Elsevier

Lithos

Volume 265, 15 November 2016, Pages 148-158
Lithos

FTIR thermochronometry of natural diamonds: A closer look

https://doi.org/10.1016/j.lithos.2016.09.021Get rights and content

Highlights

  • High quality infrared data for diamonds are presented, including line-scans and maps.

  • A new method to constrain the thermal history of zoned diamonds in the mantle is presented.

  • The problems with data in the literature that use whole diamonds to obtain temperatures in the mantle are explained.

  • The spatial distribution of the 3107 cm 1 hydrogen defect is shown to shed light on the nature of diamond formation.

  • Future perspectives for infrared studies of diamonds are presented.

Abstract

Fourier Transform Infrared (FTIR) spectroscopy is a commonly-used technique for investigating diamonds, that gives the most useful information if spatially-resolved measurements are used. In this paper we discuss the best way to acquire and present FTIR data from diamonds, using examples from Murowa (Zimbabwe), Argyle (Australia) and Machado River (Brazil). 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 are presented. Model mantle residence temperatures are calculated from the concentration of A and B nitrogen-containing defects in the diamonds using known times of annealing in the mantle. A new, two-stage thermal annealing 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 analysed 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 VN3H hydrogen defects associated with the 3107 cm 1 vibration does not follow the same pattern as nitrogen defects, and an enrichment of VN3H 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.

Introduction

Diamonds and their inclusions are some of the most valuable samples for elucidating the large scale geological history of our planet because they are the deepest sourced samples available for study, and in many cases, amongst the oldest. They form a key part of Earth's deep carbon cycle (Shirey et al., 2013). The very slow chemical diffusion rates in diamond combined with high resistance to changes in external physical and chemical environments result in diamonds behaving like time capsules over much of Earth history. Individual diamond crystals can therefore preserve a wealth of information on the nature of their growth conditions and subsequent storage in the mantle. While there have been significant developments in the study and interpretation of mineral and fluid inclusions in diamonds in recent years (e.g. Stachel and Harris, 2008, Thomson et al., 2016, Weiss et al., 2015), there is still much to be learned from closer examination of the diamonds themselves.

Diamonds usually show zoned internal growth structures that can be complex with octahedral, cuboid or sectorial growth, multiple growth centres, dissolution and overgrowth features (e.g. Bescrovanov, 1992, Bulanova et al., 2005). These features, which contain valuable information on the history of diamond growth, can be imaged with techniques such as photoluminescence, cathodoluminescence and optical birefringence. Spatial discontinuities in isotopic compositions and infrared absorption features can also commonly be observed. (e.g. Bulanova et al., 2010, Craven et al., 2009, Smart et al., 2011, Thomson et al., 2014). These zoning patterns provide an opportunity to extract a unique history of the nature of diamond forming fluids and can play an important part in constraining mantle processes such as metasomatism, input of subducted materials and changes in mantle conditions over geological time.

One measurement that is simple to perform and is therefore an essential part of nearly all systematic studies of diamonds is Fourier Transform Infrared (FTIR) spectroscopy. FTIR spectra are used mainly to determine the nitrogen concentration and the distribution of N between different defect centres, although several other parameters can also be measured. In many cases measurements are taken through whole diamonds, the assumption being that some sort of average or representative numbers are obtained. Of course, in the case of valuable gemstones this is the only option, as destruction of the diamond by slicing or breaking into smaller pieces is too costly. However, as we will show in this paper, reducing the available information to a single set of parameters not only misses the opportunities offered by more detailed sampling, but may in fact produce misleading information. Some previous work has used spatially-resolved FTIR on zoned diamonds and it has sometimes been shown that multiple episodes of growth are recorded with a single zoned diamond (e.g. de Vries et al., 2013a, Griffin et al., 1995, Palot et al., 2013, Pearson et al., 1999, Taylor et al., 1995). Furthermore some studies report FTIR maps where some specific feature, such as the intensity at a particular wavenumber or the area of a peak are displayed for a grid of points (e.g. Bulanova et al., 2002, Howell et al., 2012a, Lu et al., 2012, Spetsius et al., 2015). However, none of these studies have taken full advantage of spatially resolved FTIR and, crucially, none has gone as far as to show maps of calculated temperatures.

The data presented in this study are from a range of spectra collected in our laboratory at the University of Bristol over the last five years and are selected to illustrate the opportunities and pitfalls of FTIR of diamond. The aims of this paper are: (i) to illustrate the heterogeneity that is common in natural diamonds; (ii) to provide guidance on the methods to extract the maximum information from the spatial distribution of N in diamonds; (iii) to describe a method for modeling the thermal histories of heterogeneous diamonds; (iv) to discuss the errors introduced by conventional FTIR methods on diamonds; (v) to show how the spatial distribution of the major hydrogen defect in natural diamonds provides important evidence for diamond growth conditions and (vi) to stimulate further developments in the use of spatially resolved FTIR spectroscopy of diamond.

Section snippets

Previous work on nitrogen defects and aggregation rates

Various absorption peaks in infrared spectra of diamonds were recognized as N-related as far back as the 1950s (e.g. Kaiser and Bond, 1959). A huge amount of work subsequently went into establishing the absorption envelope for each of the possible infrared-active N-related defects, the quantitative relationships between the area of the peaks in the infrared spectra and the concentrations of the different N defects (e.g. Boyd et al., 1994, Boyd et al., 1995) and determining the rate of

Sample preparation

Infrared spectroscopy, as implemented here, is a transmission technique that involves focusing an infrared beam using a parabolic mirror. If the sample is very thin, the spatial resolution can be on the order of a few microns (the diffraction limit). However, in practice there is an optimum thickness for a slice of diamond. If the slice is too thin the signal-to-noise ratio becomes problematic for fast acquisition of maps with many pixels, and interference fringes resulting from multiple

Line scans of diamonds from the Murowa kimberlite, Zimbabwe

Mur 70 is a diamond from Murowa, Zimbabwe that has an octahedral external morphology and contains a peridotitic garnet inclusion (Bulanova et al., in review-a). Fig. 1 shows the results of a line scan across a central plate prepared from diamond Mur 70 plotted in the conventional way with log N vs %1aB. This style of presentation is the standard one used in the geological literature, and demonstrates that the annealing temperature implied by most points is about 1140 °C, but it is of limited

Thermal modeling

Most of the diamonds we have studied are heterogeneous in terms of nitrogen concentration (Ntot); the examples described in this paper are not unusual. In some cases there is a gradual variation of Ntot, in others there are sharp discontinuities and, frequently, there is oscillatory zoning. Many diamonds show all of these types of zoning. Often, the variation in A and B defect concentrations that accompanies variation in Ntot is such that each pixel gives the same model temperature. In other

Future work

The approach taken here shows that there is a wealth of information that can be extracted from spatially resolved FTIR studies of diamonds that most previous studies have not used. The detailed variation in nitrogen and VN3H defect concentrations and the core-to-rim variation in model temperature provide valuable constraints on the history of diamond growth and storage in the mantle. However the limitation in terms of a unique thermal history for individual diamonds is that there is an

Acknowledgments

We thank Andy McKay and Ed Wibberley for their contributions to diamond preparation and data collection, the University of Bristol for funding the purchase of the iN10MX, Thermo Scientific for help and support, Rio Tinto for providing diamonds, De Beers and NERC for providing a studentship to LS. We are grateful to Mike Walter, Antony Burnham, Andrew Thomson and the rest of the Bristol diamond group for stimulating discussions and NERC for provision of grant NE/J008583/1.

We thank Ingrid Chinn,

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