This project aims to understand primary compositions of mantle-derived carbonatite magmas and carbonaterich components in association with alkali silicate magmas. The study is based on a set of representative samples from renowned occurrences of carbonatites.

In 2019, we continued research on perovskite accumulations in the Afrikanda alkaline–ultramafic complex (Kola Peninsula, NW Russia) and had our paper accepted for publication in Contributions to Mineralogy and Petrology. We aimed to understand the development of the perovskite-rich zones in the olivinites, clinopyroxenites and silico-carbonatites by studying polymineralic inclusions hosted in perovskite and magnetite. The abundance of inclusions varies across the three perovskite textures, with numerous inclusions hosted in the fine-grained equigranular perovskite, fewer inclusions in the coarse-grained interlocked perovskite and rare inclusions in the massive perovskite. A variety of silicate, carbonate, sulfide, phosphate and oxide phases are assembled randomly and in variable proportions in the inclusions. These observations reveal that the inclusions are not bona fide melt inclusions, but represent host media trapped during subsolidus sintering of magmatic perovskite. The continuation of the sintering process resulted in the coarsening of inclusion-rich subhedral perovskite into inclusion-poor anhedral and massive perovskite. We advocate the importance of inclusion studies for interpreting the origin of oxide minerals and their associated economic deposits and suggest that the formation of large-scale accumulations of minerals in other oxide deposits may be a result of annealing of individual disseminated grains.

With three papers published, Naomi Potter was admitted to the degree of Doctor of Philosophy.

This project aims to understand primary compositions of mantle-derived carbonatite magmas and carbonate-rich components in association with alkali silicate magmas. The study is based on a set of representative samples from renowned occurrences of carbonatites.

In 2018, our research on perovskite accumulation in the Afrikanda alkaline–ultramafic complex (Kola Peninsula, NW Russia) was published in Contributions to Mineralogy and Petrology (Potter et al.). This paper presents textural and compositional varieties of perovskite, which is a common accessory mineral in many mafic and ultramafic rocks, but perovskite deposits are rare, and studies of perovskite ore deposits are correspondingly scarce. We classify perovskite into three types based on crystal morphology, inclusion abundance, composition, and zonation. Perovskite in some rocks is represented by fine-grained, equigranular, monomineralic clusters and networks, but has fine- to coarse-grained interlocked and massive textures in other rocks. Electron backscatter diffraction reveals that some perovskite grains are composed of multiple subgrains and may represent stages of crystal rotation, coalescence and amalgamation. We propose that exclusively sub-solidus processes can lead to the development of coarse-grained and massive perovskite. A combination of characteristic features identified in the Afrikanda perovskite (equigranular crystal mosaics, interlocked irregular-shaped grains, and massive zones) is observed in other oxide ore deposits, particularly in layered intrusions of chromitites and intrusion-hosted magnetite deposits and suggests that the same amalgamation processes may be responsible for some of the coarse-grained and massive textures observed in oxide deposits worldwide.

This project aims to understand primary compositions of mantle-derived carbonatite magmas and carbonate-rich components in association with alkali silicate magmas. The study is based on a set of fresh samples representing worldwide occurrences of carbonatites.

In 2017, our research on liquid-liquid immiscibility in the Oldoinyo Lengai 1993 lava was published in Chemical Geology (Potter et al.). This paper presents petrographic evidence of liquid immiscibility between silicate, carbonate, chloride, and fluoride melt phases. Several textural features preserved in the silicate spheroids, melt inclusions, and carbonatite groundmass have been identified, which exhibit evidence of silicate-carbonate, carbonate-carbonate and carbonate-halide immiscibility. Rapid quenching of the lava facilitated fortuitous preservation of the end products of these multistage liquid immiscibility processes within the groundmass.

We continue to resolve the origin of intraplate carbonatitic to alkaline volcanism using examples from western Africa. The first radiometric and isotopic results for the Catanda complex, which hosts the only extrusive carbonatites in Angola, are reported by Giuliani et al. in Geology. Apatite (U-Th-Sm)/He and phlogopite ⁴⁰Ar/³⁹Ar ages of Catanda aillikite lavas indicate eruption at ca. 500–800 ka, more than 100 m.y. after emplacement of abundant kimberlites and carbonatites in this region. The lavas share similar high-μ (HIMU)-like Sr-Nd-Pb-Hf isotope compositions with other young mantle-derived volcanics from Africa (e.g., Northern Kenya Rift; Cameroon Line). The position of the Catanda complex in the Lucapa corridor, a long-lived extensional structure, suggests a possible tectonic control for the volcanism. The complex is also located on the Bié Dome, a broad region of fast Pleistocene uplift attributed to mantle upwelling. Seismic tomography models indicate convection of deep hot material beneath regions of active volcanism in Africa, including a large area encompassing Angola and northern Namibia.

The project is aimed at understanding primary compositions of mantle-derived carbonatite magmas and carbonate-rich components in association with alkali silicate magmas. The study is based on a set of fresh samples representing worldwide occurrences of carbonatites.

In 2016, research by PhD student Naomi Potter on the liquid-liquid immiscibility in the Oldoinyo Lengai 1993 lava was published in Chemical Geology. This paper presents petrographic evidence of liquid immiscibility between silicate, carbonate, chloride, and fluoride melt phases. Several textural features preserved in the silicate spheroids, melt inclusions, and carbonatite groundmass have been identified, which exhibit evidence of silicate-carbonate, carbonate-carbonate and carbonate-halide immiscibility. Rapid quenching of the lava facilitated fortuitous preservation of the end products of these multi-stage liquid immiscibility processes within the groundmass.

The project is aimed at understanding primary compositions of mantle-derived carbonatite magmas and carbonate-rich components in association with alkali silicate magmas. The study is based on a set of fresh samples representing worldwide occurrences of carbonatites. Currently, this project is funded by an ARC Discovery grant.

Research published in Nature Communications presented evidence for the alkaline nature of parental carbonatite melts, based on studies of magnetite-hosted melt inclusions. A linked study published in Geology reported the occurrence of primary halite in mantle-derived carbonatite magmas. Recent results on Angolan carbonatites have demonstrated that they were emplaced at 0.65±0.05 Ma, thus linking their origin to the present-day re-activation of the Cretaceous Lucapa rift in Angola.

A series of experiments was aimed at constraining the causes of the rapid ascent of carbonate-rich melts to the surface, which suggested that carbonate-silicate liquid immiscibility had significant implications for magma evolution through melt-crystal reactions, liquid unmixing, effervescence of CO₂ and related dramatic decreases in viscosity.