Magma in mantle has deep impact
Magma
forms far deeper than geologists previously thought, according to new research
at Rice University. A group led by geologist Rajdeep Dasgupta put very small
samples of peridotite under very large pressures in a Rice laboratory to
determine that rock can and does liquify, at least in small amounts, as deep as
250 kilometers in the mantle beneath the ocean floor. He said this explains
several puzzles that have bothered scientists.
Dasgupta is lead author of the paper to be published this week
in Nature.
The
mantle is the planet's middle layer, a buffer of rock between the crust -- the
top 5 miles or so -- and the core. If one could compress millions of years of
observation down to minutes, the mantle would look like a rolling mass of
rising and falling material. This slow but constant convection brings materials
from deep within the planet to the surface -- and occasionally higher through
volcanoes.
The
Rice team focused on mantle beneath the ocean because that's where the crust is
created and, Dasgupta said, "the connection between the interior and
surface world is established." Silicate melts -- aka magma -- rise with
the convective currents, cool and spread out to form the ocean crust. The
starting point for melting has long been thought to be at 70 kilometers beneath
the seafloor.
That
has confounded geologists who suspected but could not demonstrate the existence
of deeper silicate magma, said Dasgupta, an assistant professor of Earth
science at Rice.
Scientists
determine the mantle's density by measuring the speed of a seismic wave after
an earthquake, from its origin to other points on the planet. These waves
travel faster through solids than liquids, and geologists have been surprised
to detect waves slowing down through what should be the mantle's express lane.
"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."
The
research also offers clues to the bulk electrical conductivity of the oceanic
mantle, he said. "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."
Because
humans have not yet dug deep enough to sample the mantle directly -- though
some are trying -- researchers have to extrapolate data on rocks carried up to
the surface. A previous study by Dasgupta determined that melting in Earth's
deep upper mantle is caused by the presence of carbon dioxide. The present
study shows that carbon not only leads the charge to make carbonate fluid but
also helps to make silicate magma at significant depths.
The
researchers also found carbonated rock melts at significantly lower
temperatures than non-carbonated rock. "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."
In
Dasgupta's high-pressure lab at Rice, volcanic rocks are windows to the
planet's interior. The researchers crushed tiny rock samples that contain
carbon dioxide to find out how deep magma forms.
"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."
They
use powerful hydraulic presses to partially melt "rocks of interest"
that contain tiny amounts of carbon to simulate what they believe is happening
under equivalent pressures in the mantle. "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.
"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."
Co-authors
of the paper are Rice graduate student Ananya Mallik and postdoctoral
researcher Kyusei Tsuno; Research Professor Anthony Withers and Marc
Hirschmann, the George and Orpha Gibson Chair of Earth and Planetary Sciences,
at the University of Minnesota, and Greg Hirth, a professor of geological
sciences at Brown University.
The
study was supported by the National Science Foundation and a Packard Fellowship
to Dasgupta.
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