The mantle – the mostly solid, rocky part of Earth’s interior – is about 60 degrees Celsius hotter than previously thought, a new study has found.
The findings led by Woods Hole Oceanographic Institution (WHOI) in the US could change how scientists think about many issues in Earth science including how ocean basins form
A 60-degree increase may not sound like a lot compared to a molten mantle temperature of more than 1,400 degrees Celsius, researchers said.
However, the result is significant as a hotter mantle would be more fluid, helping to explain the movement of rigid tectonic plates, they said.
Since it is not possible to measure the mantle’s temperature directly, geologists have to estimate it through laboratory experiments that simulate the high pressures and temperatures inside the Earth.
Water is a critical component of the equation: the more water (or hydrogen) in rock, the lower the temperature at which it will melt.
The peridotite rock that makes up the upper mantle is known to contain a small amount of water.
To figure out how the water content of mantle rock affects its melting point, Emily Sarafian, a graduate student in the MIT-WHOI Joint Programme conducted a series of lab experiments.
She used a piston-cylinder apparatus, a machine that uses electrical current, heavy metal plates, and stacks of pistons in order to magnify force to recreate the high temperatures and pressures found deep inside the Earth.
Following standard experimental methodology, Sarafian created a synthetic mantle sample. She used a known, standardised mineral composition and dried it out in an oven to remove as much water as possible.
Sarafian modified her starting sample by adding spheres of a mineral called olivine, which occurs naturally in the mantle.
The spheres were large enough for Sarafian to analyse their water content using secondary ion mass spectrometry (SIMS).
From there, she was able to calculate the water content of her entire starting sample.
To her surprise, she found it contained approximately the same amount of water known to be in the mantle.
Based on her results, Sarafian concluded that mantle melting had to be starting at a shallower depth under the seafloor than previously expected.
To verify her results, Sarafian turned magnetotellurics – a technique that analyses the electrical conductivity of the crust and mantle under the seafloor.
Reconciling the temperatures and pressures Sarafian measured in her experiments with the melting depth from an earlier study led her to a startling conclusion: The oceanic upper mantle must be 60 degrees Celsius hotter than current estimates.