New clues about the rise of Earth's continents
One popular explanation for properties that result in dry land is
unlikely according to new experiments

Date:
May 4, 2023
Source:
Smithsonian
Summary:
New research deepens the understanding of Earth's crust by testing
and ultimately eliminating one popular hypothesis about why
continental crust is lower in iron and more oxidized compared to
oceanic crust. The iron- poor composition of continental crust is
a major reason why vast portions of the Earth's surface stand above
sea level as dry land, making terrestrial life possible today. The
study uses laboratory experiments to show that the iron-depleted,
oxidized chemistry typical of Earth's continental crust likely did
not come from crystallization of the mineral garnet, as a popular
explanation proposed in 2018.


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FULL STORY ========================================================================== Continents are part of what makes Earth uniquely habitable for life among
the planets of the solar system, yet surprisingly little is understood
about what gave rise to these huge pieces of the planet's crust and
their special properties. New research from Elizabeth Cottrell, research geologist and curator of rocks at the Smithsonian's National Museum of
Natural History, and lead study author Megan Holycross, formerly a Peter
Buck Fellow and National Science Foundation Fellow at the museum and now
an assistant professor at Cornell University, deepens the understanding of Earth's crust by testing and ultimately eliminating one popular hypothesis about why continental crust is lower in iron and more oxidized compared
to oceanic crust. The iron-poor composition of continental crust is a
major reason why vast portions of the Earth's surface stand above sea
level as dry land, making terrestrial life possible today.

The study, published today in Science, uses laboratory experiments to show
that the iron-depleted, oxidized chemistry typical of Earth's continental
crust likely did not come from crystallization of the mineral garnet,
as a popular explanation proposed in 2018.

The building blocks of new continental crust issue forth from the
depths of the Earth at what are known as continental arc volcanoes,
which are found at subduction zones where an oceanic plate dives beneath
a continental plate. In the garnet explanation for continental crust's iron-depleted and oxidized state, the crystallization of garnet in the
magmas beneath these continental arc volcanoes removes non-oxidized
(reduced or ferrous, as it is known among scientists) iron from the
terrestrial plates, simultaneously depleting the molten magma of iron
and leaving it more oxidized.

One of the key consequences of Earth's continental crust's low iron
content relative to oceanic crust is that it makes the continents less
dense and more buoyant, causing the continental plates to sit higher atop
the planet's mantle than oceanic plates. This discrepancy in density and buoyancy is a major reason that the continents feature dry land while
oceanic crusts are underwater, as well as why continental plates always
come out on top when they meet oceanic plates at subduction zones.

The garnet explanation for the iron depletion and oxidation in continental
arc magmas was compelling, but Cottrell said one aspect of it did not
sit right with her.

"You need high pressures to make garnet stable, and you find this low-iron magma at places where crust isn't that thick and so the pressure isn't
super high," she said.

In 2018, Cottrell and her colleagues set about finding a way to test
whether the crystallization of garnet deep beneath these arc volcanoes
is indeed essential to the process of creating continental crust as is understood. To accomplish this, Cottrell and Holycross had to find ways to replicate the intense heat and pressure of the Earth's crust in the lab,
and then develop techniques sensitive enough to measure not just how much
iron was present, but to differentiate whether that iron was oxidized.

To recreate the massive pressure and heat found beneath continental
arc volcanoes, the team used what are called piston-cylinder presses
in the museum's High-Pressure Laboratory and at Cornell. A hydraulic piston-cylinder press is about the size of a mini fridge and is mostly
made of incredibly thick and strong steel and tungsten carbide. Force
applied by a large hydraulic ram results in very high pressures on tiny
rock samples, about a cubic millimeter in size. The assembly consists
of electrical and thermal insulators surrounding the rock sample, as
well as a cylindrical furnace. The combination of the piston-cylinder
press and heating assembly allows for experiments that can attain the
very high pressures and temperatures found under volcanoes.

In 13 different experiments, Cottrell and Holycross grew samples of
garnet from molten rock inside the piston-cylinder press under pressures
and temperatures designed to simulate conditions inside magma chambers
deep in Earth's crust.

The pressures used in the experiments ranged from 1.5 to 3 gigapascals --
that is roughly 15,000 to 30,000 Earth atmospheres of pressure or 8,000
times more pressure than inside a can of soda. Temperatures ranged from
950 to 1,230 degrees Celsius, which is hot enough to melt rock.

Next, the team collected garnets from Smithsonian's National Rock
Collection and from other researchers around the world. Crucially, this
group of garnets had already been analyzed so their concentrations of
oxidized and unoxidized iron were known.

Finally, the study authors took the materials from their experiments
and those gathered from collections to the Advanced Photon Source at the
U.S. Department of Energy's Argonne National Laboratory in Illinois. There
the team used high- energy X-ray beams to conduct X-ray absorption spectroscopy, a technique that can tell scientists about the structure
and composition of materials based on how they absorb X-rays. In this
case, the researchers were looking into the concentrations of oxidized
and unoxidized iron.

The samples with known ratios of oxidized and unoxidized iron provided
a way to check and calibrate the team's X-ray absorption spectroscopy measurements and facilitated a comparison with the materials from their experiments.

The results of these tests revealed that the garnets had not incorporated enough unoxidized iron from the rock samples to account for the levels
of iron- depletion and oxidation present in the magmas that are the
building blocks of Earth's continental crust.

"These results make the garnet crystallization model an extremely unlikely explanation for why magmas from continental arc volcanoes are oxidized
and iron depleted," Cottrell said. "It's more likely that conditions
in Earth's mantle below continental crust are setting these oxidized conditions." Like so many results in science, the findings lead to more questions: "What is doing the oxidizing or iron depleting?" Cottrell
asked. "If it's not garnet crystallization in the crust and it's something about how the magmas arrive from the mantle, then what is happening in
the mantle? How did their compositions get modified?" Cottrell said
that these questions are hard to answer but that now the leading theory
is that oxidized sulfur could be oxidizing the iron, something a current
Peter Buck Fellow is investigating under her mentorship at the museum.

This study is an example of the kind of research that museum scientists
will tackle under the museum's new Our Unique Planet initiative,
a public-private partnership, which supports research into some of
the most enduring and significant questions about what makes Earth
special. Other research will investigate the source of Earth's liquid
oceans and how minerals may have served as templates for life.

This research was supported by funding from the Smithsonian, the National Science Foundation, the Department of Energy and the Lyda Hill Foundation.

* RELATED_TOPICS
o Earth_&_Climate
# Geology # Earth_Science # Earthquakes # Volcanoes
o Fossils_&_Ruins
# Early_Climate # Origin_of_Life # Anthropology #
Charles_Darwin
* RELATED_TERMS
o Continental_crust o Crust_(geology) o Structure_of_the_Earth
o Lithosphere o Ocean o Volcano o Quartz o Earth

========================================================================== Story Source: Materials provided by Smithsonian. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. Megan Holycross, Elizabeth Cottrell. Garnet crystallization does not
drive oxidation at arcs. Science, 2023; 380 (6644): 506 DOI:
10.1126/ science.ade3418 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/05/230504155638.htm

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