Natural diamonds form deep in Earth’s mantle and can remain there for billions of years. Brought to the surface in kimberlite magmas, natural diamonds are important geological samples since they represent both products and long-term residents of the mantle. Many natural diamonds contain inclusions, which are small quantities of other minerals trapped in resilient carbon crystal of diamond. These inclusions are geologic snapshots of Earth’s geologic past, and, combined with chemical and structural analysis of the diamond itself, provide information about the conditions under which the diamond (and associated inclusions) formed.
In a recent paper, Galina Bulanova (University of Bristol, UK) and colleagues report a striking generational zonation of carbon isotopes and mineral inclusion compositions within a single diamond from the Mir kimberlite pipe (Russia) . The core of the diamond, depleted in 13C, formed at 180 km depth during a time of active subduction at the margins of the Siberian craton 2.1 billion years ago. The intermediate and rim zones of the diamond, with normal mantle carbon isotopes, formed a billion years later at a depth of 120 km, during extension and uplift of the sub-Siberian mantle.
The diamond thus records stages of growth, separated in space and time, in distinct geodynamic environments. This study, combining optical mineralogy, isotope and trace element geochemistry, and spectroscopy provides an ideal example of the geologic history contained within a single diamond when a wide spectrum of collaborative methods are brought to bear to study a single stone.
Image: Mir Diamond Mine, Siberia, Russia. Wikimedia Commons.