Diamond Formation Depth
Diamond Formation Depth
From the lithospheric mantle to the lower Earth: where diamonds are born
Diamond formation depth refers to the range of depths within the Earth's mantle at which diamond crystals nucleate and grow. The overwhelming majority of gem-quality and industrial diamonds crystallise within the lithospheric mantle, at depths of roughly 150 to 200 kilometres beneath the surface, where pressures exceed 5 gigapascals (GPa) and temperatures range from approximately 1,100 to 1,400 °C. A smaller but scientifically extraordinary population — the so-called sublithospheric or "superdeep" diamonds — originates at depths surpassing 300 kilometres, and in some documented cases beyond 600 kilometres, in the transition zone and lower mantle. Formation depth is not merely an academic curiosity: it governs the pressure–temperature history recorded in a diamond's inclusion assemblage, its trace-element chemistry, and, in certain cases, its colour and structural characteristics.
The Lithospheric Mantle: The Primary Diamond Factory
The lithospheric mantle is the rigid, chemically depleted lower portion of the tectonic plate. Beneath ancient, tectonically stable continental regions known as cratons, this lithosphere extends to exceptional depths — sometimes exceeding 200 kilometres — and has remained relatively undisturbed for billions of years. These deep cratonic roots provide the thermal and chemical environment in which diamond is the stable polymorph of carbon, rather than graphite.
At the relevant depths, pressures of 5 to 7 GPa and temperatures in the range of 1,100–1,400 °C place the system firmly within the diamond stability field on the carbon phase diagram. Carbon is introduced into the mantle primarily through two mechanisms: subduction of organic-rich or carbonate-bearing oceanic crust, and primordial carbon already present in the mantle from Earth's accretion. Isotopic studies of diamond carbon (δ¹³C values) reveal a broad range, consistent with both mantle-derived and recycled crustal carbon sources.
Diamonds forming at lithospheric depths typically contain mineral inclusions belonging to two broad parageneses. Peridotitic diamonds — the more common type — carry inclusions such as olivine, enstatite, diopside, pyrope garnet, and chromite, reflecting crystallisation within harzburgite or lherzolite host rock. Eclogitic diamonds contain omphacitic pyroxene, almandine-pyrope garnet, and occasionally coesite or kyanite, indicating formation within subducted oceanic crustal material that has been transformed to eclogite at depth. The distinction between these two parageneses has direct implications for understanding carbon cycling in the deep Earth.
Sublithospheric Diamonds: Windows into the Deep Earth
A subset of diamonds, estimated to constitute roughly 1–2 per cent of all diamonds studied in detail, bear inclusions that are inconsistent with lithospheric pressures. These sublithospheric diamonds carry phases that are stable only at depths of 300 kilometres or greater, placing their origin in the mantle transition zone (approximately 410–660 km depth) or the lower mantle (below 660 km).
Key indicator minerals found as inclusions in superdeep diamonds include:
- Ringwoodite — a high-pressure polymorph of olivine (Mg,Fe)₂SiO₄ stable between roughly 520 and 660 km depth. The 2014 report in Nature of a ringwoodite inclusion within a diamond from Juína, Brazil, provided the first direct evidence of water stored in the transition zone, with the inclusion containing approximately 1.5 per cent water by weight.
- Ferropericlase (magnesiowüstite) — a (Mg,Fe)O phase characteristic of the lower mantle, commonly found in diamonds from Juína and Kankan, Guinea.
- Bridgmanite (formerly perovskite-structured MgSiO₃) — the most abundant mineral in the lower mantle by volume, occasionally recovered as a retrogressed inclusion in superdeep diamonds.
- CaSiO₃-perovskite — a calcium silicate phase stable at lower-mantle pressures, found as inclusions that revert to wollastonite or other phases upon decompression.
The Juína region of Mato Grosso, Brazil, and the Kankan kimberlite field of Guinea are among the most productive known sources of superdeep diamonds. Diamonds from the Cullinan mine (Premier mine) in South Africa have also yielded sublithospheric inclusion assemblages. Research published in Gems & Gemology and allied journals has progressively refined the pressure–temperature estimates for these stones using thermobarometric calculations on their inclusion phases.
Pressure–Temperature Conditions and the Carbon Phase Diagram
The stability of diamond relative to graphite is defined by a well-established phase boundary. At mantle geotherms relevant to cratonic lithosphere, diamond becomes the stable phase at pressures above approximately 4.5 GPa at 1,000 °C, rising to around 6 GPa at 1,400 °C. This boundary, combined with the geothermal gradient of a given craton, defines the diamond window — the depth interval within which diamond can form and persist without converting to graphite during slow cooling.
Sublithospheric conditions impose far greater pressures: the transition zone operates at roughly 13–24 GPa, and the lower mantle at pressures exceeding 24 GPa. At these extremes, carbon behaviour and diamond morphology may differ from lithospheric counterparts. Some superdeep diamonds exhibit unusual nitrogen aggregation states or anomalously low nitrogen contents, possibly reflecting the distinct thermal histories and residence times at extreme depth.
Transport to the Surface: Kimberlite and Lamproite Magmas
Diamonds are thermodynamically unstable at the Earth's surface and would eventually convert to graphite under ambient conditions, albeit at an imperceptibly slow rate. Their survival depends on rapid transport from mantle depths to the surface. This is accomplished by kimberlite magmas — volatile-rich, potassic ultramafic melts that ascend through the lithosphere at high velocity, entraining mantle xenoliths and xenocrysts (including diamonds) along the way. The ascent is sufficiently rapid — estimated at metres per second in the upper portions of the conduit — to prevent graphitisation and to limit resorption of diamond crystals by the host magma.
Lamproite magmas serve an analogous role at a smaller number of localities, most notably the Argyle pipe in Western Australia, which was a lamproite-hosted deposit and the world's foremost source of pink diamonds until its closure in 2020.
The surface expressions of these eruptions are diatremes — carrot-shaped volcanic pipes filled with kimberlite or lamproite breccia. The age of kimberlite emplacement is distinct from the age of the diamonds themselves: diamonds at Kimberley, South Africa, for instance, were emplaced by kimberlites approximately 90 million years ago, yet their formation ages, determined by dating silicate or sulphide inclusions using Re–Os or Sm–Nd isotopic systems, commonly range from 1 to 3.5 billion years. The diamonds are ancient passengers in a relatively young vehicle.
Influence of Formation Depth on Gem Characteristics
For the gemmologist and trade professional, formation depth has practical relevance beyond pure science. The inclusion assemblage of a diamond — whether peridotitic, eclogitic, or sublithospheric — can be characterised by advanced spectroscopic and microanalytical techniques, providing provenance information that complements conventional origin assessment. Certain coloured diamonds owe their hue in part to trace elements or structural defects whose distribution is influenced by the pressure–temperature conditions of formation: boron-bearing type IIb blue diamonds, for example, are now understood from research published in Nature (2018) to be predominantly superdeep in origin, with their boron derived from subducted oceanic crust.
The nitrogen aggregation state — the degree to which nitrogen atoms have diffused from isolated (type Ib) to paired (type IaA) to larger platelet-associated (type IaB) configurations — is a function of both temperature and time. Diamonds with anomalously low aggregation for their apparent age may have resided at cooler lithospheric temperatures, while those with high aggregation may reflect prolonged storage at elevated temperatures closer to the base of the lithosphere.