rd., Bristol, BS8 1RJ, U.K.
AbstractFTIR is a commonly-used technique for investigating diamonds, but much of the most useful information is lost if spatially resolved measurements are not used. In this study we show examples of FTIR core-to-rim line scans, maps with high spatial resolution and maps with high spectral resolution that are fitted to extract the spatial variation of different nitrogen and hydrogen defects. Model residence temperatures are calculated from the concentration of A and B centres using known times of annealing in the mantle, and a new, two-stage aggregation model is presented that better constrains the thermal history of the diamond and that of the mantle lithosphere in which the diamond resided. The effect of heterogeneity within the analyzed FTIR volume is quantitatively assessed and errors in model temperatures that can be introduced by studying whole diamonds instead of thin plates are discussed. The spatial distribution of hydrogen associated with the 3107 cm -1 vibration does not follow the same pattern as nitrogen, and an enrichment of hydrogen at the boundary between pre-existing diamond and diamond overgrowths is observed. There are several possible explanations for this observation including a change in chemical composition of diamond forming fluid during growth or kinetically controlled uptake of hydrogen.
Platelets in diamond are extended planar defects that are thought to be generated during the nitrogen aggregation process in type Ia diamonds. They were subjected to intensive research during the 1980s and 1990s but the techniques used for observation of defects in diamond have improved since that time and new insights can be gained by further study. This study combines high resolution Fourier Transform Infrared (FTIR) analysis, with an emphasis on the main platelet peak, and transmission electron microscopic (TEM) imaging. By performing TEM and FTIR analyses on volumes of diamond that were closely spatially related it is shown that the average platelet diameter, D, follows the relationship D = a x−b where x is the position of the platelet peak in the infrared spectrum, a is a constant and b is the minimum position of the platelet peak. The best fit to the data is obtained if a value of b = 1360 cm-1 is used, giving a fitted value of a = 221. The observed variation in infrared (IR) peak width can also be explained in terms of this relationship. Additionally, platelet morphology was found to vary according to diameter with large platelets being more elongated. The tendency to become more elongated can be described by the empirical equation AR = 11.9 D+19.6 + 0.4 where AR is the aspect ratio. Using the relationships established here, it will be possible to study platelet abundance and size as a function of parameters such as nitrogen concentration, nitrogen aggregation and diamond residence time in the mantle. This work therefore will open up new methods for constraining the geological history of diamonds of different parageneses and from different localities.
The behaviour of platelets in natural diamonds and the development of a new mantle thermometer. Contributions to Mineralogy and Petrology, 173(5), [39].
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.