[meteorite-list] New Clues to the Early Solar System From Ancient Meteorites (Diogenites)

[Ron Baalke]

http://carnegiescience.edu/news/new_clues_early_solar_system_ancient_meteorites  

New clues to the early Solar System from ancient meteorites
Carnegie Institution for Science
July 23, 2012

Washington, D.C. - In order to understand Earth's earliest
history - its formation from Solar System material into the present-day
layering of metal core and mantle, and crust - scientists look to
meteorites. New research from a team including Carnegie's Doug Rumble
and Liping Qin focuses on one particularly old type of meteorite called
diogenites. These samples were examined using an array of techniques,
including precise analysis of certain elements for important clues to
some of the Solar System's earliest chemical processing. Their work is
published online July 22 by Nature Geoscience.

At some point after terrestrial planets or large bodies accreted from
surrounding Solar System material, they differentiate into a metallic
core, asilicate mantle, and a crust. This involved a great deal of
heating. The sources of this heat are the decay of short-lived
radioisotopes, the energy conversion that occurs when dense metals are
physically separated from lighter silicate, and the impact of large
objects. Studies indicate that the Earth's and Moon's mantles may have
formed more than 4.4 billion years ago, and Mars's more than 4.5 billion
years ago.

Theoretically, when a planet or large body differentiates enough to form
a core, certain elements including osmium, iridium, ruthenium, platinum,
palladium, and rheniumâ - known as highly siderophile elements - are
segregated into the core. But studies show that mantles of the Earth,
Moon and Mars contain more of these elements than they should.
Scientists have several theories about why this is the case and the
research team - which included lead author James Day of Scripps
Institution of Oceanography and Richard Walker of the University of
Maryland - set out to explore these theories by looking at diogenite
meteorites.

Diogenites are a kind of meteorite that may have come from the asteroid
Vesta, or a similar body. They represent some of the Solar System's
oldest existing examples of heat-related chemical processing. What's
more, Vesta or their other parent bodies were large enough to have
undergone a similar degree of differentiation to Earth, thus forming a
kind of scale model of a terrestrial planet.

The team examined seven diogenites from Antarctica and two that landed
in the African desert. They were able to confirm that these samples came
from no fewer than two parent bodies and that the crystallization of
their minerals occurred about 4.6 billion years ago, only 2 million
years after condensation of the oldest solids in the Solar System.
Examination of the samples determined that the highly siderophile
elements present in the diogenite meteorites were present during
formation of the rocks, which could only occur if late addition or
"accretion" of these elements after core formation had taken place. This
timing of late accretion is earlier than previously thought, and much
earlier than similar processes are thought to have occurred on Earth,
Mars, or the Moon.

Remarkably, these results demonstrate that accretion, core formation,
primary differentiation, and late accretion were all accomplished in
just over 2 to 3 million years on some parent bodies. In the case of
Earth, there followed crust formation, the development of an atmosphere,
and plate tectonics, among other geologic processes, so the evidence for
this early period is no longer preserved.

"This new understanding of diogenites gives us a better picture of the
earliest days of our Solar System and will help us understand the
Earth's birth and infancy," Rumble said. "Clearly we can now see that
early events in planetary formation set the stage very quickly for
protracted subsequent histories."
 
This work was supported by NASA.