June Editorial      

Using diamonds to find out about platinum.

Platinum group elements (PGEs) are more commonly known as platinum metals. They have outstanding catalytic properties and are often used as pollution-control devices in cars. On the other hand platinum metals are highly resistant to wear and tarnish, and platinum in particular makes fine jewellery. Other distinctive properties of platinum metals include resistance to chemical attack, excellent high-temperature characteristics, and stable electrical properties. All of these properties have industrial applications. With both jewellers and an increasing number manufacturers wanting PGEs it is not surprising that these metals are a very sought-after commodity. The six platinum group metals (ruthenium, rhodium, palladium, osmium, iridium, and platinum) have similar physical and chemical properties, and tend to occur together in the same mineral deposits. Unfortunately, another characteristic feature which all PGEs share is rarity. Platinum, which is the most abundant PGE, is 30 times rarer than gold.

The 2.05-billion-year-old Bushveld Complex in South Africa is the world's largest single PGE repository and was therefore the focus of a recent joint research project involving the University of Cape Town and Carnegie Institution of Washington. The aim was to unravel where the PGEs in the Bushveld Complex came from. The tools used for the job were 2 billion year-old diamonds which had been mined nearby. When diamonds are formed, traces of PGEs ('inclusions') can be found trapped within them. These rare metal traces give the diamonds their unique isotopic 'signatures' which help geologists determine where the diamonds were formed and how old they are. In reverse, diamonds from a particular region can help with understanding the origin of local PGEs. Stephen H. Richardson of the University of Cape Town and Steven B. Shirey of the Carnegie Institution studied PGE inclusions in about 20 diamonds collected near the Bushveld Complex. They found that the isotopic signatures of the PGEs in the diamonds and Bushveld ore minerals match, showing the main source of Bushveld platinum to be mantle, not crust falling into the magma chamber as previously thought.

Their results which have just been published in the journal Nature (Nature 453, 910-913 (12 June 2008) ) contradicts the earlier belief that the source of PGE deposits was the continental crust. The studies also help to explain the richness of PGEs in this region. As Stephen Richardson explained: "The old subcontinental mantle has a higher PGE content than the crust and there is more of it for Bushveld magmas to traverse and pick up the PGEs found in the ores."

Beyond providing insight into the source of PGEs in the Bushveld complex, these results will also help in understand how PGEs are concentrated under the earth's surface, which should help in finding new sources. That is certainly good news for miners, industrialists and jewellers.