Sep 15 2011 | Posted by SSSandy

Minerals from ocean-floor rocks found in ultra-deep diamonds

Mineral inclusions discovered in diamonds prove that surface rocks can be subducted into the deep part of the Earth’s mantle. The isotopic composition of the diamonds confirms that recycling of crustal materials, including carbon, extends into the lower mantle.

The theory of plate tectonics is at the centre of our understanding of how the Earth works. It has been known for decades that new crust is formed at mid-ocean ridges and that this crust is subducted as plates dive underneath other plates in regions such as the Pacific Ring of Fire and descend into the Earth’s mantle. What is not so well known is the fate of these subducted plates.

In this week's edition of the journal Science, scientists from the University of Bristol (Professor Michael Walter, Dr Simon Kohn, Dr Galina Bulanova, Mr Christopher Smith), Universidade de Brasilia (Professor Débora Araujo), and the Carnegie Institution of Washington (Dr Eloise Gaillou, Dr Junyue Wang, Dr Andrew Steele, Dr Steven Shirey), show that oceanic crust can make its way right down to the lower mantle (deeper than 660km) and then be transported back to the surface.

The samples studied are tiny inclusions of minerals trapped in diamonds from the Juina region of Brazil. Diamonds are extremely durable, and so they make excellent hosts for the trapped minerals they contain. However, the original minerals can change as the pressure and temperature conditions of the diamond change, and the inclusions record that history.

Professor Walter and colleagues discovered, for the first time, a set of mineral inclusions with compositions matching the entire mineral assemblage characteristic of oceanic crust subducted into the lower mantle (depth greater than ~ 700 km). Trapped originally as single mineral phases, the inclusions become multi-mineral assemblages upon uplift. The authors suggest that the diamonds were transported from the lower to upper mantle via large-scale upwelling beneath Brazil during the Cretaceous Era, possibly in a mantle plume. The diamonds were ultimately exhumed rapidly to the surface in kimberlite magmas (kimberlites are the main volcanic rock to transport diamonds to the surface).

The authors also observe that four of the six diamonds studied have extremely low amounts of 13C, a feature never previously seen in diamonds from the lower mantle. Low 13C is consistent with an origin of the carbon in oceanic crust at the Earth’s surface. These and future results from investigations of diamonds and their inclusions could transform our understanding of the oxidation state, volatile content, and geological history of the lower mantle. They certainly mean that recycling of crustal materials, including carbon, is not limited to the upper mantle but extends deeper into the lower mantle.

Dr Simon Kohn said: “The amazing thing about the diamonds from Juina is that each new batch we study provides something unexpected. As we investigate them in more detail with new techniques they continue to give up more of their remarkable secrets.”

Professor Michael Walter said: “Inclusions in diamonds are fantastically useful for studying the inaccessible part of the deep Earth. It’s a bit like studying extinct insects in amber. Although we can’t extract DNA and grow dinosaurs, we can extract their chemical compositions and tell where they formed by growing minerals in the lab at extreme conditions.”

Professor Débora Araujo said: “It is really exciting to see Brazilian diamonds playing such an important role in this scientific breakthrough. Samples from this region have been investigated for several years and yet we are not running out of exciting new discoveries. We are all very pleased to be involved in such a successful international collaboration”

Dr Steven Shirey said: “I find it astonishing that we can use the tiniest of mineral grains to show some of the largest scale motions of the Earth's mantle.”