Kimberlites

Kimberlites, while relatively rare, are perhaps the most interesting of igneous rocks and have accordingly attracted attention disproportionate to their abundance. They are the source of most diamonds and bring us direct sampling of Earth’s mantle (otherwise only available indirectly by geophysical methods). They are confined to the ancient cratons of the continents: the oldest and thickest parts of the continental crust (where the geothermal gradient is low) and are now known to occur on all the continents. They were not formed in the earliest phases of Earth’s history. Emplaced explosively in carrot‐shaped pipes, they are pyroclastic rocks close to the surface (often removed by erosion) but become hypabyssal (dykes and sills) at depth. Kimberlites belong to the ultramafic group of rocks with high MgO. They are consequently rich in olivine and lack feldspar. They are rich in volatiles, such as water and carbonate, and the above‐mentioned mantle xenoliths and megacrysts, including eclogite, harzburgite, olivine, mica, zircon, diamond and others, as well as crustal xenoliths. Kimberlites are thus hybrid rocks and the nature of the parent magmas is difficult to define. Related rocks, which may contain diamonds, are lamproites and ultramafic lamprophyres. A recent article by Tony Waltham in Geology Today discussed kimberlites and especially their diamonds; here we concentrate on kimberlites themselves with lesser emphasis on other diamondiferous rocks.