Oldest Earth mantle reservoir discovered

August 11, 2010

Researchers have found a primitive Earth mantle reservoir on Baffin Island in the Canadian Arctic. Geologist Matthew Jackson and his colleagues from a multi-institution collaboration report the finding--the first discovery of what may be a primitive Earth mantle--this week in the journal Nature.

The Earth's mantle is a rocky, solid shell that is between the Earth's crust and the outer core, and makes up about 84 percent of the Earth's volume. The mantle is made up of many distinct portions or reservoirs that have different chemical compositions.

Scientists had previously concluded that the Earth was slightly older than 4.5 billion years old, but had not found a piece of the Earth's primitive mantle.

Until recently, researchers generally thought that the Earth and the other planets of the solar system were chondritic, meaning that the mantle's chemistry was thought to be similar to that of chondrites--some of the oldest, most primitive objects in the solar system. Assuming a chondritic model of the Earth, a piece of the primitive mantle would have certain isotope ratios of the chemical elements of helium, lead and neodymium.

The model that the Earth was chondritic was called into question with a discovery five years ago by a team at the Carnegie Institution of Washington, which suggested the ratio of neodymium on Earth was higher than what would be expected if the Earth were indeed chondritic.

That finding changed the neodymium ratio expected in the primitive mantle and in turn, changed where researchers should be looking to find evidence of a primitive mantle. According to the lead author, Matthew Jackson, "We had been looking under the wrong rock."

Since many of the ancient rocks have melted over time, finding a piece of the primitive mantle means studying lavas. Lavas retain the same isotopic composition of the rocks that have melted into the lava. Therefore, testing the lava's composition is identical to testing the original rock's composition.

When the assumption about the neodymium ratio was altered, Jackson and his colleagues knew they should take a look at lava samples from Baffin Island, since those samples contained the correct ratios of helium and neodymium. They discovered that the lavas also had the correct ratio for lead. The lead isotopes suggest that the samples from Baffin Island date the lava's mantle source reservoir to between 4.55 and 4.45 billion years old, only a little younger than the age of the Earth. The lava sample comes from an ancient rock that melted 62 million years ago.

When the researchers studied the composition of the lava found at Baffin Island, they discovered that the sample had the correct ratios of all three chemical elements--helium, lead, and the new non-chronditic neodymium ratio. This discovery suggests that the sample from Baffin Island is the first evidence for the oldest mantle reservoir.

This study challenges the idea that the Earth has a chondritic primitive mantle and according to Matthew Jackson is, "suggesting an alternative." One possibility, according to Jackson, is that "the early Earth went through a differentiation event and the Earth's crust was extracted from the early mantle and is now hidden in the deep earth; the hidden crust and the mantle found on Baffin Island would sum to chondritic."
-end-
This discovery will help researchers understand the composition of the original, early Earth. This research was supported by the National Science Foundation and the Carnegie Institution of Washington.

National Science Foundation

Related Solar System Articles from Brightsurf:

Ultraviolet shines light on origins of the solar system
In the search to discover the origins of our solar system, an international team of researchers, including planetary scientist and cosmochemist James Lyons of Arizona State University, has compared the composition of the sun to the composition of the most ancient materials that formed in our solar system: refractory inclusions in unmetamorphosed meteorites.

Second alignment plane of solar system discovered
A study of comet motions indicates that the Solar System has a second alignment plane.

Pressure runs high at edge of solar system
Out at the boundary of our solar system, pressure runs high.

What a dying star's ashes tell us about the birth of our solar system
A UA-led team of researchers discovered a dust grain forged in a stellar explosion before our solar system was born.

What scientists found after sifting through dust in the solar system
Two recent studies report discoveries of dust rings in the inner solar system: a dust ring at Mercury's orbit, and a group of never-before-detected asteroids co-orbiting with Venus, supplying the dust in Venus' orbit.

Discovered: The most-distant solar system object ever observed
A team of astronomers has discovered the most-distant body ever observed in our solar system.

Discovery of the first body in the Solar System with an extrasolar origin
Asteroid 2015 BZ509 is the very first object in the Solar System shown to have an extrasolar origin.

First interstellar immigrant discovered in the solar system
A new study has discovered the first known permanent immigrant to our solar system.

A star disturbed the comets of the solar system in prehistory
About 70,000 years ago, when the human species was already on Earth, a small reddish star approached our solar system and gravitationally disturbed comets and asteroids.

Scientists detect comets outside our solar system
Scientists from MIT and other institutions, working closely with amateur astronomers, have spotted the dusty tails of six exocomets -- comets outside our solar system -- orbiting a faint star 800 light years from Earth.

Read More: Solar System News and Solar System Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.