Studying ancient Earth's geochemistry
Image of
southwest Greenland. (Credit: Jacques Descloitres, MODIS Land Rapid Response
Team, courtesy of NASA Visible Earth)
|
Researchers still have much to learn about the volcanism that
shaped our planet's early history. New evidence from a team led by Carnegie's
Frances Jenner demonstrates that some of the tectonic processes driving
volcanic activity, such as those taking place today, were occurring as early as
3.8 billion years ago. Their work is published in Geology. Upwelling and melting of
Earth's mantle at mid-ocean ridges, as well as the eruption of new magmas on
the seafloor, drive the continual production of the oceanic crust. As the
oceanic crust moves away from the mid-ocean ridges and cools it becomes denser
than the underlying mantle. Over time the majority of this oceanic crust sinks
back into the mantle, which can trigger further volcanic eruptions. This
process is known as subduction and it takes place at plate boundaries.
Volcanic
eruptions that are triggered by subduction of oceanic crust are chemically
distinct from those erupting at mid-ocean ridges and oceanic island chains,
such as Hawaii. The differences between the chemistry of magmas produced at
each of these tectonic settings provide 'geochemical fingerprints' that can be
used to try to identify the types of tectonic activity taking place early in
Earth's history.
Previous
geochemical studies have used similarities between modern subduction zone
magmas and those erupted about 3.8 billion years ago, during the Eoarchean era,
to argue that subduction-style tectonic activity was taking place early in
Earth's history. But no one was able to locate any suites of volcanic rocks
with compositions comparable to modern mid-ocean ridge or oceanic island magmas
that were older than 3 billion years and were also free from contamination by
continental crust.
Because
of this missing piece of the puzzle, it has been ambiguous whether the
subduction-like compositions of volcanic rocks erupted 3.8 billion years ago
really were generated at subduction zones, or whether this magmatism should be
attributed to other processes taking place early in Earth's history.
Consequently, evidence for subduction-related tectonics earlier than 3 billion
years ago has been highly debated in scientific literature.
Jenner
and her team collected 3.8 billion-year-old volcanic rocks from Innersuartuut,
an island in southwest Greenland, and found the samples have compositions
comparable to modern oceanic islands, such as Hawaii.
"The
Innersuartuut samples may represent the world's oldest recognized suite of
oceanic island basalts, free from contamination by continental crust,"
Jenner said. "This evidence strengthens previous arguments that subduction
of oceanic crust into the mantle has been taking place since at least 3.8
billion years ago."
Source: Carnegie
Institution
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Posted by Unknown
on Saturday, January 19, 2013.
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