Kimberlite and Lamproite: The Final Vehicle from the Depths

Most natural diamonds spend the greater part of their geological history hidden in the mantle. For them to become accessible to people, something exceptional must occur: deep magma must cut through a diamond-bearing part of the mantle, entrain the crystals, and carry them toward the surface before they are destroyed.

The most important such transporter is kimberlite. A smaller number of important deposits are associated with lamproite.

Fundamental rule For most diamonds, kimberlite is a vehicle, not a factory. The ages of a diamond and its kimberlite often differ by billions of years.

What kimberlite is

Kimberlite is a rare, deep-derived, volatile-rich ultramafic igneous rock that may contain a large proportion of material entrained from the mantle and crust.

The resulting rock is therefore hybrid. It may contain:

• crystals formed from kimberlite magma;
• olivine and other xenocrysts from the mantle;
• xenoliths of peridotite and eclogite;
• fragments of the surrounding crust;
• secondary minerals formed during alteration and cooling;
• diamonds as xenocrysts.

The chemistry of the original kimberlite melt therefore cannot simply be read from a piece of altered rock at the surface.

Source and generation of kimberlite magma

Kimberlite melts originate very deep in the mantle and are rich in volatile components, especially CO₂ and H₂O. The precise mechanisms by which they form and their geodynamic triggers remain active areas of research.

Spatial and temporal relationships with rifting, craton margins, supercontinent cycles, and deep-mantle processes show that there is no single simple “kimberlite formula.”

It is important to separate:

• the generation of kimberlite melt;
• the entrainment of diamond from the lithosphere;
• rapid ascent;
• eruption and pipe formation.

These are distinct stages of the same system.

One important model of acceleration involves a reaction between carbonate melt and orthopyroxene in the lithospheric mantle. Such a reaction can reduce CO₂ solubility, promote exsolution of a gas phase, and increase buoyancy. The model is chemically and physically plausible, but it should not be presented as the only possible mechanism for every kimberlite.

Ascent: rapid, but not at a constant speed

No kimberlite eruption from the geological past has been measured directly with instruments. Ascent rates are estimated from models, diffusion profiles in minerals, the preservation of xenoliths, textures, decompression, and magma physics.

Older models often emphasized very high rates of several meters per second. More recent diffusion chronometry shows that, in at least some kimberlite systems, the deeper stages of the journey may proceed more slowly—for example, on the order of hundredths to tenths of a meter per second—with later acceleration toward shallower levels and eruption.

The most accurate statement is therefore:

Over geological timescales, kimberlite rises exceptionally rapidly compared with most magmatic processes, but its speed is not constant and no single value applies to all depths and deposits.

[VISUAL 10.1: Multistage ascent of kimberlite—deep segregation/flow → fracture transport → acceleration and gas expansion → eruption]

Why rapid transport matters

Diamond leaves the region in which it is stable and enters hot, reactive magma under progressively lower pressure. The longer the contact lasts, the greater the potential for:

• resorption;
• oxidation;
• surface graphitization;
• loss of mass;
• mechanical damage.

Experimental models show that higher ascent rates can substantially improve diamond preservation, but specific thresholds depend on temperature, magma composition, and the model. Values such as “must be faster than X m/s” must therefore not be used as a universal geological rule.

Resorption: the transporter can change the passenger

Kimberlite need not create a diamond in order to alter its surface.

During interaction with magma and fluids, a crystal may:

• lose sharp edges;
• change from an octahedral toward a more rounded dodecahedroidal form;
• develop etch pits and other surface features;
• lose some of its mass;
• fracture through mechanical processes.

Importantly, some resorption may also occur before entrainment by kimberlite, during earlier metasomatic events in the mantle. Surface morphology is therefore not a simple chronometer of ascent.

The detailed interpretation of rough surfaces belongs to Chapter 13.

Xenoliths: cargo from the depths

Kimberlite may lift mantle fragments ranging in size from tiny pieces to large xenoliths. This demonstrates how physically powerful the transport is and how effectively the magma can entrain material from different depths.

A diamond enclosed in a xenolith may be partly shielded from direct contact with the magma, but the xenolith may break apart during ascent and release the crystal.

From dike to pipe

A kimberlite system is not necessarily a single regular “carrot.” It may include:

• deep feeder dikes;
• sill-like or lateral intrusions;
• multiple successive conduits;
• diatremes;
• crater and surface deposits;
• multiple phases of magmatic and volcaniclastic infill.

Erosion may remove the upper part of the system and leave only its deeper root. The present shape of a deposit is therefore not necessarily the original shape of the volcano.

The terms root zone, diatreme, and crater zone are useful in an idealized description, but real pipes may diverge from that model, branch, crosscut older phases, and contain different magmatic or volcaniclastic facies. Modern volcanology therefore describes geometry and facies for the specific body rather than forcing them into a single scheme.

[VISUAL 10.2: Idealized kimberlite system—feeder dike, root zone, diatreme, crater, and erosion level]

Magmatic and phreatomagmatic fragmentation

As magma approaches the surface, decreasing pressure promotes gas exsolution. Fragmentation may result primarily from the expansion of magmatic volatiles, but in some systems interaction between magma and groundwater may also be important.

Real kimberlite pipes often preserve multiple eruptive phases and different textures. It is therefore not scientifically responsible to describe a single pipe with one universal eruption model without considering the local geology.

“Blue ground” and “yellow ground”

The historical mining terms blue ground and yellow ground describe differently altered or oxidized parts of kimberlite ore, especially in the South African mining tradition.

They are not modern universal petrographic categories of kimberlite. In technical writing, they should be used as historical or mining terms, not as a fundamental rock classification.

Lamproite is not kimberlite

Lamproite is a distinct group of alkaline, volatile-rich igneous rocks. It differs chemically and mineralogically from kimberlite, although rare lamproites can also transport diamonds.

The best-known example is Australia’s Argyle, whose AK1 diamond system is associated with lamproite volcanism. AK1 is a complex volcanic system whose lamproite emplacement is bracketed by recent U–Pb geochronology between approximately 1311 ± 9 Ma and 1257 ± 15 Ma, while an important population of Argyle’s eclogitic diamonds has been dated to approximately 1580 ± 30 Ma. This example also shows that the transporting magma and the diamond belong to different geological events.

Argyle is important because it demonstrated that deposits of exceptional economic significance need not follow the simplest classic rule of “Archean craton + kimberlite.” The deposit lies in a complex Proterozoic orogenic setting.

Argyle did not create the pink color

Pink and brown Argyle diamonds are associated with deformation of the crystal lattice and a long deep-Earth history before lamproite transport. The lamproite carried the crystals; it did not simply “give” them their pink color during eruption.

The connection between color and plastic deformation is discussed in detail in Chapters 35 and 46.

Why few kimberlites become mines

Finding kimberlite is not enough to establish an economic deposit.

It is necessary to assess:

• whether it contains diamonds;
• their concentration;
• the size distribution;
• the proportion of gemologically valuable stones;
• the geometry and volume of the ore;
• dilution by surrounding rock;
• the depth and conditions of extraction;
• processing costs;
• the market value of production.

Grade, often expressed in carats per metric ton, is only one part of the equation. Two ores with the same grade may have entirely different economic values if their diamonds differ in size, color, clarity, and value distribution.

The detailed economics of mining are beyond the scope of this chapter, but it is important to separate geological diamond content from economic viability.

Indicator minerals and exploration

Kimberlite may break down through erosion, and its resistant minerals may be transported through soil, rivers, or glaciers. Explorers therefore trace suites of indicator minerals back toward a possible source.

Such a trail may lead to a hidden pipe, but it does not in itself prove that the source is economically diamond-bearing.

Could kimberlites erupt today?

Kimberlites are known geologically from different periods of Earth’s history, but no modern kimberlite eruption has been directly observed. Descriptions of what such an eruption would look like therefore derive from rocks, deposit structures, physical models, and analogies—not from a historically recorded event.

This is a good example of the boundary between a strongly supported geological model and direct observation.

Chapter summary

• Kimberlite is the principal natural transporter of diamonds from the deep mantle toward the surface.
• Diamonds are generally far older than the kimberlite that carries them.
• The resulting kimberlite is a hybrid rock containing a large proportion of entrained mantle and crustal material.
• Kimberlite ascent is geologically rapid, but its rate is not constant and there is no single universal value.
• Rapid transport helps preserve diamond but does not prevent all resorption.
• Some surface features may form during kimberlite transport, and others earlier in the mantle.
• A kimberlite system may include dikes, feeder zones, diatremes, craters, and multiple eruptive phases.
• Blue ground and yellow ground are historical mining terms, not universal modern rock classes.
• Lamproite is a distinct igneous group and may be diamond-bearing.
• Argyle is a key example of a major lamproite-hosted diamond deposit.
• Lamproite did not cause the pink color of Argyle diamonds; the color is associated with their deep deformation history.
• Grade alone does not determine the economic value of a deposit.
• No modern kimberlite eruption has been directly observed, so details of eruption dynamics are inferred from geological evidence and models.

[VISUAL 10.3: From the deep mantle to a mine—diamond entrainment → ascent → resorption → pipe/lamproite → erosion and exploration]