Quantitative analysis of trace element concentrations in some gem-quality diamonds

Sublithospheric diamond ages and the supercontinent cycle

Subduction related to the ancient supercontinent cycle is poorly constrained by mantle samples. Sublithospheric diamond crystallization records the release of melts from subducting oceanic lithosphere at 300-700 km depths1,2 and is especially suited to tracking the timing and effects of deep mantle processes on supercontinents. Here we show that four isotope systems (Rb-Sr, Sm-Nd, U-Pb and Re-Os) applied to Fe-sulfide and CaSiO3 inclusions within 13 sublithospheric diamonds from Juína (Brazil) and Kankan (Guinea) give broadly overlapping crystallization ages from around 450 to 650 million years ago. The intracratonic location of the diamond deposits on Gondwana and the ages, initial isotopic ratios, and trace element content of the inclusions indicate formation from a peri-Gondwanan subduction system. Preservation of these Neoproterozoic-Palaeozoic sublithospheric diamonds beneath Gondwana until its Cretaceous breakup, coupled with majorite geobarometry3,4, suggests that they accreted to and were retained in the lithospheric keel for more than 300 Myr during supercontinent migration. We propose that this process of lithosphere growth-with diamonds attached to the supercontinent keel by the diapiric uprise of depleted buoyant material and pieces of slab crust-could have enhanced supercontinent stability.

Diamond fingerprinting for source discrimination using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and Fourier transform infrared spectrometry (FTIR)

The Kimberley Process Certification Scheme (KPCS) was established in 2000 as a means of controlling the flow of conflict diamonds, mostly, from the African continent. In 2013, the KPCS imposed an embargo on diamonds from the Central African Republic (CAR). Since then the embargo has been lifted in certain prefectures of the country, however, smuggling is suspected from non-compliant areas.

Three parcels of diamonds suspected to have mining origins in the CAR, were analysed. These diamonds were investigated for their morphological and chemical characteristics, to establish a diamond fingerprint and to determine if these diamonds had the same fingerprint as previously analysed diamonds from CAR or the Democratic Republic of the Congo (DRC). The analyses of these diamonds were included in the already established diamond database of rough diamonds from the African continent.

The morphological characteristics identified included the mass (ct), colour, surface coatings, dominant, secondary and tertiary form, shape, breakage, inclusions, abrasion and surface features that are specific to octahedral, dodecahedral and cubic shapes. The morphological characteristics determined from the diamonds revealed that morphology alone cannot be used as a discriminatory method for diamond fingerprinting.

Fourier transform infrared spectroscopy (FTIR) identified the nitrogen concentration and aggregation state of that N. This allowed for the typing of the diamonds as Type I (containing N) and Type II (containing no measureable N). The concentration of N in the three parcels is less than 600 ppm. Further classification of Type I diamonds was performed according to the N aggregation state as single, double or four-fold. The vast majority of diamonds show a combination of nitrogen aggregation states while few were classified as Type II. Fourier transform IR showed no discernible trends between the current study and the established database.

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used as a means of determining the trace element concentrations of ⁶⁹Ga, ⁸⁸Sr, ⁸⁹Y, ⁹⁰Zr, ⁹³Nb, ¹³³Cs, ¹³⁷Ba, ¹³⁹La, ¹⁴⁰Ce, ¹⁴¹Pr, ¹⁴⁶Nd, ¹⁴⁷Sm, ¹⁵³Eu, ¹⁵⁷Gd, ¹⁵⁹Tb, ¹⁶³Dy, ¹⁶⁵Ho, ¹⁶⁶Er, ¹⁶⁹Tm, ¹⁷²Yb, ¹⁷⁵Lu, ¹⁷⁸Hf, ¹⁸¹Ta, ²³²Th and ²³⁸U. Laser ablation ICP-MS determined that not all elements produce statistically viable data, however, the data could still be used to discern trace element differences and trends among the parcels. In the current set of diamonds, laser ablation ICP-MS data for parcels A and B showed an excellent agreement with each other as well as those from diamonds previously analysed from CAR. None of the three parcels showed any similarity to data from Bria River or the DRC. It is concluded that the diamonds from parcels A and B are very likely to have mining origins in the same area in the CAR, whereas parcel C is distinct and of possible mixed origin.

An Evaluation of the Potential for Determination of the Geographic Origin of Ruby and Sapphire Using an Expanded Trace Element Suite Plus Sr–Pb Isotope Compositions

The geographic origin of gem corundum has emerged as one of its major value factors. Combined with gemological observations, trace element analysis is a powerful tool for the determination of corundum provenance. However, owing to similar properties and features of gem corundum from different localities, but similar geological settings, and very low levels of many trace elements in gem corundum, the determination of geographic origin remains challenging. In this study, we present trace elements compositions determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for rubies and blue sapphires from several different localities of geologically similar deposits: high-Fe amphibolite-type rubies, low-Fe marble-type rubies, and metamorphic blue sapphires. In addition, we determined Sr and Pb isotopic ratios by offline laser ablation sampling followed by thermal ionization mass spectroscopy (TIMS). By applying new and existing elemental discrimination schemes and the multivariate statistical method linear discriminant analysis (LDA), we show that, in addition to the commonly used discriminators Mg, Fe, V, Ti, and Ga, the elements Ni, Zr, Cr, and Zn show potential for geographic origin determination. Amphibolite-type rubies from different localities can be discriminated using Sr and Pb isotope ratios, whereas the discrimination of marble-type ruby and metamorphic blue sapphires is limited. Our results re-emphasize the challenge of geographic origin determination and the need for a more powerful discriminatory tool.

Primordial and recycled helium isotope signatures in the mantle transition zone

Isotope compositions of basalts provide information about the chemical reservoirs in Earth's interior and play a critical role in defining models of Earth's structure. However, the helium isotope signature of the mantle below depths of a few hundred kilometers has been difficult to measure directly. This information is a vital baseline for understanding helium isotopes in erupted basalts. We measured He-Sr-Pb isotope ratios in superdeep diamond fluid inclusions from the transition zone (depth of 410 to 660 kilometers) unaffected by degassing and shallow crustal contamination. We found extreme He-C-Pb-Sr isotope variability, with high ³He/⁴He ratios related to higher helium concentrations. This indicates that a less degassed, high-³He/⁴He deep mantle source infiltrates the transition zone, where it interacts with recycled material, creating the diverse compositions recorded in ocean island basalts.