Igneous Rock Associations 32. Carbonatites: A Review with Emphasis on Geochemical and Petrogenetic Aspects

Carbonatites are defined as an igneous rock containing > 50% carbonate minerals, although not all rocks labelled as carbonatite fit strict mineralogical criteria. Carbonatites are rare, very unusual in terms of mineralogy and geochemistry, compositionally variable and extraordinarily diverse. These enigmatic rocks are also highly prospective, hosting large resources of Nb, REE, phosphates and other commodities now classed as ‘critical minerals’. Ideas on the genesis and evolution of carbonatite are divergent and many observations or hypotheses remain controversial. This review focuses mostly on geochemical aspects, notably as applied to models for the generation of carbonatite parental magmas, and the wider implications of such data. These rare and fascinating rocks provide insights into the secular evolution of the sub-continental mantle, the role of metasomatism at both crustal and mantle levels, and the nature of deeper parts of the mantle. Carbonatites are known from all the continents and extend back in time to ~ 3.0 Ga. Only two clear examples are known from the oceanic realm, both associated with oceanic islands located atop mantle plumes. Continental carbonatite occurrences, along with kimberlite intrusions, are the deepest direct probes of the Earth’s subcontinental mantle. Their parental magmas, regardless of how generated, have very low viscosities and wetting angles, and can migrate rapidly towards the surface. They are also highly efficient metasomatic agents at crustal levels, where such effects can be directly studied, and they are inferred to have similar effects at mantle depths. Only one active carbonatite volcano is known (Oldoinyo Lengai, Tanzania) and extrusive carbonatites are much rarer than intrusive varieties in the geological record. Carbonatites are broadly classified into Ca-rich (calciocarbonatite), Mg-rich (magnesiocarbonatite) and Fe-rich (ferrocarbonatite) varieties using mineralogy and chemistry. A rare Na-rich variety (natrocarbonatite) is defined largely at Oldoinyo Lengai but is likely ephemeral on geological time scales and under-represented in the geological record. In most areas, carbonatites are spatially and temporally associated with alkaline silicate igneous suites, which usually include silica-undersaturated types, and may include carbonate-rich rocks. The radiogenic isotope compositions of carbonatites suggest that parental melts were generated by small-volume partial melting of the mantle. Their Sr–Nd–Pb isotope signatures resemble those of oceanic island basalt (OIB) and indicate the involvement of specific mantle ‘components’ or reservoirs identified from studies of global OIB data. This suggests that these same hypothetical mantle components, believed to represent deeper regions of the mantle, also exist beneath continental regions. The near absence of carbonatites in the modern oceanic realm may indicate that thickened lithosphere beneath the continents exerts important controls on carbonatite genesis and/or ascent. The radiogenic isotope signatures of carbonatites seem to exclude significant contributions from the shallower ‘depleted mantle’ reservoir that is the dominant lithospheric source for mid-ocean ridge basalt (MORB). This argues against various models proposing that carbonatites partly represent recycled continental crustal material (and associated carbon) returned to the mantle by subduction. Stable isotope data for carbonatites are less diagnostic than radiogenic isotope data, but also hint at deep-seated, primitive mantle sources and similarly constrain potential contributions from recycled crustal materials. However, interpretation of stable isotope data is invariably complicated by the widespread fluid-related alteration within and around individual carbonatite complexes. Intense discussion of carbonatites and their precise origins is likely to continue but radiogenic isotope data and the growing stable isotope database favour models in which parental melts were sourced from deeper (sublithospheric) regions of the mantle. The petrological and geochemical complexity of individual carbonatite suites reflect later processes that likely include crystal fractionation, liquid immiscibility, interaction with wall rocks and other factors operating closer to their eventual sites of emplacement. Understanding of these intricate and linked processes remains incomplete, but they are obviously important for petrologists and economic geologists. The common association between carbonatites and alkaline silicate igneous rocks is unlikely to be coincidental, but it does not necessarily indicate a direct sequential linkage. However, observational and experimental evidence suggests that carbonated silicate magma can in some cases fractionate to carbonatite-like compositions or exsolve immiscible carbonate magma. From a wider perspective, carbonatites provide probes into large parts of the deep subcontinental mantle and may give insight into fundamental mantle differentiation events in Hadean (> 4.0 Ga) and later Archean (~ 3.0 Ga) times. Carbonatites also provide a mechanism by which carbon can be transferred from the deep Earth to the surface and (ultimately) to the atmosphere as CO2, so they may influence or mediate long-term climate variations. Despite their very limited extent on geological maps, carbonatites may be generated on a semicontinuous basis at mantle depths and likely represent a critical and influential component of the wider Earth System. There are many economic and scientific reasons to expand carbonatite research.