Diamonds that formed deep in the Earth’s mantle contain clues of chemical reactions that occurred on the seafloor and can help geoscientists understand how material is exchanged between the planet’s surface and its depths.
A new study published in Science Advances confirms that serpentinite – a rock that forms from peridotite, the main type of rock in Earth’s mantle, when water penetrates cracks in the ocean floor – can carry surface water as far as 700 kilometers deep by plate tectonics.
During subduction, nearly all tectonic plates that make up the seafloor eventually bend and slide down into the mantle, which can recycle surface materials such as water, into the Earth.
Serpentine contained inside subducting plates may be one of the most significant, yet poorly known, geochemical pathways by which surface materials are captured and sent into the Earth’s depths. The presence of deeply-subducted serpentines was previously suspected – based on research conducted by Carnegie and GIA about the origin of blue diamonds and to the chemical composition of erupted mantle material that makes up mid-ocean ridges, seamounts and ocean islands. However, until now, evidence demonstrating this pathway had not been fully confirmed.
They found this evidence by studying a type of large diamond that forms between 360 and 750 kilometers down, at least as deep as the transition zone between the upper and lower mantle. Sometimes they contain inclusions of tiny minerals that indicate what is happening at these extreme depths.
The inclusion point to magnetite and other iron-rich minerals formed when oceanic plate peridotite transformed to serpentine on the seafloor. This rock was eventually subducted hundreds of kilometers down into the mantle transition zone.
These findings confirm a long-suspected pathway for deep-Earth recycling.
https://phys.org/news/2021-03-deep-diamonds-evidence-deep-earth-recycling.html