[meteorite-list] Cosmochemists Find Rare Element Curium in Meteorite

[Ron Baalke]

http://news.uchicago.edu/article/2016/03/04/cosmochemists-find-evidence-rare-element-early-solar-system

 
Cosmochemists find evidence of rare element in early solar system

'Curious Marie' sample leads to critical detection of curium in meteorite
University of Chicago
March 4, 2016

UChicago scientists have discovered evidence in a meteorite that a rare 
element, curium, was present during the formation of the solar system. 
The finding ends a 35-year-old debate on its possible presence in the 
early solar system, and plays a crucial role in reassessing models of 
stellar evolution and synthesis of elements in stars. Details of the discovery 
appear in the March 4 edition of Science Advances.

"Curium is an elusive element. It is one of the heaviest-known elements, 
yet it does not occur naturally because all of its isotopes are radioactive 
and decay rapidly on a geological time scale," said the study's lead 
author, Francois Tissot, PhD'15, now a postdoctoral fellow at the Massachusetts 
Institute of Technology.

And yet Tissot and his UChicago co-authors, Profs. Nicolas Dauphas and 
Lawrence Grossman, have found evidence of curium in an unusual ceramic 
inclusion they called "Curious Marie,' taken from a carbonaceous meteorite. 
Curium became incorporated into the inclusion when it condensed from the 
gaseous cloud that formed the sun in the early solar system.

Curious Marie and curium are named after Marie Curie, whose pioneering 
work laid the foundation of the theory of radioactivity. Curium was only 
discovered in 1944, by Glenn Seaborg and his collaborators at the University 
of California, Berkeley, who bombarded atoms of plutonium with alpha particles 
(atoms of helium) to synthesize a new, very radioactive element.

To chemically identify this new element, Seaborg and his collaborators 
studied the energy of the particles emitted during its decay at the Metallurgical 
Laboratory at UChicago (which later became Argonne National Laboratory). 
The isotope they had synthesized was the very unstable curium-242, which 
decays in a half-life of 162 days.

On Earth today, curium exists only when manufactured in laboratories or 
as a byproduct of nuclear explosions. Curium could have been present in 
the early solar system as a product of massive star explosions that happened 
before the solar system was born.

"The possible presence of curium in the early solar system has long 
been exciting to cosmochemists, because they can often use radioactive 
elements as chronometers to date the relative ages of meteorites and planets," 
said Dauphas, the Louis Block Professor in Geophysical Sciences.

The longest-lived isotope of curium (Cm-247) decays over time into an 
isotope of uranium (U-235). Therefore, a mineral or a rock formed early 
in the solar system would have incorporated more Cm-247 than a similar 
mineral or rock that formed later, after Cm-247 had decayed. If scientists 
were to analyze these two hypothetical minerals today, they would find 
that the older mineral contains more U-235 (the decay product of Cm-247 
than the younger mineral).

"The idea is simple enough, yet, for nearly 35 years, scientists have 
argued about the presence of Cm-247 in the early solar system," Tissot 
said.

long wait to detect curium 

Early studies in the 1980s found large excesses of U-235 in any meteoritic 
inclusions they analyzed, and concluded that curium was very abundant 
when the solar system formed. More refined experiments conducted by James 
Chen and Gerald Wasserburg, SB'51, SM'52, PhD'54, at the California 
Institute of Technology showed that these early results were spurious, 
and that if curium was present in the early solar system, its abundance 
was so low that state-of-the-art instrumentation would be unable to detect 
it.

Scientists had to wait until a new, higher-performance mass spectrometer 
was developed to successfully identify, in 2010, that tiny excesses of 
U-235 could be the smoking gun for the presence of Cm-247 in the early 
solar system.

"That was an important step forward, but the problem is, those excesses 
were so small that other processes could have produced them," Tissot 
noted.

Models predict that curium, if present, was in low abundance in the early 
solar system. Therefore, the excess U-235 produced by the decay of Cm-247 
cannot be seen in minerals or inclusions that contain large or even average 
amounts of natural uranium. One of the challenges was thus to find a mineral 
or inclusion likely to have incorporated a lot of curium but containing 
little uranium.

With the help of Grossman, professor emeritus in geophysical sciences, 
the team was able to identify and target a specific kind of meteoritic 
inclusion rich in calcium and aluminum. These calcium, aluminum-rich inclusions 
are known to have a low abundance of uranium and likely to have high curium 
abundance. One of these inclusions - Curious Marie - contained an extremely 
low amount of uranium.

"It is in this very sample that we were able to resolve an unprecedented 
excess of U-235," Tissot said. "All natural samples have a similar 
isotopic composition of uranium, but the uranium in Curious Marie has 
6 percent more U-235 - a finding that can only be explained by live Cm-247 
in the early solar system."

Thanks to this sample, the research team was able to calculate the amount 
of curium present in the early solar system and compared it to other heavy 
radioactive elements such as iodine-129 and plutonium-244. They found 
that all these isotopes could have been produced together by a single 
process in stars.

"This is particularly important because it indicates that as successive 
generations of stars die and eject the elements they produced into the 
galaxy, the heaviest elements are produced together, while previous work 
had suggested that this was not the case," Dauphas explained.

The finding of naturally occurring curium in meteorites closes the loop 
opened 70 years ago by the discovery of manmade curium, and it provides 
a new constraint, which modelers can now incorporate into complex models 
of stellar nucleosynthesis and galactic chemical evolution to further 
understand how elements like gold were made in stars.

Citation: "Origin of uranium isotope variations in early solar nebula 
condensates" by F.L.H. Tissot, N. Dauphas, and L. Grossman, Science 
Advances, March 4, 2016. DOI: Vol. 2, No. 3, March 4, 2016, DOI: 10.1126/sciadv.1501400.

Funding: National Aeronautics and Space Administration, National Science 
Foundation