Ancient Mantle Rocks Under Baffin Island
Scientists have discovered a new window into the Earth's violent past. Geochemical evidence from volcanic
rocks collected on Baffin Island in the Canadian Arctic suggests that beneath it lies a region of the Earth's
mantle that has largely escaped the billions of years of melting and geological churning that has affected the
rest of the planet. Researchers believe the discovery offers clues to the early chemical evolution of the Earth.
Revealing the Composition of Earth's Early Mantle
The newly identified mantle "reservoir," as it is called, dates from just a few tens of million years after the
Earth was first assembled from the collisions of smaller bodies. This reservoir likely represents the composition
of the mantle shortly after formation of the core, but before the 4.5 billion years of crust formation and recycling
modified the composition of most of the rest of Earth's interior.
"This was a key phase in the evolution of the Earth," says co-author Richard Carlson of the Carnegie Institution's
Department of Terrestrial Magnetism. "It set the stage for everything that came after. Primitive mantle such as that we
have identified would have been the ultimate source of all the magmas and all the different rock types we see on Earth today."
Clues from Helium Isotopes
Carlson and lead author Matthew Jackson (a former Carnegie postdoctoral fellow, now at Boston University), with
colleagues, using samples collected by coauthor Don Francis of McGill University, targeted the Baffin Island
rocks, which are the earliest expression of the mantle hotspot now feeding volcanic eruptions on Iceland,
because previous study of helium isotopes in these rocks showed them to have anomalously high ratios of
helium-3 to helium-4.
Helium-3 is generally extremely rare within the Earth; most of the mantle's supply
has been outgassed by volcanic eruptions and lost to space over the planet's long geological history. In
contrast, helium-4 has been constantly replenished within the Earth by the decay of radioactive uranium and
thorium. The high proportion of helium-3 suggests that the Baffin Island lavas came from a reservoir in the
mantle that had never previously outgassed its original helium-3, implying that it had not been subjected to
the extensive chemical differentiation experienced by most of the mantle.
Rocks Formed Before Plate Tectonics Began
The researchers confirmed this conclusion by analyzing the lead isotopes in the lava samples, which date the
reservoir to between 4.55 and 4.45 billion years old. This age is only slightly younger than the Earth itself.
The early age of the mantle reservoir implies that it existed before melting of the mantle began to create the
magmas that rose to form Earth's crust and before plate tectonics allowed that crust to be mixed back into the mantle.
Many researchers have assumed that before continental crust formed the mantle's chemistry was similar to that
of meteorites called chondrites, but that the formation of continents altered its chemistry, causing it to
become depleted in the elements, called incompatible elements, which are extracted with the magma when melting
occurs in the mantle. "Our results question this assumption," says Carlson. "They suggest that before continent
extraction, the mantle already was depleted in incompatible elements compared to chondrites, perhaps because of
an even earlier Earth differentiation event, or perhaps because the Earth originally formed from building blocks
depleted in these elements."
Earth's Original Crust
Of the two possibilities, Carlson favors the early differentiation model, which would involve a global magma
ocean on the newly-formed Earth. This magma ocean produced a crust that predated the crust that exists today.
"In our model, the original crust that formed by the solidification of the magma ocean was buoyantly unstable
at Earth's surface because it was rich in iron," he says. "This instability caused it to sink to the base of the
mantle, taking the incompatible elements with it, where it remains today."
Some of this deep material may have remained liquid despite the high pressures, and Carlson points out that
seismological studies of the deep mantle reveal certain areas, one beneath the southern Pacific and another
beneath Africa, that appear to be molten and possibly chemically different from the rest of the mantle. "I'm
holding out hope that these seismically imaged areas might be the compositional complement to the "depleted"
primitive mantle that we sample in the Baffin Island lavas," he says.
The paper is published in the August 12 issue of Nature.
|Landsat GeoCover image of Baffin Island. Light blue areas are ice cover. NASA image. Enlarge Image. Learn how to interpret GeoCover images.
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