Komatiite

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Komatiite is a very magnesium-rich volcanic rock that comes from the Earth’s mantle. It is ultramafic and crystallized from lava with at least 18 percent magnesium oxide (MgO). Because of its high magnesium and low silicon, potassium and aluminum, it is often called a picritic rock. A distinctive feature of komatiite is the spinifex texture—long, blade-like crystals of olivine and pyroxene that grow in a thermal gradient during rapid cooling.

Komatiites are rare and most are from the very early Earth. The great majority formed during the Archaean Eon, about 4.0 to 2.5 billion years ago. This scarcity of younger examples is thought to reflect a hotter mantle in the Archaean, when higher temperatures allowed the unusually magnesium-rich melts to form and erupt.

The melts that produced komatiite were extremely hot, with eruption temperatures estimated up to or above 1600°C. Such high temperatures made the lava very fluid, so it flowed much like water, creating very thin sheets or channels instead of thick lava flows. When komatiite erupted underwater or near surface, it often formed pillow lavas and lava tubes rather than broad, extensive flows.

Geographically, komatiites are mostly found in ancient geological regions known as Archaean shields and greenstone belts. The youngest known komatiites come from Gorgona Island off Colombia, while a rare Proterozoic example is found in the Winnipegosis belt of Manitoba, Canada.

Chemically, komatiites reflect high degrees of partial melting of the mantle. They typically build rocks with high MgO and low potassium and other incompatible elements. Geochemists divide komatiites into two groups based on aluminum and titanium in the rock: aluminium undepleted komatiites (AUDK, Group I) and aluminium depleted komatiites (ADK, Group II). These groups were once thought to form from different depths of melting, but recent studies suggest a single komatiite flow can mix magmas with a range of compositions, which can blur the distinction between the two groups.

Komatiites are related to other ultra-mafic rocks, including boninite, and they provide clues about very hot mantle conditions in the early Earth. The pristine volcanic minerals in komatiites commonly include forsteritic olivine, calcium-rich pyroxenes, plagioclase, and chromite. Some komatiites show cumulate textures, where early crystals accumulate as the lava solidifies. The spinifex crystals indicate rapid crystallization in a high-temperature melt, while other rocks in the same flow can show deeply magnesian, cumulate textures.

Because all known komatiites have undergone metamorphism, they are often called metakomatiites. Metamorphic minerals in komatiites fall into two broad groups, depending on the fluids present during metamorphism. Carbonated metakomatiites form minerals like talc, magnesite and tremolite, while hydrated metakomatiites are dominated by serpentine, chlorite and brucite. The exact mineral mix inside a metakomatiite records the pressure and composition of the metamorphic fluids the rock experienced.

In terms of volcanic structure, komatiite eruptions are interpreted as forming shield-like volcanic edifices, but with an initial highly fluid, channelized flow that creates a basal magnesian olivine adcumulate zone and thinner, sheeted flow facies outward from the vent. As eruptions progress, the magma becomes less magnesian and the landscape shifts toward a more typical shield-volcano architecture.

A major reason komatiites are important today is their association with nickel-copper sulfide ore deposits. The Kambalda region in Western Australia, where nickel sulfide ore was first discovered on a komatiite, helped establish the economic value of these rocks. Today, komatiite-related nickel deposits contribute a significant portion of the world’s nickel production, with major mining areas in Australia, Canada and South Africa, and notable occurrences in other regions as well.

In short, komatiite is a rare, ancient, highly magnesian lava rock that flowed when Earth was much hotter. Its distinctive spinifex crystals, exceptionally high melting temperatures, and links to valuable mineral deposits make it an important key to understanding Earth’s early mantle and volcanic history.