FTIR thermochronometry of natural diamonds: A closer look

Abstract

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.

Deep Carbon

“…34,35 Only recently has the full potential of FTIR spectroscopy for determining the thermal history of a diamond been recognized. The major recent developments have been: (1) improvements in the methods for acquiring and processing FTIR spectroscopy maps 34 ; (2) a better understanding of the temperature history that is available from zoned diamonds 36 ; and (3) unlocking the abundant information that is provided by the FTIR spectroscopy signal of plateletsplanar defects created with B centers during nitrogen aggregation. 37,38 Figure 5.1a shows a map of "model temperatures" made up by automated fitting of several thousand FTIR spectra in a map of a diamond from Murowa, Zimbabwe.…”

“…(b) Modeling the possible combinations of temperature and time that could explain the FTIR spectroscopy characteristics of a zoned diamond from Murowa. 36 spectroscopy measurements on platelets, we now have a much better understanding of the meaning of the FTIR spectroscopy platelet peak characteristics (position, area, width, and symmetry) and how platelets evolve with thermal history. 37,38 In addition to diamonds with regular platelet behavior and irregular, platelet-degraded behavior, we have identified a new class of sub-regular diamonds with anomalously small platelets that have experienced unusually low mantle temperatures (below about 1120 C).…”

“…Additional data are required to confirm this view. The relative abundance of the minor isotope of sulfur, 36 S, is also affected by both mass-dependent reactions (related to microbial cycling; e.g.…”

The Unique Nature of Argyle Fancy Diamonds: Internal Structure, Paragenesis, and Reasons for Color

“…Hence, seemingly lower temperatures in the rim of some diamonds could imply that the rim is significantly younger than the core. For further explanation, see text and Kohn et al (2016).…”

“…Thus, in these cases, the rim is most likely significantly younger than the core. This issue is discussed by Kohn et al (2016), who refer to such temperatures as model temperatures. Using two stages of aggregation, the time gap between core and rim growth can be estimated.…”