Flawless diamonds have long been sought as gemstones for their beauty and clarity. For a geologist, however, diamonds filled with mineral deposits (or inclusions) provide a chance to see a frozen snapshot of extreme chemical and physical processes that have occurred in the earths mantle. Researchers in the United Kingdom, Germany, United States, and Brazil have characterized inclusions in diamonds from Brazil to determine the ultimate origin of the mineral deposits [Walter, et al., Nature (2008) 454, 622].
The rocks were mined from an area near Juína, Brazil containing kimberlite, an igneous rock formed deep in the earths mantle known to be rich in diamonds. With a focused synchrotron X-ray beam less than half the width of a human hair the team determined that the inclusions were largely silicate perovskites composed of CaSiO3 along with some CaTiO3.
The geochemistry suggested that the inclusions came from subducted carbonate oceanic crust that had been recycled into the mantle. Oceanic crust is constantly replenished by magma surfacing at mid-ocean ridges. Over millions of years, the crust reacts with saltwater to form carbonate minerals which eventually sink back into the mantle in regions called subduction zones. Carbonates tend to lower the melting point of the magma it encounters. It is thought that this melt is loaded with elements that carry the crustal ‘flavour’.
Results, backed up by further experiments enabled the research team to show that the diamonds and their perovskite inclusions had indeed crystallised from very small-degree melts in the Earth's mantle