Komatiite

Komatiite is a rare and distinctive type of ultramafic volcanic rock. It is defined by its origin in the Earth's mantle and its extremely high magnesium content. Specifically, a rock is classified as komatiite if its lava contains at least 18 wt% magnesium oxide (MgO). Because of this high magnesium, scientists also classify it as a picritic rock. These rocks are unique because they have very low levels of silicon, potassium, and aluminium.

Studying komatiites helps geologists understand the thermal history of our planet.

The formation of komatiite requires a specific and intense melting process. It is created through high degrees of partial melting, which means more than 50% of the source rock melts. This process produces a magma with a very high melting point. Calculated eruption temperatures can reach or even exceed 1600 °C. For comparison, common basaltic lavas usually erupt at temperatures between 1100 and 1250 °C. This intense heat makes the resulting lava extremely fluid. While basaltic lava flows like honey or treacle, komatiitic lava has a viscosity close to that of water. This allows it to flow swiftly and create very thin flows, sometimes only 10 mm thick.

Komatiites are categorized into two geochemical classes based on their aluminium and titanium levels. The first class is aluminium undepleted komatiite (AUDK), also known as Group I. The second class is aluminium depleted komatiite (ADK), or Group II. Scientists define these groups using their Al2O3/TiO2 ratios. It was once thought that these groups represented different depths of melting. In ADK, high pressure might prevent garnet from melting in the source. In AUDK, melting likely occurs at a lesser depth. However, recent studies of fluid inclusions in chrome spinels suggest a single flow might actually mix different parental magmas.

Most komatiites are ancient relics from the Archaean Eon, occurring between 4.03 and 2.5 billion years ago. They are rarely found in younger Proterozoic or Phanerozoic rocks. This age restriction is likely due to the cooling of the Earth's mantle over time. The early Earth had much higher heat production than it does today. This heat came from residual energy left over from planetary accretion. Additionally, the early mantle had a greater abundance of radioactive isotopes. Specifically, shorter-lived isotopes like uranium 235 produced significant decay heat.

Geographically, these rocks are mostly found in Archaean shield areas and greenstone belts. The rock was named after its type locality along the Komati River in South Africa. Other notable locations include the Abitibi greenstone belt in Ontario, Canada. Rare examples also exist in the Winnipegosis komatiite belt in Manitoba, Canada, and on the island of Gorgona near Colombia.

These sites provide essential data on how mantle plumes or subduction zones functioned billions of years ago.

A defining feature of many komatiites is their unique spinifex texture. This texture consists of large, dendritic, or blade-like crystals of olivine and pyroxene. The name comes from the Australian grass, Triodia, which grows in similar clumps. This texture forms due to the rapid crystallization of olivine within a thermal gradient in the upper part of a lava flow.

In a typical flow, the mineralogy changes from the bottom to the top. The base often contains olivine cumulates, while the upper zones show the spinifex texture. Some samples may even show a volcanic glass chill zone at the very top.

Because they are so old, almost all known komatiites have undergone metamorphism. This means they have been changed by heat and fluids, and are technically called metakomatiites. These changes are often categorized as either hydration or carbonation. Carbonated komatiites form when the partial pressure of carbon dioxide (XCO2) is above 0.5. This process creates minerals like talc, magnesite, and tremolite. Hydrated komatiites form when XCO2 is below 0.5 and water is present. This results in minerals such as serpentine, chlorite, and brucite. These metamorphic processes can significantly alter the original texture and mineral makeup of the rock.