Mineral inclusions trapped within natural diamonds studied by high-energy diffraction

Diamonds are the deepest and oldest terrestrial samples reaching the Earth’s surface after a long journey through the Earth’s mantle. They form between about 120 and 1000 km depth and can incorporate minerals at great depths before bringing them to the surface, allowing us to study directly ”deep fragments of our planet” inaccessible by any other means. Thanks to the study of such fragments we can understand in which type of rocks diamonds were formed and at which depth and temperature within the Earth. However, only 1% of diamonds contain mineral inclusions suitable for scientific investigations and thus a non-destructive approach is very often crucial.

Using single-crystal X-ray diffraction at the Extreme Conditions beamline (I15) at Diamond Light Source, researchers were able to study the crystal structure of clinopyroxenes, Ca-Mg-Fe-Al-rich silicates, one of the most abundant minerals found as inclusions in diamonds. The inclusions were still trapped in their diamond hosts and were located several hundred micrometres inside. Thanks to the high-energy (60 keV) X-rays used in this study, it was possible to determine in which kind of rock the diamonds grew.

Lithospheric diamonds, which represent about 94% of the entire diamond population, generally belong to three main groups: peridotitic (~65%), eclogitic (~33%) and websteritic (~2%)1. Among the inclusions found in lithospheric diamonds, clinopyroxenes principally belong to the peridotitic and eclogite rock types. These inclusions are mainly of diopsidic composition (~CaMgSi2O6) or omphacitic composition (~Ca0.5Na0.5Mg0.5Al0.5Si2O6). The diopside-rich clinopyroxenes are peridotitic in origin whereas the omphaciterich compositions are eclogitic. Ca-rich clinopyroxenes in diamonds mainly crystallised in space group C2/c (only rarely they crystallise in P2/n and this occurs for low-temperature crystallisation, which does not apply to diamonds studied in this work) and are characterised by four distinct crystallographic sites: the M2-site is mainly occupied by Ca and Na, the M2`-site is a split position with respect to the M2 and is generally occupied by Fe and Mg, the M1-site is generally occupied by Mg, Fe, Al, Cr and the T-site is almost entirely occupied by Si. Based on a very wide chemical-composition database obtained by the analysis of clinopyroxenes extracted from diamonds1, clinopyroxenes can be tentatively classified mainly into three groups (data in atoms per formula unit a.p.f.u.). The first are the peridotitic clinopyroxenes with Ca > 80% and Na < 20% and the second are eclogitic clinopyroxenes with 50% < Ca < 60% and 40% < Na < 50%. A third group of clinopyroxenes exists, the websteric, but they are extremely rare (<2%).

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Figure 1: An optical microscope image of the diamonds investigated in our study (modified from 4). The different types of inclusions are indicated by the arrows.

If chemical information on the inclusions, still trapped in their diamond hosts, can be obtained it will allow the rock type in which diamonds formed to be determined without destroying the rare diamond samples. The best way to obtain quantitative information about the cation distribution over the crystallographic sites is to use single-crystal X-ray diffraction. However, the complexity of this technique is very high due to the small size of the inclusions (about 100 mm) entrapped in millimeter-sized natural diamonds. In this study three inclusions within three different diamonds have been investigated (Fig.1).

The study has been carried out for the first time using high-energy synchrotron X-ray radiation. Using the high-energy X-rays of the beamline I15, made the X-ray absorption from the relatively large diamond hosts negligible and provided extremely high-quality structural information about the inclusions, which can thus be used to obtain the inclusion chemistry by determining the site occupancy factors. Experiments were performed using a focused beam of λ = 0.20675 Å (E = 59.966 keV), collimated down by a 20 mm pinhole. Data were collected with an Atlas CCD detector (Rigaku) scanning φ and ω with a width of 1° and treated using the CrysAlisPRO software package (Rigaku). The crystal structures were refined using SHELXL2, starting from the atomic coordinates provided by Nestola et al.3 with neutral scattering curves. The M2’ position was refined for all three clinopyroxenes obtaining a significant improvement in the agreement factor R1. The occupancies of the M2 and M1 crystallographic sites were allowed to vary in order to obtain indications about the chemical compositions.

The high quality of the intensity data allowed not only an anisotropic refinement of the crystal structures of the three inclusions, but also to refine the occupancy factors at each crystallographic site. The quality of these data is comparable to that of a clinopyroxene crystals collected in air; the number of unique reflections is 15 - 18 times higher than the number of refined parameters and almost 98% of the unique reflections have observed structurefactors (Fo). The number of unique reflections and their large intensity/sigmaratios resulted in refinement agreement factors, R1’s, between 2.8% and 3.9%. Considering that such inclusions are embedded in millimetre thick diamond hosts, these results demonstrate the great potential of this experimental approach for studying inclusions.

The mean atomic number (m.a.n.) obtained for each crystallographic site within a crystal provides robust information about the chemical composition of the inclusions and does not depend on any possible residual pressure exerted by the diamond host on the inclusions. The high quality of the crystal structure refinements allowed us to determine reliable m.a.n. values for the M2 and M1 crystallographic sites (the site occupancy at the T site is not considered as it is completely occupied by Si)4.

It is almost impossible to determine the cation distribution at the M1-site for those clinopyroxenes from the m.a.n values alone as this site could be occupied by six different elements and some of them could even be in a different oxidation state (e.g. Fe2+ or Fe3+). However, combining the m.a.n. and the average M1-O bond distances in addition to the information about the cation distribution of the M2-site, it was confirmed that two of the inclusions are diopsidic whereas the third one is eclogitic in composition (Fig. 1).

In summary, this research showed, for the first time, that single-crystal X-ray diffraction at high energy (60 keV) is likely the best approach to investigate crystalline compounds of any crystal size located deeply within natural large diamonds. High-quality crystal structure data collected on such mineral inclusions in diamonds allowed us to determine the primary source rocks in which diamonds crystallised billions of years ago.

References:

  1. Stachel, T. et al. The origin of cratonic diamonds–constraints from mineral inclusions. Ore Geol. Rev. 34, 5–32, doi: 10.1016/j.oregeorev.2007.05.002 (2008).
  2. Sheldrick, G.M. Crystal structure refinement with SHELXL. Acta Cryst C, 71, 3-8, doi: 10.1107/S2053229614024218 (2015).
  3. Nestola, F. et al. High-pressure behavior of Ca/Na clinopyroxenes: The effect of divalent and trivalent 3d-transition elements. Am Mineral 95 (5-6), 832–838, doi: 10.2138/am.2010.3396 (2010)
  4. Nestola, F. et al. Source assemblage types for cratonic diamonds from X-ray synchrotron diffraction. Lithos 265, 334-338, doi: 10.1016/j.lithos.2016.07.037 (2016).

Funding acknowledgement:

The work has been supported by the ERC Starting Grant to FN (grant agreement n° 307322). The authors wish to acknowledge beamline I15 at Diamond Synchrotron Light Source where the data were collected during the experiment EE7616. MA has been supported by the S.I.R.-M.I.U.R. grant MILE DEEp (grant number RBSI140351).

Corresponding author:

Professor Fabrizio Nestola, University of Padova, fabrizio.nestola@unipd.it

Related publication:

Nestola F, Alvaro M, Casati MN, Wilhelm H, Kleppe AK, Jephcoat AP, Domeneghetti MC, Harris JW. Source assemblage types for cratonic diamonds from X-ray synchrotron diffraction. Lithos 265, 334-338, doi:10.1016/j.lithos.2016.07.037 (2016).

Publication keywords:

Clinopyroxene; Synchrotron; Peridotite; Eclogite