Kimberlite: A Unique Probe into the Deep Earth

With all due respect to basalt, and I appreciate a granite as much as the next person, kimberlite is hard to beat. But what is a kimberlite? Kimberlites may be classified as igneous rocks but it is difficult to know how exactly to describe them in terms of magma, at least in any conventional sense of the word. Kimberlites tap the deepest recesses of our planet that we can sample. Propelled by a formidable volatile load, kimberlite melts transit hundreds of kilometers of mantle and crust, perhaps in just a few days, to form unique ballistic deposits at Earth’s surface. Kimberlites accumulate and transport ripped-up bits from throughout most, if not all, of their ascent path, including diamond, that classic gem of desire with its remarkable qualities that have fueled a global market. Indeed, much of our understanding of kimberlite is owed to the intrepid explorers who searched for and studied elusive diamond deposits. Adding to the veil of petrological complexity kimberlites are often pervasively altered by fluids, some of which were magmatic but some of which were not. The study of kimberlites over many decades has revealed glimpses of their origins and the paths by which they have travelled.

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v15n6 From the Editors

The copy of Elements you are holding in your hands (or reading online) is the result of the creativity and expertise of our 18 participating socities, authors, editors, reviewers, graphic designers, business and administrative staff, print and shipping vendors, and advertisers. Every issue represents hundreds of hours of effort by many individuals working together for a common goal … to deliver Elements to you, the reader. Elements is a joint endeavour. Each year, in our final issue, we take a moment to extend our appreciation to those that brought Elements to life. This year is no different.

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Carbon — Beautiful, Essential, Deadly

The unique physical chemistry of carbon confers an extraordinary ability to form molecules that are variously beautiful (think diamond), essential (think living cells), and toxic (think greenhouse gas). Nowhere is this split personality more evident than in the enigmatic igneous clan of kimberlites, the topic for this issue of Elements. No one who has set eyes on a cut diamond, especially the delicate pink stones from soon-to-close Argyle Mine in Western Australia (see photo to the right), can fail to be awestruck at Nature’s capacity for beauty. Kimberlite magmas that bring diamonds to the surface are carbon-fuelled, whether by methane through a complex series of redox melting reactions (see Foley et al. 2019 this issue p. 393), or by carbon dioxide exsolving from kimberlite melt at sub-crustal depths and propelling it explosively to the surface (see Russell et al. 2019 this issue p. 405). We have yet to witness a kimberlite erupt – the last known eruption, in Tanzania, was ten thousand years ago – but we can be fairly sure that the greenhouse gas delivery of a single kimberlite pipe in full flow was pretty substantial. For kimberlites, carbon is both passenger and propellant.

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Diamond Exploration and Resource Evaluation of Kimberlites

Kimberlite rocks and deposits are the eruption products of volatile-rich, silica-poor ultrabasic magmas that originate as small-degree mantle melts at depths in excess of 200 km. Many kimberlites are emplaced as subsurface cylindrical-to-conical pipes and associated sills and dykes. Surficial volcanic deposits of kimberlite are rare. Although kimberlite magmas have distinctive chemical and physical properties, their eruption styles, intensities and durations are similar to conventional volcanoes. Rates of magma ascent and transport through the cratonic lithosphere are informed by mantle cargo entrained by kimberlite, by the geometries of kimberlite dykes exposed in diamond mines, and by laboratory-based studies of dyke mechanics. Outstanding questions concern the mechanisms that trigger and control the rates of kimberlite magmatism.

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Kimberlite Volcanology: Transport, Ascent, and Eruption

Kimberlite rocks and deposits are the eruption products of volatile-rich, silica-poor ultrabasic magmas that originate as small-degree mantle melts at depths in excess of 200 km. Many kimberlites are emplaced as subsurface cylindrical-to-conical pipes and associated sills and dykes. Surficial volcanic deposits of kimberlite are rare. Although kimberlite magmas have distinctive chemical and physical properties, their eruption styles, intensities and durations are similar to conventional volcanoes. Rates of magma ascent and transport through the cratonic lithosphere are informed by mantle cargo entrained by kimberlite, by the geometries of kimberlite dykes exposed in diamond mines, and by laboratory-based studies of dyke mechanics. Outstanding questions concern the mechanisms that trigger and control the rates of kimberlite magmatism.

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Dating Kimberlites: Methods and Emplacement Patterns Through Time

Key to deciphering the origin and tectonic setting of kimberlite magmatism is an accurate understanding of when they formed. Although determining absolute emplacement ages for kimberlites is challenging, recent methodological advances have contributed to a current database of >1,000 precisely dated kimberlite occurrences. Several profound findings emerge from kimberlite geochronology: kimberlites were absent in the first half of Earth history; most kimberlites were emplaced during the Mesozoic; kimberlite magma formation may be triggered by a variety of Earth processes (deep mantle plumes, subduction of oceanic lithosphere, continental rifting); and enhanced periods of kimberlite magmatism coincide with supercontinent breakup.

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Kimberlites from Source to Surface: Insights from Experiments

High-pressure experiments are unconvincing in explaining kimberlites as direct melts of carbonated peridotite because the appropriate minerals do not coexist stably at the kimberlite liquidus. High-pressure melts of peridotite with CO2 and H2O have compositions similar to kimberlites only at pressures where conditions are insufficiently oxidizing to stabilize CO2: they do not replicate the high K2O/Na2O of kimberlites. Kimberlite melts may begin their ascent at ≈300 km depth in reduced conditions as melts rich in MgO and SiO2 and poor in Na2O. These melts interact with modified, oxidized zones at the base of cratons where they gain CO2, CaO, H2O, and K2O and lose SiO2. Decreasing CO2 solubility at low pressures facilitates the incorporation of xenocrystic olivine, resulting in kimberlites’ characteristically high MgO/CaO.

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Kimberlites as Geochemical Probes of Earth’s Mantle

Kimberlites are ultrabasic, Si-undersaturated, low Al, low Na rocks rich in CO2 and H2O. The distinctive geochemical character of kimberlite is strongly influenced by the nature of the local underlying lithospheric mantle. Despite this, incompatible trace element ratios and radiogenic isotope characteristics of kimberlites, filtered for the effects of crustal contamination and alteration, closely resemble rocks derived from the deeper, more primitive, convecting mantle. This suggests that the ultimate magma source is sub-lithospheric. Although the composition of primitive kimberlite melt remains unresolved, kimberlites are likely derived from the convecting mantle, with possible source regions ranging from just below the lithosphere, through the transition zone, to the core–mantle boundary.

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What is a Kimberlite? Petrology and Mineralogy of Hypabyssal Kimberlites

Hypabyssal kimberlites are subvolcanic intrusive rocks crystallised from mantle-derived magmas poor in SiO2 and rich in CO2 and H2O. They are complex, hybrid rocks containing significant amounts of mantle-derived fragments, primarily olivine with rare diamonds, set in a matrix of essentially magmatic origin. Unambiguous identification of kimberlites requires careful petrographic examination combined with mineral compositional analyses. Melt inclusion studies have shown that kimberlite melts contain higher alkali concentrations than previously thought but have not clarified the ultimate origin of these melts. Because of the hybrid nature of kimberlites and their common hydrothermal alteration by fluids of controversial origin (magmatic and/or crustal), the composition of primary kimberlite melts remains unknown.

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Kimberlites: From Deep Earth to Diamond Mines

Kimberlites are rare, enigmatic, low-volume igneous rocks. They are highly enriched in magnesium, volatiles (CO2 and H2O) and incompatible trace elements and are thought to be the most deeply derived (>150 km) magmatic rocks on Earth. Kimberlites occur in ancient and thick continental lithosphere, forming intrusive sheets and composite pipes, commonly in clusters. Despite their rarity, kimberlites have attracted considerable attention because they entrain not only abundant mantle fragments but also diamonds, which can provide a uniquely rich picture of the deep Earth. This issue summarises current thinking on kimberlite petrology, geochemistry, and volcanology and outlines the outstanding questions on the genesis of kimberlites and associated diamond mines.

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