Mass wins the race toward cool—and leaves a clue to igneous rock formation
the crash-car derby between heavy and light isotopes vying for the
coolest spots as magma turns to solid rock, weightier isotopes have an
edge, research led by Case Western Reserve University shows.
This tiny detail may offer clues to how igneous rocks form.
molten rock cools along a gradient, atoms want to move towards the cool
end. This happens because hotter atoms move faster than cooler atoms
and, therefore, hotter atoms move to the cool region faster than the
cooler atoms move to the hot region.
all isotopes of the same element want to move towards the cool end, the
big boys have more mass and, therefore, momentum, enabling them to keep
moving on when they collide along the way.
as if you have a crowded, sealed room of sumo wrestlers and geologists
and a fire breaks out at one side of the room,” said Daniel Lacks,
chemical engineering professor and lead author of the paper. “All will
try to move to the cooler side of the room, but the sumo wrestlers are
able to push their way through and take up space on the cool side,
leaving the geologists on the hot side of the room.”
worked with former postdoctoral researcher Gaurav Goel and geology
professor James A. Van Orman at Case Western Reserve; Charles J. Bopp IV
and Craig C. Lundstrum, of University of Illinois, Urbana; and Charles
E. Lesher of the University of California at Davis. They described their
theory and confirming mathematics, computer modeling, and experiments
in the current issue of Physical Review Letters.
Lacks, Van Orman and Lesher also published a short piece in the current issue of Nature, showing how their findings overturn an explanation based on quantum mechanics, published in that journal last year.
theoretical understanding of thermal isotope separation in gases was
developed almost exactly 100 years ago by David Enskog, but there is as
yet not a similar full understanding of this process in liquids,” said
Frank Richter, who is the Sewell Avery Distinguished Professor at the
University of Chicago and a member of the National Academy of Sciences.
He was not involved in the research. “This work by Lacks et al. is an
important step towards remedying this situation.”
This separation among isotopes of the same element is called fractionation.
have been able to see fractionation of heavy elements in igneous rocks
only since the 1990s, Van Orman said. More sensitive mass spectrometers
showed that instead of a homogenous distribution, the concentration
ratio of heavy isotopes to light isotopes in some igneous rocks was up
to 0.1% higher than in other rocks.
One way of producing this fractionation is by temperature.
understand how this happens, the team of researchers created a series
of samples made of molten magnesium silicate infused with elements of
different mass, from oxygen on up to heavy uranium.
samples, called silicate melts, were heated at one end in a standard
lab furnace, creating temperature gradients in each. The melts were then
allowed to cool and solidify.
scientists then sliced the samples along gradient lines and dissolved
the slices in acid. Analysis showed that no matter the element, the
heavier isotopes slightly outnumbered the lighter at the cool end of the
Computer simulations of the atoms, using classical mechanics, agreed with the experimental results.
process depends on temperature differences and can be seen whether the
temperature change across the sample is rapid or gradual,” Lacks said.
diffusion through gases was one of the first methods used to separate
isotopes, during the Manhattan Project. It turns out that isotope
fractionation through silicate liquids is even more efficient than
can occur inside the Earth wherever a sustained temperature gradient
exists,” Van Orman said. “One place this might happen is at the margin
of a magma chamber, where hot magma rests against cold rock. Another is
nearly 1,800 miles inside the Earth, at the boundary of the liquid core
and the silicate mantle.”
researchers are now adding pressure to the variables as they
investigate further. This work was done at atmospheric pressure but
where the Earth’s core and mantle meet, the pressure is nearly 1.4
and Van Orman are unsure whether high pressure will result in greater
or lesser fractionation. They can see arguments in favor of either.
Isotope Fractionation by Thermal Diffusion in Silicate Melts