Raman spectroscopy confirms an ocean's worth of water in the Earth's lower mantle

In recent years, Renishaw Raman microscopes located at two prestigious geological institutions, have been used for the non-destructive analysis of inclusions in ultradeep diamonds from the Earth's lower mantle. Diamond inclusions are an important chemically inert vessel, preserving the original structure and chemistry of mineral fragments formed under high pressure. Petrological studies using Raman spectroscopy have produced an increasing body of evidence confirming that large amounts of water exist in the Earth's mantle.

How much water exists within the Earth?

The water content in the Earth's mantle is critical to the dynamics of the Earth's interior.¹ Water lowers the internal friction of rocks and plays a key role in geological phenomena such as earthquakes, water weakening, dehydration embrittlement, fracturing, and mantle convection. Accurate quantification of the water content in the Earth's mantle is important, as a small deviation can significantly change our understanding of geological processes. Deep in the Earth, a discontinuity occurs between the mantle transition zone and the lower mantle. Scientists can detect this global 660 km boundary by the abrupt changes in density and seismic wave velocities. Few samples exist from the 660 km boundary, so the chemical composition of these different depths is still debated. Questions remain about whether volatile chemicals like water can move across the transition zone boundary into the lower mantle. Understanding the flux of water across this boundary also informs our understanding of the global water cycle.

First hints of a hydrous transition zone in the Earth's mantle

In 2013, terrestrial ringwoodite was discovered within a diamond inclusion from Juina, Brazil.¹ Ringwoodite is a high-pressure phase of (Mg,Fe)₂SiO₄. Ringwoodite has the same chemical composition as olivine, but with a spinel structure, due to increased pressure. The spinel structure of ringwoodite enables it to store a massive amount of water in the Earth's mantle. Previously, ringwoodite had only ever been synthesized in laboratories or observed in meteorites. Raman spectroscopy of terrestrial ringwoodite was first performed at the Geoscience Institute, Goethe University, Germany, using a Renishaw RM1000 Raman microscope. This identified the diamond inclusion as ringwoodite from the 660 km boundary. Next, Fourier transform infrared (FTIR) spectroscopy indicated approximately 1.5% water content, which approaches water saturation within the transition zone. At the time, it was unclear if the hydrous content from this sample is broadly representative of the transition zone.