Journey to the Mantle: The Petrology of Kimberlite Pipes and the Genesis of Diamonds

The petrology of kimberlite pipes reveals a dramatic story of deep-earth processes and the formation of diamonds. Kimberlite is a complex, hybrid ultramafic rock, rich in volatiles like CO₂ and H₂O, that originates from partial melting in the mantle. Its defining characteristic is a texture comprising large, ancient crystals (macrocrysts and xenocrysts) such as olivine, pyrope garnet, and diamond, all set within a fine-grained groundmass that crystallized during its eruption. These macrocrysts are not native to the kimberlite magma but are fragments of mantle rock sampled during its ascent. The formation of kimberlite pipes is a catastrophic event. Driven by the rapid expansion of dissolved gases, the magma rockets towards the surface at supersonic speeds, creating deep, carrot-shaped diatremes. This rapid transit is essential, as it transports diamonds through regions of lower pressure so quickly that they do not have time to revert to graphite. The diamonds themselves are relics of an ancient Earth, formed 1 to 3.5 billion years ago under the immense pressure and temperature found 150-250 km beneath the oldest continental cores, or cratons. They reside in this mantle 'storage' for eons until a much younger kimberlite eruption provides them with a high-speed elevator to the surface, making kimberlite the ultimate geological messenger.

1. Introduction to Kimberlite: Earth's Diamond Messenger

Kimberlite is more than just a rock; it is a geological messenger, a high-speed elevator from the depths of the Earth's mantle. It is the primary igneous host rock for most of the world's diamonds, making its study—petrology—fundamental to understanding both deep-earth processes and economic geology. Kimberlites are exceptionally rare, with their violent, explosive emplacement providing a unique window into the sub-continental lithospheric mantle (SCLM), a region otherwise inaccessible. This text explores the intricate petrology of kimberlite, the violent formation of the pipes that contain it, and the ancient origin story of its precious cargo: diamonds.

2. The Petrology of Kimberlite: A Hybrid Mantle Rock

Kimberlite is defined as a potassic, ultramafic, volatile-rich hybrid igneous rock. Its complex nature stems from its origin as a small-degree partial melt in the asthenosphere, which then interacts with and incorporates a vast amount of material from the mantle and crust during its rapid ascent.

Mineralogical Composition: Kimberlite petrology is characterized by a distinctive inequigranular texture, featuring large crystals (macrocrysts) set within a fine-grained groundmass.

• Macrocrysts and Xenocrysts: These are large (0.5 mm to >10 cm) crystals that are often rounded and out of equilibrium with the magma that transported them. They are fragments of dissociated mantle rocks (xenoliths) like peridotite and eclogite. Key macrocrysts include:

Olivine: The most abundant mineral, often altered to serpentine. Phlogopite Mica: A potassium-bearing mica, indicating the potassic nature of the magma. Pyrope Garnet: Chromium-rich (purple-red) and calcium-rich varieties are crucial diamond indicator minerals (e.g., G9/G10 garnets). Cr-diopside: A bright green clinopyroxene. Ilmenite: An iron-titanium oxide. Diamond: The most famous xenocryst. It is entirely foreign to the kimberlite magma and is simply carried by it.

• Groundmass: The fine-grained matrix that crystallized from the kimberlite melt during its final emplacement. It is rich in minerals that reflect the magma's volatile-rich character, including:

Serpentine and Calcite: Secondary minerals formed by the alteration of olivine and the exsolution of CO2 from the melt. Perovskite (CaTiO₃): A key primary magmatic mineral, rich in trace elements. * Spinel and Apatite: Common accessory minerals.

This hybrid mineralogy tells a story: the macrocrysts are ancient passengers, while the groundmass is the vehicle that brought them to the surface.

3. Emplacement of Kimberlite Pipes: A Supersonic Eruption

Kimberlites erupt to the surface not as conventional lava flows, but in violent, gas-driven explosive events that form deep, carrot-shaped structures known as diatremes or pipes. This process is critical for the preservation of diamonds.

The Diatreme Model: A typical kimberlite pipe can be divided into three main zones:

1. Crater Facies: The uppermost, surface expression, often eroded away. It consists of pyroclastic material (tuff rings) and sediments deposited in a crater lake.
2. Diatreme Facies: The main, carrot-shaped body of the pipe, extending from near the surface to depths of 1-2 km. It is filled with a chaotic mixture of fragmented kimberlite, mantle xenoliths, and country rock torn from the pipe walls. The characteristic rock type is Tuffisitic Kimberlite Breccia (TKB), formed by a gas-solid fluidization process during the eruption.
3. Root Zone (Hypabyssal Facies): The feeder system below the diatreme, consisting of kimberlite dikes and sills. The rock here is more coherent and magmatic, having cooled before the main explosive phase.

The Emplacement Mechanism: The process begins with a small-volume melt deep in the mantle (>150 km). This magma is super-critically rich in dissolved volatiles, primarily CO₂ and H₂O. As it ascends, pressure decreases, causing these volatiles to exsolve catastrophically. This rapid gas expansion propels the magma upwards at estimated speeds of 10-30 m/s, potentially reaching supersonic velocities in the upper crust. This extreme speed is crucial; it prevents the diamonds, which are metastable at the surface, from converting back to their low-pressure polymorph, graphite.

4. The Genesis of Diamonds: Ancient Mantle Relics

A common misconception is that diamonds are formed within the kimberlite magma. In reality, diamonds are ancient xenocrysts, formed billions of years before the kimberlite eruption that brought them to the surface.

The Diamond Stability Field: Carbon has two main polymorphs: graphite (stable at low pressure) and diamond (stable at high pressure). Diamonds can only crystallize under the immense pressures and high temperatures found in a specific region of the upper mantle known as the Diamond Stability Field. This region typically exists at depths of 150-250 km, exclusively beneath the oldest, coldest, and thickest parts of continents: Archean cratons.

Age and Origin: By analyzing mineral inclusions trapped inside diamonds (e.g., tiny garnets, olivines), scientists can use radiometric dating techniques (e.g., Sm-Nd, Rb-Sr) to determine their age. These studies reveal that most diamonds are between 1 and 3.5 billion years old. In contrast, their host kimberlites are typically much younger, ranging from tens to hundreds of millions of years old. For example, the diamonds from the Kimberley mines in South Africa are ~3 billion years old, while the kimberlite pipes are only ~90 million years old.

This vast age gap proves that diamonds form and are stored in the deep lithospheric root of cratons for eons. A much later, unrelated thermal event generates the kimberlite magma, which, on its frantic journey to the surface, rips through this diamond-bearing region, incorporating the diamonds and their host-rock fragments into the melt.

Carbon Sources and Paragenesis: Isotopic studies of diamond carbon (δ¹³C) show two main sources: a 'peridotitic' source, with carbon from the primordial mantle, and an 'eclogitic' source, with carbon believed to originate from subducted oceanic crust that was recycled back into the mantle. This highlights the complex geological cycles involved in diamond formation.

Browse public study notes