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Sulphide INclusions in DIAmonds: A Window into The Earth’s Interior Through Time

Periodic Reporting for period 1 - SINDIA (Sulphide INclusions in DIAmonds: A Window into The Earth’s Interior Through Time)

Reporting period: 2019-04-01 to 2021-03-31

Diamonds containing mineral inclusions are unique tools to study the chemical history and evolution of the Earth’s interior. Among mineral inclusions in diamond, sulphides are the most abundant and contain critical evidence regarding the time of formation of diamonds, yet they are the least understood. Sulphides are typically included into diamond as high-temperature monosulfide solid solution (Mss) but re-equilibrate to an assemblage of exsolved phases at low-temperature. This exsolution causes fractionation of major and trace elements within the inclusions and unless the inclusions can be extracted and studied in their entirety, this process limits the ability to extract accurate isotopic information for geochronology.

In order to obtain the most accurate, high fidelity chemical and age information from sulphides it is fundamental that the inclusions used for age determinations are well characterized, while in the diamonds, with in-situ techniques. To this end, SINDIA was designed to attain three specific scientific objectives: i) comprehensive description of untreated multi-phase sulphide inclusions in diamonds; ii) reconstruction of the Mss original bulk composition; iii) definition of the true isotopic ratios to retrieve reliable diamond ages.

By combining for the first-time a thorough, non-destructive, in-situ characterization of sulphide inclusions in diamonds with homogenization experiments and isotopic analyses, we were able to accurately determine the composition of the sulphides, reconstruct their genetic processes and asses the validity of the mostly used dating system.
SINDIA followed a multi-step approach whereby micro-X-ray diffraction, tomographic techniques and fluorescence were combined to deeply characterize and map the spatial distribution of sulphide inclusions within the diamond host. A suite of 25 sulphide inclusions in diamonds from two Canadian Cratons were investigated in-situ to understand their mineralogy, original composition and crystallographic relationship with respect to their diamond host to assess whether the mineral inclusions formed at the same time as the diamond (syngenetic), or whether the diamonds enclosed pre-existing mineral grains (protogenetic). Single crystal X-ray diffraction analyses allowed the collection of crystallographic data on inclusions in diamonds down to 30-40 μm in size and most importantly allowed quantifying the whole inclusion without compromising it via polishing, providing a true “bulk” analysis. To this end, single-crystal data collections were converted into 2D powder patterns to perform Rietveld refinements of the diffraction pattern of the polycrystalline assemblage, offering an alternative approach to quantifying the relative amount of each sulphide phase present in the analysed volume and to derive the original composition of the Mss stable at high temperature in the mantle. These analyses have also provided, for the first-time, crystallographic evidence supporting a protogenetic origin of sulphide inclusions. The accurate bulk composition of the Mss determined here was then combined with the most recent sulphide experimental data and the formation condition (based on the available inclusion geo¬thermobarometry) of the investigated diamonds, to interpret the state of the sulphide inclusions at the time of incorporation into the diamond. Our results indicate that the sulphide inclusions were encapsulated in diamonds as solid grains and are therefore protogenetic supporting the findings from the crystallographic orientation relationships. Precisely characterized sulphide inclusions were also homogenised at high temperatures using a gas mixing furnace, and rapidly quenched to recover the original (Mss). In doing so, the major element composition of the sulphide inclusions was made uniform, by heating, while a gas was injected to avoid decomposition of the diamond. After homogenization, X-ray diffraction data confirmed a change from a polycrystalline assemblage of exsolved sulphide phases (pyrrothite, pentlandite and chalcopyrite) to single-crystal Mss. The new findings were finally coupled with a diffusion model in order to place constraints on the reliability of absolute ages derived from diamond inclusions. The results indicated that Re-Os equilibration is sufficiently fast in sulphides inclusions at mantle temperatures, confirming that even if the minerals are protogenetic, they can define true isotopic ratios and therefore provide true ages of diamond.
This project has provided the unique opportunity to perform unprecedented studies aimed at addressing fundamental questions in the Earth Sciences. One of the main scientific achievements of this project is that for the first time single-crystal X-ray diffraction was used to determine the bulk composition of sulphide inclusions in diamond, which are typically re-equilibrated to a low-temperature assemblage, and determined the protogenetic origin of the inclusion with respect to the host. As sulphides are the most common inclusions in diamond, and Re-Os age dating of sulphide inclusions are used to infer diamond formation ages, unravelling the relationship between the inclusion and host is of fundamental importance. To establish under what circumstances a geologically meaningful age is obtained we used diffusion data to calculate the time it would take the for the inclusions to come to equilibrium with the fluids from which their host diamonds crystallized. We reached the conclusion that this time is quite rapid, geologically speaking, and therefore that even pre-existing sulphides have the ability to form isochronous relationships and thus provide meaningful ages for geochronology. Our study has tackled a problem that has caused scientific controversy for the last 25 years, since the first inclusion age dating studies, and therefore provides a main advancement to the field. These findings also have implications for Re-Os dating of mantle samples in general (not limited to diamond inclusions) as the rapid Os equilibration times for fluid/sulphide interaction at mantle tempera¬tures provide also a way to constrain the timing and scale of fluid migration in mantle rocks at 100–200 km depths, conditions far beyond those for fluid processes during metamorphism in crustal rocks.

This study has thus opened a new line of research into the nature of sulphide inclusions in diamonds and the results obtained within this project will be an important reference point for future studies in different fields, spanning from mineralogy to mineral physics, petrology, geochemistry and geodynamics. The proposed study has provided international visibility to the researcher and the University of Padova by producing fundamental scientific advances in this new field of research and by publishing a fundamental scientific contribution in a highly ranked international journal. Furthermore, the establishment of collaborations with external universities and research institutions has made the host institution a leading Department of international reference in this new and exciting field of research.
Sulphide inclusion in diamond V2-17 investigated in this project