Scientists decipher the role of carbon and the break-up of continents


Date:
June 3, 2020
Source:
University of New Mexico
Summary:
An international collaboration has led scientists to new insights
into the storage and dynamic transfer of carbon below thick and
very old continental crust.



FULL STORY ========================================================================== University of New Mexico (UNM) Professor of Earth and Planetary Sciences,
Dr.

Tobias Fischer and Syracuse University research fellow (now University
of Auckland Lecturer), Dr. James Muirhead led an international team of interdisciplinary researchers to investigate the role of carbon in the
break-up of continents.


==========================================================================
This work, much of which has been funded by grants from the National
Science Foundation, is a culmination of research efforts that started
with former students from UNM and other US, French, Tanzanian and Kenyan universities.

The collaboration, which also included scientists from New Mexico Tech,
the University of Oregon, University of Dar Es Salaam, Seoul National University, University of Tokyo, University of Alberta, Macquarie
University, Goethe University and Universite' de Montpellier II, led
to new insights into the storage and dynamic transfer of carbon below
thick and very old continental crust currently published in the journal
Nature titled, Displacement of cratonic mantle and lithospheric channeling concentrates deep carbon during continental rifting.

It was first recognized by former UNM student, now assistant professor
at Seoul National University, Dr. Hyunwoo Lee, that the East African
Rift and continental rifts in general are significant sources of carbon degassed from the Earth's mantle to the atmosphere. While later work
by other groups showed that CO2 emissions from the East African Rift
are variable along its 3,000 km extent, the question remained "where
does all this carbon come from and how is it so efficiently released?" Subsequent work by Fischer and collaborator Professor Stephen Foley from Macquarie University, Australia, proposed a model in which the degassing
CO2 is ultimately sourced from carbon that has accumulated over billions
of years at the base of the thick old cratonic lithosphere located in
the center and edge of the East African Rift.

"The model suggests that this accumulated carbon originates from
subducting oceanic plates and deep mantle plumes," said Fischer. "These processes could deliver sufficient carbon to the bottom of very thick
and billion year old continental lithosphere to explain the high CO2
fluxes observed in the actively deforming part of the rift." However,
the model proposed by Fischer and Foley could not explain how this deep
CO2 managed to leak out from the actively extending part of the rift,
which is exactly where the current work connects the dots.



========================================================================== Muirhead and Fischer together with Master's student Amani Laizer from University of Dar Es Salaam in Tanzania and geophysics Ph.D. student
Sarah Jaye Oliva from Tulane University returned to Tanzania in 2018 and collected data and samples in locations where active rifting, i.e. where
the plates move apart, intersect the old thick craton that lies above a
mantle plume. Gas samples were collected from hot springs in this region
that have never been sampled before.

The analyses of these samples within the context of already existing
data from the earlier work showed a striking difference in chemical
composition of the gases that are released from the active rift and the
craton. Craton gases are entirely crustal with no sign of any mantle
gases, including CO2. Nitrogen and crustal helium dominate these craton
gases. Rift gases on the other hand are stuffed with mantle CO2 and have
a strong mantle helium isotope signature.

Measured mantle CO2 fluxes are close to zero on the craton but surge in
the adjacent actively extending rift.

"Right at the boundary between the craton and the deforming rift sits
the world's only currently erupting carbonatite volcano, Oldoinyo
Lengai," said Fischer. "This volcano erupts lavas that are so liquid
they move like motor oil. The reason for this is that they are devoid
of the silica that makes up most igneous rocks but contain about 30
percent carbon, a staggeringly high amount that gives the rock its name carbonatite. Looking back in geologic time, it turns out that there are
many carbonatite volcanoes right at the edge of the Tanzania craton, but
they are just not currently active." This distribution of carbonatites
led the team to propose a mechanism that causes the lateral migration
of the deep cratonic lithosphere where all that stored solid carbon is
located, into the mantle at the edges of the craton.

Geophysical data acquired and analyzed by Tulane University and
Universite' de Montpellier II image a steep step in plate thickness
at the craton edge. The geophysicists led by Professor Cindy Ebinger,
Drs. Sarah Oliva and Professor Christel Tiberi proposed that this step
enhances formation of melt and explains the concentration of magma
that carries the excess CO2, as well as the spatial distribution of
sometimes damaging earthquakes that open cracks for the CO2 to rise to
the surface. This would explain the striking difference in CO2 release
and source as documented by the surface measurements.

This conceptual model also fits into quantitative physical models
developed by Dr. Jolante van Wjik, professor at New Mexico Tech and
Dr. Claire Currie, professor at University of Alberta, which shows that unusually thick and low density mantle rocks beneath a craton will be
swept laterally by mantle flow, moving toward the thinner plate beneath
the continental rift.

This material transfer may enhance melt production. Therefore, the
research team concluded, lateral migration of deep cratonic lithosphere
soaked with ancient accumulated carbon is ultimately responsible for carbonatite volcanism and the on-going continental break-up in this
region of East Africa.


========================================================================== Story Source: Materials provided by University_of_New_Mexico. Original
written by Steve Carr.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Muirhead, J.D., Fischer, T.P., Oliva, S.J. et al. Displaced cratonic
mantle concentrates deep carbon during continental rifting. Nature,
2020 DOI: 10.1038/s41586-020-2328-3 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200603120540.htm https://www.sciencedaily.com/releases/2020/06/200603120540.htm

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