Iron-rich rocks unlock new insights into Earth's planetary history
Study suggests ancient microorganisms helped cause massive volcanic
events

Date:
May 25, 2023
Source:
Rice University
Summary:
A new study suggests iron-rich ancient sediments may have helped
cause some of the largest volcanic events in the planet's history.


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FULL STORY ========================================================================== Visually striking layers of burnt orange, yellow, silver, brown
and blue-tinged black are characteristic of banded iron formations,
sedimentary rocks that may have prompted some of the largest volcanic
eruptions in Earth's history, according to new research from Rice
University.

The rocks contain iron oxides that sank to the bottom of oceans long ago, forming dense layers that eventually turned to stone. The study published
this week in Nature Geoscience suggests the iron-rich layers could connect ancient changes at Earth's surface -- like the emergence of photosynthetic
life -- to planetary processes like volcanism and plate tectonics.

In addition to linking planetary processes that were generally thought
to be unconnected, the study could reframe scientists' understanding
of Earth's early history and provide insight into processes that could
produce habitable exoplanets far from our solar system.

"These rocks tell -- quite literally -- the story of a changing
planetary environment," said Duncan Keller, the study's lead author and
a postdoctoral researcher in Rice's Department of Earth, Environmental
and Planetary Sciences.

"They embody a change in the atmospheric and ocean chemistry."
Banded iron formations are chemical sediments precipitated directly
from ancient seawater rich in dissolved iron. Metabolic actions of microorganisms, including photosynthesis, are thought to have facilitated
the precipitation of the minerals, which formed layer upon layer over
time along with chert (microcrystalline silicon dioxide). The largest
deposits formed as oxygen accumulated in Earth's atmosphere about 2.5
billion years ago.

"These rocks formed in the ancient oceans, and we know that those
oceans were later closed up laterally by plate tectonic processes,"
Keller explained.

The mantle, though solid, flows like a fluid at about the rate that
fingernails grow. Tectonic plates -- continent-sized sections of the
crust and uppermost mantle -- are constantly on the move, largely as a
result of thermal convection currents in the mantle. Earth's tectonic
processes control the life cycles of oceans.

"Just like the Pacific Ocean is being closed today -- it's subducting
under Japan and under South America -- ancient ocean basins were destroyed tectonically," he said. "These rocks either had to get pushed up onto continents and be preserved -- and we do see some preserved, that's
where the ones we're looking at today come from -- or subducted into
the mantle." Because of their high iron content, banded iron formations
are denser than the mantle, which made Keller wonder whether subducted
chunks of the formations sank all the way down and settled in the lowest
region of the mantle near the top of Earth's core. There, under immense temperature and pressure, they would have undergone profound changes as
their minerals took on different structures.

"There's some very interesting work on the properties of iron oxides at
those conditions," Keller said. "They can become highly thermally and electrically conductive. Some of them transfer heat as easily as metals
do. So it's possible that, once in the lower mantle, these rocks would
turn into extremely conductive lumps like hot plates." Keller and his co-workers posit that regions enriched in subducted iron formations
might aid the formation of mantle plumes, rising conduits of hot rock
above thermal anomalies in the lower mantle that can produce enormous
volcanoes like the ones that formed the Hawaiian Islands. "Underneath
Hawaii, seismological data show us a hot conduit of upwelling mantle,"
Keller said.

"Imagine a hot spot on your stove burner. As the water in your pot is
boiling, you'll see more bubbles over a column of rising water in that
area. Mantle plumes are sort of a giant version of that." "We looked at
the depositional ages of banded iron formations and the ages of large
basaltic eruption events called large igneous provinces, and we found
that there's a correlation," Keller said. "Many of the igneous events --
which were so massive that the 10 or 15 largest may have been enough to resurface the entire planet -- were preceded by banded iron formation deposition at intervals of roughly 241 million years, give or take 15
million. It's a strong correlation with a mechanism that makes sense."
The study showed that there was a plausible length of time for banded
iron formations to first be drawn deep into the lower mantle and to then influence heat flow to drive a plume toward Earth's surface thousands
of kilometers above.

In his effort to trace the journey of banded iron formations, Keller
crossed disciplinary boundaries and ran into unexpected insights.

"If what's happening in the early oceans, after microorganisms chemically change surface environments, ultimately creates an enormous outpouring
of lava somewhere else on Earth 250 million years later, that means these processes are related and 'talking' to each other," Keller said. "It also
means it's possible for related processes to have length scales that are
far greater than people expected. To be able to infer this, we've had to
draw on data from many different fields across mineralogy, geochemistry, geophysics and sedimentology." Keller hopes the study will spur further research. "I hope this motivates people in the different fields that it touches," he said. "I think it would be really cool if this got people
talking to each other in renewed ways about how different parts of the
Earth system are connected." Keller is part of the CLEVER Planets:
Cycles of Life-Essential Volatile Elements in Rocky Planets program,
an interdisciplinary, multi-institutional group of scientists led by
Rajdeep Dasgupta, Rice's W. Maurice Ewing Professor of Earth Systems
Science in the Department of Earth, Environmental and Planetary Sciences.

"This is an extremely interdisciplinary collaboration that's looking at
how volatile elements that are important for biology -- carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur -- behave in planets, at how
planets acquire these elements and the role they play in potentially
making planets habitable," Keller said.

"We're using Earth as the best example that we have, but we're trying to
figure out what the presence or absence of one or some of these elements
might mean for planets more generally," he added.

Cin-Ty Lee, Rice's Harry Carothers Wiess Professor of Geology, Earth, Environmental and Planetary Sciences, and Dasgupta are co-authors on
the study.

Other co-authors are Santiago Tassara, an assistant professor at Bernardo O'Higgins University in Chile, and Leslie Robbins, an assistant professor
at the University of Regina in Canada, who both did postdoctoral work
at Yale University, and Yale Professor of Earth and Planetary Sciences
Jay Ague, Keller's doctoral adviser.

NASA (80NSSC18K0828) and the Natural Sciences and Engineering Research
Council of Canada (RGPIN-2021-02523) supported the research.

* RELATED_TOPICS
o Plants_&_Animals
# Nature # Fish # Marine_Biology
o Earth_&_Climate
# Geology # Earth_Science # Earthquakes
o Fossils_&_Ruins
# Origin_of_Life # Fossils # Early_Climate
* RELATED_TERMS
o Mesopotamia o Decade_Volcanoes o Earthquake_liquefaction o
Toba_catastrophe_theory o Timeline_of_environmental_events o
Iron_Age o Krakatoa o Caldera

========================================================================== Story Source: Materials provided by Rice_University. Original written
by Silvia Cernea Clark.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Duncan S. Keller, Santiago Tassara, Leslie J. Robbins, Cin-Ty
A. Lee, Jay
J. Ague, Rajdeep Dasgupta. Links between large igneous province
volcanism and subducted iron formations. Nature Geoscience, 2023;
DOI: 10.1038/ s41561-023-01188-1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/05/230525140951.htm

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