Why Don't Many Volcanoes Create Diamonds?

Understanding the Formation of Diamonds in Volcanic Settings

The occurrence of diamonds in volcanic rocks, particularly kimberlites and lamproites, is a fascinating subject in geology. Despite the connection often made between volcanoes and diamond formation, it is important to clarify that not all volcanoes generate diamonds. In fact, many volcanoes do not produce diamonds because of the unique conditions necessary for diamond formation. Let’s delve into what leads to diamond creation and why it doesn’t happen in most volcanic eruptions.

The Role of Kimberlites and Lamproites

Kimberlites and Lamproites: Specialized Volcanic Rocks

Kimberlites and lamproites are specialized types of volcanic rocks that play a crucial role in the transport of diamonds from their deep mantle sources to the Earth’s surface. These rocks are created in specific geological conditions and environments that favor the occurrence of diamonds. Kimberlites are small to large, pipe-like intrusions that originate from the deep mantle, while lamproites are similar but typically form larger, more extensive intrusions. Both types of rocks can carry diamonds as they rise to the surface, making them valuable in the search for diamond deposits.

Why Don't Most Volcanoes Create Diamonds?

Conditions for Diamond Formation in Magma

The vast majority of volcanic eruptions involve magma produced by shallow melting processes, typically at depths of 30-100 kilometers. This magma is stored in magma chambers at depths of 3-10 kilometers, where temperatures range between 800-1200°C. Given these conditions, diamonds are highly unstable and tend to sublimate or resorb into CO2 or graphite, a process that occurs above 800°C at low pressure. Therefore, the high temperatures and lower pressures within most volcanic environments make it almost impossible for diamonds to form or survive.

The Importance of Depth and Pressure

For diamonds to form, the conditions must be both high pressure and lower temperature. This is especially true for diamond stability at depths of around 120-200 kilometers in the mantle, where the critical conditions for diamond formation are met. The lower melting point and high density of magma at such depths allow for the preservation of diamonds. However, this is far below the usual depths of volcanic eruptions, making diamond formation unlikely.

Magmatic Origins and Rapid Ascent

Magma Composition and Ascent

For diamonds to be transported from the deep mantle to the Earth’s surface, the magma involved must have the right composition. Volcanoes that generate diamonds often have magmas that are highly volatile, containing significant amounts of CO2 and H2O. These volatile components allow the magma to ascend explosively, especially in the final stages of the eruption. The explosiveness of the ascent helps maintain the high pressures necessary for diamond formation, and the rapid cooling at the surface helps preserve the diamonds.

Depth and Composition of Magmas

The magmatic origins of diamond-bearing systems must start at depths of around 120-200 kilometers, sometimes extending to depths of 600 kilometers. This deep origin is necessary to intersect the diamond-bearing lithosphere and bring diamonds to the surface. The magmas must also ascend quickly, often within hours, before the diamonds sublimate or resorb into CO2 or graphite. The rapid ascent is a critical factor in ensuring that diamonds are not decomposed or lost during the eruption process.

The Role of Continental Lithosphere

Continental Lithosphere and Diamond Stability

In addition to the magmatic and chemical conditions, the continental lithosphere must have specific characteristics to generate diamonds. The presence of an "old, cold, and thick" continental lithosphere is crucial for creating the conditions where diamonds can form and remain stable. This type of lithosphere provides the necessary pressure and low temperature environment for diamond crystallization. The cold lithosphere prevents the decompression and resorption of diamonds, allowing them to remain in a stable form and be transported to the surface.

Formation of Diamonds in Continental Lithosphere

The process of diamond formation in continental lithosphere begins with the deep-seated magma rising and intersecting the diamond-bearing layers. The rapid ascent and high pressure maintain the stability of diamonds, and the cold surface environment provides the necessary cooling for preservation. This combination of factors is what makes kimberlites and lamproites such valuable geological indicators of diamond-bearing areas.

Conclusion

While many volcanoes are celebrated for their dramatic eruptions and volcanic landscapes, not all of them generate diamonds. The unique combination of depth, pressure, and temperature, as well as the specialized composition of magmas, is what allows diamonds to form and survive in certain volcanic environments. The role of specialized rocks like kimberlites and lamproites and the specific geological and chemical conditions they require for diamond formation emphasizes the rarity and value of diamond-bearing volcanic deposits.

Keywords: Volcanoes, Diamonds, Kimberlites, Lamproites, Magmatic Eruption