Geology, Physics

Understanding magma in the mantle: rocks melt at greater depth than previously thought

Magma forms much deeper than geologists previously believed, according to a new study conducted by Rice University.

Magma and Crust

 

mantle

The group led by geologist Rajdeep Dasgupta put very small samples of peridotite under very large pressures, to find out if the rock can liquify, at least in small amounts, as deep as 250 km beneath the ocean floor. Peridotite is the dominant rock of the Earth’s mantle above a depth of about 400 km, and it was previously believed that the mineral doesn’t liquify above that depth.

mantle structure

The Rice team focused on mantle beneath oceans because that is where the crust is typically formed; in a much simplified version, the silicate melts (magma) rise with the convective currents, cool as they reach the crust and then solidify to create new crust. The starting point for melting has long been thought to be at 70 kilometers beneath the seafloor.

Geophysics

In order to determine the mantle’s density and properties, geophysicists used seismic information; seismic waves from earthquakes travel the globe much like sound waves, and by measuring their speed, we can estimate some the medium’s properties. These waves travel faster in solids (especially in denser solids), and slower in liquids; the first questions arose here.

Seismologists have observed anomalies in their velocity data as deep as 200 kilometers beneath the ocean floor,” Dasgupta said. “Based on our work, we show that trace amounts of magma are generated at this depth, which would potentially explain that.”

seismic wave

So it didn’t really add up – and this wasn’t the only clue. Geophysicists have also struggled to explain the bulk electrical conductivity of the oceanic mantle – something which was observed, but couldn’t really be figured out.

“The magma at such depths has a high enough amount of dissolved carbon dioxide that its conductivity is very high,” Dasgupta said. “As a consequence, we can explain the conductivity of the mantle, which we knew was very high but always struggled to explain.”

The thing is, we cannot really dig down to the mantle – this is miles away from happening, both figuratively and literally, so we have to rely on indirect measurements (seismology, electric measurements, etc), lab experiments and surface extrapolations. Another interesting they found in this experiment was that carbonated rocks melt at significantly lower temperatures than non-carbonated.

“This deep melting makes the silicate differentiation of the planet much more efficient than previously thought,” Dasgupta said. “Not only that, this deep magma is the main agent to bring all the key ingredients for life — water and carbon — to the surface of the Earth.”

Volcanic windows

However, Dasgupta believes that volcanic rocks are the key to understanding our planet’s mantle.

“Our field of research is called experimental petrology,” he said. “We have all the necessary tools to simulate very high pressures (up to nearly 750,000 pounds per square inch for these experiments) and temperatures. We can subject small amounts of rock samples to these conditions and see what happens.”

A surfaced volcanic rock - peridotite

A surfaced volcanic rock – peridotite

To subject the rocks to these hellish conditions, they use massive hydraulic presses.

“When rocks come from deep in the mantle to shallower depths, they cross a certain boundary called the solidus, where rocks begin to undergo partial melting and produce magmas,” Dasgupta said. “Scientists knew the effect of a trace amount of carbon dioxide or water would be to lower this boundary, but our new estimation made it 150-180 kilometers deeper from the known depth of 70 kilometers,” he said.

These findings have major implications for all planetary sciences:

“What we are now saying is that with just a trace of carbon dioxide in the mantle, melting can begin as deep as around 200 kilometers. And when we incorporate the effect of trace water, the magma generation depth becomes at least 250 kilometers. This does not generate a large amount, but we show the extent of magma generation is larger than previously thought and, as a consequence, it has the capacity to affect geophysical and geochemical properties of the planet as a whole.”

The paper will be published this week in Nature

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  • TheMatrixDNA

    If the nuclear reactions of the core makes the core growing, these locations of magma formation also must change over time. In fact, our scientific understanding about all aspects of astronomy and details of our planet is only beginning. “…so we have to rely on indirect measurements (seismology, electric measurements, etc), lab experiments and surface extrapolations…) Is it, the current scientific method, based on Physics, the unique and best method for understanding our planet and the Cosmos? No,because the final product of this planet and Cosmos – biological systems – can’t be explained by Physics. The forces and elements for biological life are not being identified in our current models, but, for sure, they must be there. Then, we have Matrix/DNA Theory models, a different method making the reverse way: starting from the final product – biological systems and life – and calculating backwards, we got surprisingly different models. Matrix/DNA still has not solved all questions about this Gaya Planet because we don’t know which method Nature used for formation of this Solar system. Like in cell”s formation, there are two possible methods: the formation of the original first systems ( by symbiosis connecting different shapes produced by vital cycle producing different shapes of a unique initial body), or the formation of modern systems ( merely replication by self-recycling the first ones). It is dependable of solving this question for us to know which model describes magma formation and locations.