Geologists have discovered the oldest known pieces of Earth's mantle, rocks found along Canada's remote Labrador coast that date back at least 3.8 billion years.
Sophisticated dating techniques showed the rare rocks, which would not look unusual to the average person, were between 3.8 billion and 4.2 billion years old. Until the discovery, the most ancient samples of Earth's mantle came from South Africa's diamond mining region and dated back about 3.55 billion years.The softball-size rocks were chipped away from large, rocky formations in 1987 in the Hebron area of northern Labrador, a province in the northeastern corner of Canada. Scientists suspected they might find valuable rocks in the area because of previously detected unusual mineral deposits.
Currently, Earth consists of a superheated molten core of iron and nickel, an intermediate mantle and a rocky crust.
Most scientists think Earth and nearby planets formed about 4.5 billion years ago from material left as the cooling sun shrank. However, they do not know exactly when Earth evolved from a uniform lump of elements into its present structure.
In a study released Wednesday in the journal Nature, Kenneth Collerson of the University of California-Santa Cruz and colleagues from the University of Okalhoma in Norman and Britain's VG Iostech said the Labrador rocks may provide vital clues to the planet's chemical evolution.
"This is quite a bombshell in terms of our understanding the earth's geochemistry. Previously, we had only been able to theorize about the composition of the mantle at that time by looking at pieces of ancient crust. This is the first direct geochemical evidence for the composition and chemical evolution of the pre-3.8 billion-year-old mantle," Collerson said in a statement.
The composition of the Labrador samples indicates that the mantle apparently separated into two chemically distinct layers more than 4 billion years ago, in the first 500 million years of Earth's history.
Fragments of mantle found in South Africa and Siberia have similar compositions, Collerson said. Rocks from the moon, which some scientists think formed when an asteroid or meteorite crashed into Earth and knocked off a large chunk of its mantle, also reflect a similar chemistry, he said.
In 1989, a U.S. and Australian team reported finding the oldest known samples of the Earth's crust - 4 billion-year-old rocks found in outcroppings near Canada's Great Slave Lake.
Thomas Wright, a geologist with the National Science Foundation, said mantle-derived rocks tend to be denser and darker in color than granite-like crustal rocks.
One of the Labrador rock samples apparently was pushed upward and incorporated into the Earth's crust by collisions of land masses, researchers said. The second and older type of rock found along the Labrador coast appears to have been erupted as lava onto the crust in ancient times, they said.