A unique rock formation in China holds clues that tectonic plates subducted,
or went underneath other plates, during the Archean eon (4 billion to 2.5
billion years ago), just as they do nowadays, a new study finds.
This 2.5 billion-year-old rock, known as eclogite, is rare, forming when
oceanic crust is pushed deep into the mantle (the layer between the crust
and the core) at relatively low temperatures. This type of high-pressure,
low-temperature rock is "largely confined to subduction zones on the present
Earth," study co-lead researchers Timothy Kusky and Lu Wang, Earth
scientists at the China University of Geosciences, told Live Science in an
email.
The study reveals the oldest known eclogites from an ancient mountain belt
found in Earth's oceanic crust, the researchers said. The next-oldest rocks
of this kind — 2.1 billion-year-old rocks in the Democratic Republic of the
Congo — are about 400 million years younger, the researchers said.
While this isn't the oldest evidence of plate tectonics on record — a 2021
study, for instance, dated plate tectonics to about 3.6 billion years ago —
the new finding is a valuable data point showing that tectonic plates
subducted underneath each other in Earth's "early" days, geologically
speaking at least.
Tectonic plates — the moving slabs that make up Earth's outer crust — are
responsible for the cycling of materials and elements from deep inside Earth
to its oceans, surfaces and atmosphere. For decades, the research team has
worked to understand Earth's early history and evolution, "from the time it
formed and cooled from a molten ball of magma in space" to when it
solidified, forming a rigid outer crust that evolved to the plate-tectonic
system we have today, Kusky and Wang said.
Tectonic plates are crucial for heating the planet. Due to moving tectonic
plates, "heat is lost from the interior, much like bread floating and moving
on a pot of hot boiling stew below," they said. "Whether the transition to a
plate-tectonic Earth happened early, or whether the planet evolved through
different stages dominated by different mechanisms of heat loss, is one of
the most unresolved and debated questions in Earth sciences today."
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| A slab of Archean eclogite with red garnet and green pyroxene from Shangying, China. (Image credit: Lu Wang) |
That's why, for the past 20 years, the research team has mapped Archean eon
rocks spanning about 990 miles (1,600 kilometers) in northern China — "an
ancient mountain belt, called an orogen, recording the place where two
tectonic plates collided about 2.5 billion years ago," Kusky and Wang said.
Many features in these rocks indicate that this ancient mountain belt formed
as tectonic plates interacted with each other. For instance, fragments of
oceanic crust called ophiolites are trapped in the ancient collision zone,
as are highly deformed mixtures of rocks called mélanges (French for
"mixtures") that mark the spots where the plates collided, the researchers
said. The team also found large folded structures, called nappes, that plate
tectonics pushed from hundreds to thousands of miles away.
High pressure, low temperature
The discovery of the eclogites within the mélange reveal that a tectonic
slab of oceanic crust subducted under another plate, metamorphosing — that
is, having its composition, texture or internal structure altered by heat
and pressure — as it dove deep into the mantle.
It's rare to find eclogites from the Archean eon, which has "led to an
assertion that modern plate tectonics did not operate in the Archean," Kusky
and Wang said. "Thus, finding the eclogite, a key indicator of deep-and-cold
subduction, is very significant."
A lab analysis of the eclogites from the site revealed that they formed at a
spreading oceanic ridge about 2.5 billion years ago, were transported across
the ocean floor and then were pushed into the mantle via subduction.
Microstructures in the minerals garnet and clinopyroxene indicate that they
reached temperatures between 1,458 and 1,634 degrees Fahrenheit (792 and 890
degrees Celsius) and high pressures between 287,000 and 355,000 pounds per
square inch (19.8 and 24.5 kilobars).
These figures suggest that the eclogites subducted at least 40 miles (65 km)
deep, the researchers reported. Put another way, these findings are similar
to data from minerals found at modern subduction zones, Kusky and Wang
said.
Eventually, the pressure from the two colliding tectonic plates squeezed the
dense rocks back up to the surface, "much like a watermelon seed between
your wet fingers," Kusky and Wang said.
"I think it's a very interesting study," Nicolas Greber, a professor of
geochemistry at the Natural History Museum of Geneva and the Institute of
Geological Sciences at the University of Bern who was not involved in the
research, told Live Science. "These eclogites are important because they not
only show that subduction zones existed at that time but actually that these
subduction zones were able to be quite steep."
Even so, the findings aren't too surprising or even that new, said Roberta
Rudnick, a professor of Earth science at the University of California, Santa
Barbara who was not involved in the study. "It's not really particularly
novel, in my opinion," as other researchers have already reported on just as
ancient eclogites and eclogite minerals trapped inside diamonds that came up
through volcanic pipes that "have been very well studied over the decades,"
she told Live Science.
Rudnick added that "The whole topic of when plate tectonics start is
definitely an unsettled question. But I think most of the community would
have no problem with plate tectonics being in operation by 2.5 billion years
[ago]."
The
study was published
online April 4 in the journal Proceedings of the National Academy of
Sciences.
Originally published on
Live Science.