While the ocean’s crust is constantly forming as new magma forces its way up between diverging tectonic plates lying beneath the sea, scientists are keen to find out what happens to the outlying crust that is then pushed back into the centre of the Earth.

‘Exactly what happens to subducted oceanic crust is a long-standing question in Earth sciences,’ said the lead author of the research, Dr Michael Walter of the department of earth sciences at the University of Bristol.

Scientists know that oceanic crust sinks to 3,000 kilometres below the Earth’s surface and can stay there for billions of years, but they wanted to find out why it develops a different ‘flavour’ from the surrounding magma.

They believed it was because the crust that forms on ocean floors reacts with the sea to form carbonate minerals. These have the effect of slightly lowering the melting point of the rock, allowing it to melt, even at the high pressures found deep underground, and therefore absorb the surrounding rock’s ‘flavours’.

The authors of the report were able to confirm this theory using diamonds, which form at high pressures underground and the slowly travel up to the surface. X-ray analysis showed that they had crystallised from melted rocks, which have probably also been ‘flavouring’ the mantle deep inside the Earth.

November 2008