Measuring viscosity in the Earth’s Mantle

The earth seems to be fairly solid as we stand on its surface. We stand on solid ground and do not sink through the floors in our homes. However, underneath that solid-looking crust are layers where the rock is a liquid because the surroundings are that hot. Underneath the crust is the upper mantle, which extends down to a depth of approximately 255 miles. Most of the upper mantle is solid. Underneath the upper mantle is the transition zone, which extends from 255 miles to 410 miles in depth. At this point, rocks go through a radical transformation. The rocks’ crystalline structure changes, and they become much denser. Next is the lower mantel which extends from 410 miles to 1,678 miles below the surface of the earth. It is hotter and denser than the layers above. While it is very hot, the high pressure keeps the layer solid.

The research team of Dr. Longjian Xie from the Bavarian Research Institute of Experimental Geochemistry & Geophysics (BGI) of the University of Bayreuth have conducted extensive research into the composition of the earth. Key to their research was examining the role viscosity plays in changes to the earth’s structure over the millennia.

This international research team is the first to measure the viscosity that molten rocks display when they experience the extreme temperature and pressure conditions like those found in the lower mantle. The findings are discussed in more detail in the article “Formation of Bridgmanite-Enriched Layer at the Top Lower-Mantle during Magma Ocean Solidification” .

The research team developed a heating element based on an electrically conductive diamond to use in their work. This element assisted in their examination of material samples experiencing up to 30 giga-pascals of pressure as well as temperatures close to 3,000 degrees Celsius. Utilizing these conditions in the research was very important as they mimic those conditions that could be found in the lower mantle early in the earth’s history. As well, the composition of the material samples chosen was similar to the minerals that are found in the lower mantle.

The melting processes taking place under these pressure and temperature conditions were filmed using an extremely fast camera. From there, the viscosity of the molten samples was measured. The data collected supported the assumption agreed upon by many scientists that a bridgmanite-enriched rock layer formed approximately 621 miles below the surface of the earth. This rock layer formed during the earth’s early history at the border of the upper mantle.

While scientists have known about the mineral bridgmanite for several decades, it was just recently named. The mineral is found deep beneath the surface of the earth, but it was identified and studied within a meteorite when that meteorite collided with the earth. Believed to be the most abundant mineral in the earth, scientists have studied it indirectly when measuring changes in earthquake waves as the waves travel through the planet. Bridgmanite forms from intense pressure and heat like that found deep inside the earth. The conditions are so extreme that it causes atoms to arrange in a unique pattern.

The data collected indicates that the large reservoirs found in the lower mantle contain material that began in a magma ocean at a much earlier time. These materials have not changed throughout the history of the earth. Magma oceans exist during a planet’s gradual growth when that planet is partly or completely molten. Magma oceans are key components of the formation of planets as they facilitate the formation of the core of the planet. The oceans may survive anywhere from millions to tens of millions of years. Scientists generally accept the idea that magma oceans once existed on the earth. The best chemical evidence for magma oceans in the profusion of siderophile elements in the earth’s mantle that record magma ocean depths of up to 1000 kilometers during the gradual growth of the planet. Siderophile elements are the transition metals that typically sink down into the core because they easily dissolve in iron. These elements include cobalt, gold, iridium, nickel, osmium, palladium, platinum, rhenium, rhodium, and ruthenium.

The earth’s mantle, throughout the earth’s history, formed from the magma ocean. The viscosity level measurements indicate that the crystallization of the magma ocean depended largely on pressure levels. The result was a fractional crystallization, which occurred approximately 1,000 kilometers below the surface of the earth. The researchers’ viscosity measurements demonstrate that a rock layer composed mainly of bridgmanite formed at this depth because of the crystallization processes. Researchers believe that this layer of rock could be the reason for the high viscosity levels measured at the depth during previous investigations.

Further information: Longjian Xie et al, Formation of bridgmanite-enriched layer at the top lower-mantle during magma ocean solidification, Nature Communications (2020). DOI: 10.1038/s41467-019-14071-8