Florida State University geologists for Earth, Ocean, and Atmospheric Sciences have discovered how carbon-rich molten rock in the Earth’s upper mantle can affect the movement of seismic waves. The new study is giving scientists more insight into the earth’s carbon cycle. The results of the study were published in the journal Proceedings of the National Academy of Sciences.
Scientists stress the importance of their research because carbon is an important component of the planet’s habitability. And the new research is bringing them closer to understanding how solid earth could have played a role in storing and influencing the availability of carbon on our planet’s surface, thus making it habitable.
Our research gives us a better understanding of the elasticity, density, and compressibility of these rocks and their role in the Earth’s carbon cycle.
Mainak Mukherjee, Associate Professor of Geology, EOAS
Carbon, one of the main building blocks of creation for life, is widespread throughout the Earth’s upper mantle and is mainly stored as carbonate minerals as additional minerals in mantle rocks. When carbonate-rich magma breaks out to the surface, it is distinguished by its unique mud appearance. These types of eruptions occur in specific locations around the world, such as the Ol Doinyo Lengai volcano in Tanzania.
Experts believe that the presence of carbonates in rocks significantly reduces the temperature at which they melt. Carbonates that subduct into the interior of the Earth are likely to cause low melting in the Earth’s upper mantle. It plays an important role in the deep carbon cycle of the planet.
There is less free oxygen in the Earth’s mantle, available at increasing depths. As the mantle rises upward due to the process of mantle convection, slowly moving rocks that have been reduced or had less oxygen at deeper depths gradually become more oxidized at a shallower depth.
This change, depending on the depth of the oxidation state, can cause the melting of mantle rocks – redox melting, which can lead to the formation of carbon-rich molten rock – melts. These melts can affect the physical properties of the rock, which can be detected with geophysical probes.
Prior to this study, geologists knew little about the properties of these carbonate-induced partial melts, making them difficult to detect directly.
One set of clues that geologists use to better understand their science is measuring seismic waves traveling through the layers of the Earth. A type of seismic wave, a compression wave, is faster than another type known as a shear wave, but at depths between 180 and 330 kilometers from Earth, the ratio of their velocities is even higher than usual.
This increased ratio of compressional to shear waves has been a mystery to scientists and, using our research, they can explain this surprising observation.
The researchers explained that trace amounts of carbon-rich melts, roughly 0.05%, can spread ubiquitously through the deep upper mantle of the Earth, which could lead to an increased ratio of compression to shear sound speed.
To conduct the study, the scientists performed high-pressure ultrasonic and density measurements on carbonate mineral dolomite cores. These experiments were complemented by theoretical simulations to provide new insights into the fundamental physical properties of carbonate melts.
Researchers have tried to understand the elastic and transport properties of aqueous fluids, the properties of silicate melt, and molten metal in order to better understand the mass of volatiles stored in deep solid earth.
These data mean that partially molten rocks in the mantle can contain up to 80-140 parts per million carbon, which is 20 to 36 million gigatons of carbon in the deep region of the upper mantle. This makes it an essential carbon reservoir. By comparison, Earth’s atmosphere contains just over 410 ppm carbon or about 870 gigatons. Finding such a large amount of carbon under the mantle could be one of the factors that led to the origin of life on Earth.