Large irregular Type IIa and Type IIb diamonds: valuable sublithospheric gems that elude conventional indicators

Abstract

Two diamond varieties are described in this review paper whose geological origin has only recently been illuminated. The first is large irregular Type IIa (nitrogen-poor) diamonds, which includes many famous, high-quality gems such as the Cullinan and the Koh-i-Noor. This variety has been named CLIPPIR (Cullinan-like, large, inclusion poor, pure, irregular, resorbed) diamonds based on their distinguishing overall characteristics. Metallic Fe-Ni-C-S melt inclusions, which are thought to represent the growth medium, are the most common material trapped within them, followed by Ca-silicates and low-Cr majoritic garnet. Heavy iron isotope signatures show the metallic melt evolves from a subducted protolith phase such as magnetite or metal alloys formed by serpentinization. The second diamond variety is Type IIb (nitrogen-poor and boron-bearing) diamonds, which are often blue in color, such as the Hope diamond. They tend to occur in deposits containing abundant CLIPPIR diamonds. Inclusions in Type IIb diamonds range from meta-basaltic to meta-peridotitic assemblages, similar to what has been documented previously in sublithospheric diamonds, including Ca-silicates, ferropericlase, retrogressed bridgmanite, stishovite, CF phase (a Na-rich aluminosilicate), low-Cr majoritic garnet, as well as metal alloys, sulfides, and oxides. Boron isotopes support a model whereby the hydrous phases in cold slab meta-serpentinites break down as the slab warms, releasing boron and hydrous fluids that contribute to diamond growth. CLIPPIR and Type IIb diamonds are both established as sublithospheric (superdeep) and their formation involves the subduction of cold, seawater-altered oceanic lithosphere to the mantle transition zone and uppermost lower mantle. These diamonds can contribute significantly to mine revenue but are difficult to detect and predict because they do not correlate with conventional indicator minerals or with other diamonds, including micro-diamonds. Geochronology suggests that sublithospheric diamonds ascend in buoyant packages of rock and reside at the base of the continents before being sampled by kimberlites. CLIPPIR and Type IIb diamonds at surface might be accompanied by distinct sublithospheric indicator xenocrysts or signatures, though these may evolve and re-equilibrate during upper mantle storage. Exploration for sublithospheric diamond potential should focus on indicator minerals from the base of the lithosphere and the possibility of an accreted layer.