This project is aimed at understanding the relationship between kimberlites and diamonds, flood basalts and magmatic sulfides, and chromitites and related PGE mineralisation, by linking mantle structure, phase and chemical composition, isotope evolution and temperature to the melting processes.

Genetic constraints for primitive magmas (kimberlites, shoshonites) in a number of continental magmatic provinces (South Africa, Siberia and NW Canada) were presented in four publications in high-impact journals (Lithos and Contributions to Mineralogy and Petrology) during 2019.

A renowned example of a sill kimberlite complex (Benfontein, Kimberley cluster, South Africa) provided us with an excellent opportunity to examine the emplacement and evolution of intrusive kimberlite magmas. We have undertaken a detailed petrographic and melt inclusion study of the Benfontein sills and reported new perovskite and baddeleyite U/Pb ages (85.7 ± 4.4 Ma and 86.5 ± 2.6 Ma, respectively) in Lithos. The Lower Sill is characterised by carbonate-rich diapirs, which intrude into oxide-rich layers from underlying carbonate-rich levels. The general paucity of xenogenic mantle material in the Benfontein sills is attributed to its separation from the host magma during flow differentiation during lateral spreading. The low viscosity is likely responsible for non-explosive emplacement of the Benfontein sills, while the rhythmic layering is attributed to multiple magma injections. Analyses of secondary inclusions in olivine and primary inclusions in monticellite, spinel, perovskite, apatite and interstitial calcite are largely composed of Ca-Mg carbonates and, to a lesser extent, alkali‑carbonates and other phases. These inclusions probably represent the entrapment of variably differentiated parental kimberlite melts, which became progressively more enriched in carbonate, alkalis, halogens and sulfur during crystal fractionation.

Our published study (in Contributions to Mineralogy and Petrology) of the unique sulfide mineral djerfisherite (K₆(Fe,Ni,Cu)₂₅S₂₆Cl) revealed two plausible mechanisms for its formation. Djerfisherite, occurring in the groundmass of many kimberlites, kimberlite-hosted mantle xenoliths, and as a daughter inclusion phase in diamonds and kimberlitic minerals may form through replacement of pre-existing Fe–Ni–Cu sulfides by djerfisherite, which is attributed to precursor sulfides reacting with metasomatic K–Cl bearing melts/fluids in the mantle or the transporting kimberlite melt. The second scenario envisages direct crystallisation of djerfisherite from the kimberlite melt in groundmass or due to kimberlite melt infiltration into xenoliths. The occurrence of djerfisherite in kimberlites and its mantle cargo from several localities worldwide provides strong evidence that a common kimberlite melt was enriched in K and Cl. This led to suggestions that kimberlites originated from melts that were more enriched in alkalis and halogens relative to their whole-rock compositions.

Adam Abersteiner has submitted his PhD thesis for examination; it is based on nine published papers.

This project is aimed at understanding the relationship between kimberlites and diamonds, flood basalts and magmatic sulfides, and chromitites and related PGE mineralisation, by linking mantle structure, phase and chemical composition, isotope evolution and temperature to the melting processes.

Outcomes in 2018 included genetic constraints for a number of continental magmatic provinces (South Africa, Arctic Siberia, NW Canada and eastern Finland) and included several publications in high-impact journals (Chemical Geology and Journal of Petrology).

A particular emphasis was on petrologically unique Udachnaya-East kimberlite (Siberia, Russia), which contains fresh olivine, along with abundant alkali-rich carbonates, chlorides, sulfides and sulfates in the groundmass. The mineralogical and geochemical features and the compositions of melt inclusions in unserpentinised and altered kimberlite types are compared in our study. Melt inclusions hosted in olivine, monticellite, spinel and perovskite from all kimberlite varieties contain identical daughter phase assemblages that are dominated by alkali-carbonates, chlorides and sulfates/sulfides. This enrichment in alkalis, chlorine and sulfur in melt inclusions demonstrates that these elements were an intrinsic part of the parental magma. We demonstrate that ‘contamination models’ are inconsistent with petrographic, geochemical and melt inclusion data. Instead we propose that the Udachnaya-East kimberlite crystallised from an essentially H₂O-poor, Si-Na-K-Cl-S-bearing carbonate-rich melt.

The study of monticellite-rich kimberlites from Leslie pipe (Canada) and Pipe 1 (Finland) supports that CO₂ degassing in the latter stages of kimberlite emplacement into the crust is largely driven by the reaction between olivine and the carbonate melt. The proposed decarbonation reactions may be a commonly overlooked process in the crystallisation of monticellite and exsolution of CO₂, which may in turn contribute to the explosive eruption and brecciation processes that occur during kimberlite magma emplacement and pipe formation.

The origin of high-Mg melts is addressed in the study of olivine-hosted melt inclusions from picritic volcanic rocks of the Siberian Large Igneous Province. The high abundances of volatile elements, including H₂O (up to 3 wt%), fluorine, chlorine and sulfur, taken together with directly measured major and lithophile trace elements were used to estimate the compositions of primary melts and conditions of their generation in the mantle. It is advocated that high-Mg melts form by volatile fluxing of the asthenospheric mantle rather than by decompression melting under relatively dry conditions of a rising abnormally high-temperature mantle plume.

This project is aimed at understanding the relationship between kimberlites and diamonds, flood basalts and magmatic sulfides, and chromitites and related PGE mineralisation, by linking mantle structure, phase and chemical composition, isotope evolution and temperature to the melting processes.

Outcomes in 2017 included genetic constraints for a number of continental magmatic provinces (Tasmania, South Africa, Arctic Siberia, SW China, NW Canada and eastern Finland).

These outcomes, published in several high-profile publications (Ivanov et al. and Kamenetsky et al. in Chemical Geology), focused on the understanding of the origin and evolution of Large Igneous Provinces in time and space, and in particular their relationships to the so-called mantle plumes and widespread subduction.

A particular emphasis was on the Karoo-Ferrar igneous province, which is one of the largest igneous provinces on Earth. Our new isotope dilution thermal ionization mass spectrometry (ID-TIMS) single grain U-Pb ages for zircon and baddeleyite from Tasmanian dolerites combined with ID-TIMS literature single grain U-Pb ages from the Ferrar and Karoo suites are consistent with the major pulse of synchronous magmatism throughout the province lasting about 1 Ma or less for the major pulse of magmatism at the time of the Toarcian mass extinction event.

We argue that the mechanism of synchronisation of magmatism over such a short period of time along such a long distance is the major question to be answered in search of the correct model for the origin of the Karoo-Ferrar large igneous province. It cannot be reconciled with the lower mantle plume head model with the plume impingement beneath the Karoo. Plume material could not spread beneath the lithosphere at a rate of ~ 5–10 m/yr (5000 km per 0.5–1 Myr), at least based on the current knowledge of the mantle physical properties. Our preferred model for the origin of the Karoo-Ferrar large igneous province is associated with subduction of the Phoenix plate beneath the southern Gondwana.

Large datasets on kimberlites, melt inclusions in kimberlite minerals and kimberlite-hosted lithospheric peridotite and eclogite xenoliths were presented in four papers published in Chemical Geology (Abersteiner et al., Giuliani et al. and Kiseeva et al.) and Lithos (Kargin et al.).

This project is aimed at understanding the relationship between kimberlites and diamonds, flood basalts and magmatic sulfides, and chromitites and related PGE mineralisation, by linking mantle structure, composition, and temperature to the melting processes that generate these mantle-derived magmas.

Outcomes in 2016 included genetic constraints for a number of magmatic provinces on continents (Tasmania, Karoo, Arctic Siberia, Emeishan) and the ocean floor (South Atlantic and Gorgona Island).

The reconstruction of parental melts, their temperatures and the inventory of volatile elements was aided by studies of melt inclusions. Contributions from peridotitic and garnet pyroxenite mantle sources in the subcontinental lithosphere to the compositions of primary magmas were quantified using chemistry of olivine phenocrysts. It was shown that recent and modern magmas in the South Atlantic contain substantial contributions from the subcontinental lithospheric mantle that have been entrapped in the asthenosphere since the break-up of the Gondwana supercontinent. The outcomes, published in several high-profile publications, focused on the understanding of the origin and evolution of Large Igneous Provinces in time and space, and in particular their relationships to the so-called mantle plumes. Authors and journals include Ivanov et al. and Kamenetsky et al. in Chemical Geology, Husen et al. and Shimizu et al. in Geochimica et Cosmochimica Acta, and Gurenko et al. in Earth and Planetary Science Letters.

Large datasets on kimberlites and kimberlite-hosted mantle lithologies (peridotite and eclogite xenoliths) were presented in several papers published in Chemical Geology Abersteiner et al.; Giuliani et al. and Kiseeva et al.), Lithos (Giuliani et al. and Soltys et al.) and in a review in Earth and Planetary Science Letters by Kamenetsky, V.

This project is aimed at understanding the relationship between kimberlites and diamonds, and between flood basalts and sulfide mineralisation, by linking mantle structure, composition, and temperature to the melting processes that generate these mantle-derived magmas.

The outcomes have been published in several high-profile publications during the year. A paper in Nature Communications addressed the origin of Group-II kimberlites (orangeites) through a study of MARID (mica-amphibole-rutile-ilmenite-diopside) xenoliths from the Bultfontein kimberlite in South Africa. This demonstrated that orangeites can be formed during melting a MARID-rich lithospheric mantle.

An experimental study published in Geochimica et Cosmochimica Acta proposed that interaction between a silicate mantle rock and a natrocarbonatite melt results in carbonate-silicate liquid immiscibility, which forms globules of a CO₂-rich silicate melt. On decompression, the dispersed silicate melt phase ensures a continuous supply of CO₂ bubbles, which decrease density, increase buoyancy and promote the rapid ascent of the magmatic emulsion.

A paper in Geology addressed the origin of platinum-group element (PGE) mineralisation associated with primitive magmas by analysing inclusions of Pt-Fe and Os-Ir alloys in Cr-spinel phenocrysts, which are interpreted to precipitate directly from a range of melt compositions formed in supra-subduction intraplate settings. These inclusions helped to constrain melt saturation in PGE-rich phases as a function of temperature and fugacity of oxygen and sulfur.