Ringwoodite and the Deep Water Cycle

Oceans cover 71% of the Earth’s surface. They play an important role in the water cycle that circulates water through the atmosphere. But did you know that there’s also a deep water cycle? It describes how tectonic plates water carry water into the Earth’s mantle. There, it’s absorbed by a mineral called ringwoodite. Later, the water escapes the ringwoodite and travels back to the surface inside magma.

What is ringwoodite? How does it form? 

Until recently, scientists had never seen ringwoodite from inside the Earth. To study the mineral, they had to synthesize it in a lab or extract it from meteorites. In 2014, a tiny sample of the mineral was extracted from the Earth for the first time. A grain was found inside a diamond mined in Brazil. The tiny fragment was less than 40 micrometres long.

Did you know?

If you like jewellery, you may be familiar with the gemstone peridot, which is the gem form of olivine!

The diamond originally came from the transition zone of the Earth’s mantle. The transition zone is located several hundred kilometres below the surface. That’s where high temperatures and pressures create ringwoodite out of olivine, another mineral. 

Are you familiar with the theory of plate tectonics? It describes how the Earth’s surface is made up of rocky plates. These plates sit on top of the mantle. Scientists think that the olivine that became the ringwoodite in the Brazilian diamond came from one of these plates. The plate was located at the bottom of the ocean. It pulled the olivine deep below the surface. It also brought some ocean water with it. 

In the transition zone, the olivine was heated and compressed into ringwoodite. Eventually, a diamond formed around the ringwoodite. Then, a volcano brought the diamond up to the surface.

Did you know?

Traces of ringwoodite have been found in stones from the Tenham meteorite shower.

Ringwoodite can absorb water. But it can’t absorb water in liquid, solid or gas form! High temperatures and pressures don’t just transform olivine into ringwoodite. They also cause water molecules to split. This creates hydroxyl radicals (-OH). Pores in the ringwoodite absorb the hydroxyl radicals. This traps the broken-down water inside the mineral’s molecular structure.

There were hydroxyl radicals in the ringwoodite trapped in the Brazillian diamond. The weight of the water was about 1% of the total weight of the sample. Scientists compared this to the total amount of ringwoodite estimated to be in the Earth’s mantle. It looks like the amount of water deep below the surface of the planet is about equal to the amount in the oceans!

What Is Ringwoodite’s Role in the Deep Water Cycle?

The deep water cycle circulates ocean water deep below the planet’s surface and back up again.
Water is carried below the Earth’s surface at subduction zones. Subduction happens when an oceanic plate slides under a continental plate. 

Below the surface, water helps create magma and form volcanoes. When a volcano erupts, the magma reaches the surface in the form of lava.  

Researchers have looked at how magma forms in the transition zone. First, they analyzed seismic data related to earthquakes and volcanic activity. Then, they synthesized ringwoodite in their lab. They exposed the mineral to the same high temperatures and pressures found in the transition zone. The ringwoodite began to melt, releasing some of its water. The water decreased the melting point of the rock, and this allowed magma to form.

People can’t really use the water found deep below the Earth’s surface. But it’s important to understand how water travels from the surface to the mantle and back up again. 

The deep water cycle is an important way that the planet cycles its water. It also highlights how tectonic plates move and how volcanoes form. These processes show how water, gases and minerals are constantly moving between the Earth’s surface and its interior.

Did you know?

In 2018, scientists discovered the first freely formed H₂O molecules in the Earth’s mantle. The molecules were compressed within diamonds as fragments of water-ice called “ice-VII.”