Water, Water, Everywhere

This is simply amazing! It appears that there is lots of water in the mantle of the Earth at a depth of 650 miles (1000 kilometers). The amount of water is believed to be as much as three times the water in all the oceans.

How was this water discovered? The answer is in indirect evidence. Researchers (Graham Pearson, a geochemist at the University of Alberta, Canada) found a unique diamond packed in ringwoodite containing water at Juina, Brazil. In other words the diamond contained a water rich inclusion of ringwoodite. Okay, what's this?

Ringwoodite is a high-pressure phase of magnesium silicon oxide Mg2SiO4. This blue polymorphous (exists in several forms) material is formed by high temperature and pressures that exists at a depth of 525 to 660 kilometers in the Earth's mantle (the hot rock layer between the crust and the core). This is the transition zone and water cannot exist below here. The transition layer is the boundary between the lower and upper mantle.

Diamond is a crystalline form of carbon that is only produced by high temperatures and pressures that exist at depths of 140 to160 kilometers of the Earth's mantle.

So, how could this ringwoodite that's formed in the mantle hold water? The ringwoodite would have to get rid of trapped water before it sunk to the lower mantle because the temperature there would be too high. At the transition between the upper and lower mantel the ringwoodite would transform into a silicate perovskite (Mg,Fe( SiO3) and CaSiO3) producing a melt. This perovskite is a large part of the lower mantle. The Magnesium Iron silicate is the most abundant material on Earth. The theory is that this conversion of ringwoodite to perovskite is where the ringwoodite gets rid of its trapped water before sinking into the lower mantle. Thus, the vast amounts of water lie in the upper mantle.

Earth's crust gets recycled because tectonic activity pushes and pulls slabs of oceanic crust into subduction zones (that goes under). This crust is soaked with water and this is how the water gets down to the upper mantle. The tectonic process depends on this vast water ocean in the mantle. Without it we would not exist because tectonic plate movements created the landmasses that we live on. What this means is that the mantle contains a vast reservoir of water.

How did the unique diamond end up in Brazil, essentially a 525-kilometer ride to the surface? That happened because of volcanic eruptions, the same eruption that created the Brazilian diamond mine. The reason why the diamond survived the torturous trip is because diamonds are very hard, one of the hardest materials known. It's as if this amazing diamond captured some of the mantle material (ringwoodite) containing water and brought it up so that it could be discovered.

A lot of the research work to understand the material in the mantle and how it transitions was done in the lab using powerful laser and pressure chambers used to make synthetic diamonds.

My take on this is that it's exciting and proves that there is much we still don't know about the planet we live on. It also suggests that the water in our oceans may not have come from comets and asteroids but rather formed with the Earth in the beginning.

Thanks for reading.