Carbonatites

< Previous Issue October 2021 – Volume 17, Number 5 Next Issue > Download Table of Contents Digital Edition Download Issue Carbonatites Gregory M. Yaxley, Michael Anenburg, and Suzette Timmerman – Guest Editors Table of Contents EditorialFrom the EditorsTriple PointPerspectiveMeet the AuthorsThematic ArticlesSociety NewsCosmoELEMENTSParting ShotsCalendar Overview Advertisers Next Issue 2021 Topics Overview Carbonatites are rare, but important,…

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Carbonatites and Global Tectonics

Carbonatites have formed for at least the past three billion years. But over the past 700 My the incidence of carbonatites have significantly increased. We compile an updated list of 609 carbonatite occurrences and plot 387 of known age on plate tectonic reconstructions. Plate reconstructions from Devonian to present show that 75% of carbonatites are emplaced within 600 km of craton edges. Carbonatites are also associated with large igneous provinces, orogenies, and rift zones, suggesting that carbonatite magmatism is restricted to discrete geotectonic environments that can overlap in space and time. Temporal constraints indicate carbonatites and related magmas may form an ephemeral but significant flux of carbon between the mantle and atmosphere.

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The Distinctive Mineralogy of Carbonatites

The mineralogy of carbonatites reflects both the diversity of the sources of their parent magmas and their unusual chemistry. Carbonatites contain diverse suites of both primary magmatic minerals and later hydrothermal products. We present a summary of the variety of minerals found in carbonatites, and note the economic importance of some of them, particularly those that are major sources of “critical elements”, such as Nb and rare earth elements (REEs), which are essential for modern technological applications. Selected mineral groups are then discussed in detail: the REE carbonates, the alkali-rich ephemeral minerals that are rarely preserved but that may be important in the petrogenesis of carbonatites and their metasomatic haloes in adjacent rocks, and the Nb-rich oxides of the pyrochlore supergroup.

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Formation of Rare Earth Deposits in Carbonatites

Carbonatites and related rocks are the premier source for light rare earth element (LREE) deposits. Here, we outline an ore formation model for LREE-mineralised carbonatites, reconciling field and petrological observations with recent experimental and isotopic advances. The LREEs can strongly partition to carbonatite melts, which are either directly mantle-derived or immiscible from silicate melts. As carbonatite melts evolve, alkalis and LREEs concentrate in the residual melt due to their incompatibility in early crystallising minerals. In most carbonatites, additional fractionation of calcite or ferroan dolomite leads to evolution of the residual liquid into a mobile alkaline “brine-melt” from which primary alkali REE carbonates can form. These primary carbonates are rarely preserved owing to dissolution by later fluids, and are replaced in-situ by monazite and alkali-free REE-(fluor)carbonates.

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Carbonatitic Melts and Their Role in Diamond Formation in the Deep Earth

Carbonatitic high-density fluids and carbonate mineral inclusions in ­lithospheric and sub-lithospheric diamonds reveal comparable compositions to crustal carbonatites and, thus, support the presence of carbonatitic melts to depths of at least the mantle transition zone (~410–660 km depth). Diamonds and high pressure–high temperature (HP–HT) experiments confirm the stability of lower mantle carbonates. Experiments also show that carbonate melts have extremely low viscosity in the upper mantle. Hence, carbonatitic melts may participate in the deep (mantle) carbon cycle and be highly effective metasomatic agents. Deep carbon in the upper mantle can be mobilized by metasomatic carbonatitic melts, which may have become increasingly volumetrically significant since the onset of carbonate subduction (~3 Ga) to the present day.

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Evolution of Carbonatite Magmas in the Upper Mantle and Crust

Carbonatites are the most silica-poor magmas known and are amongst Earth’s most enigmatic igneous rocks. They crystallise to rocks dominated by the carbonate minerals calcite and dolomite. We review models for carbonatite petrogenesis, including direct partial melting of mantle lithologies, exsolution from silica-undersaturated alkali silicate melts, or direct fractionation of carbonated silicate melts to carbonate-rich residual melts. We also briefly discuss carbonatite–mantle wall-rock reactions and other processes at mid- to upper crustal depths, including fenitisation, overprinting by carbohydrothermal fluids, and reaction between carbonatite melt and crustal lithologies.

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Carbonatites: Contrasting, Complex, and Controversial

Carbonatites are unique, enigmatic, and controversial rocks directly sourced from, or evolved from, mantle melts. Mineral proportions and chemical compositions of carbonatites are highly variable and depend on a wide range of processes: melt generation, liquid immiscibility, fractional crystallization, and post-magmatic alteration. Observations of plutonic carbonatites and their surrounding metasomatic rocks (fenites) suggest that carbonatite intrusions and volcanic rocks do not fully represent the true compositions of the parental carbonatite melts and fluids. Carbonatites are enriched in rare elements, such as niobium and rare earths, and may host deposits of these elements. Carbonatites are also important for understanding the carbon cycle and mantle evolution.

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