[meteorite-list] Cosmochemist Discovers Potential Solution to Meteorite Mystery

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 8 Jul 2013 16:06:49 -0700 (PDT)
Message-ID: <201307082306.r68N6nEa026693_at_zagami.jpl.nasa.gov>

http://news.uchicago.edu/article/2013/07/08/cosmochemist-discovers-potential-solution-meteorite-mystery

Cosmochemist discovers potential solution to meteorite mystery

Chondrules may have formed from high-pressure collisions in early solar system

By Steve Koppes
University of Chicago
July 8, 2013

A normally staid University of Chicago scientist has stunned many of his
colleagues with his radical solution to a 135-year-old mystery in cosmochemistry.
"I'm a fairly sober guy. People didn't know what to think all of a sudden,"
said Lawrence Grossman, professor in geophysical sciences.

At issue is how numerous small, glassy spherules had become embedded within
specimens of the largest class of meteorites - the chondrites. British
mineralogist Henry Sorby first described these spherules, called chondrules,
in 1877. Sorby suggested that they might be "droplets of fiery rain" which
somehow condensed out of the cloud of gas and dust that formed the solar
system 4.5 billion years ago.

Researchers have continued to regard chondrules as liquid droplets that
had been floating in space before becoming quickly cooled, but how did
the liquid form? "There's a lot of data that have been puzzling to people,"
Grossman said.

Grossman's research reconstructs the sequence of minerals that condensed
from the solar nebula, the primordial gas cloud that eventually formed
the sun and planets. He has concluded that a condensation process cannot
account for chondrules. His favorite theory involves collisions between
planetesimals, bodies that gravitationally coalesced early in the history
of the solar system. "That's what my colleagues found so shocking, because
they had considered the idea so 'kooky,'" he said.

Cosmochemists know for sure that many types of chondrules, and probably
all of them, had solid precursors. "The idea is that chondrules formed
by melting these pre-existing solids," Grossman said.

One problem concerns the processes needed to obtain the high, post-condensation
temperatures necessary to heat the previously condensed solid silicates
into chondrule droplets. Various astonishing but unsubstantiated origin
theories have emerged. Maybe collisions between dust particles in the
evolving solar system heated and melted the grains into droplets. Or maybe
they formed in strikes of cosmic lightning bolts, or condensed in the
atmosphere of a newly forming Jupiter.

Another problem is that chondrules contain iron oxide. In the solar nebula,
silicates like olivine condensed from gaseous magnesium and silicon at
very high temperatures. Only when iron is oxidized can it enter the crystal
structures of magnesium silicates. Oxidized iron forms at very low temperatures
in the solar nebula, however, only after silicates like olivine had already
condensed at temperatures 1,000 degrees higher.

At the temperature at which iron becomes oxidized in the solar nebula,
though, it diffuses too slowly into the previously formed magnesium silicates,
such as olivine, to give the iron concentrations seen in the olivine of
chondrules. What process, then, could have produced chondrules that formed
by melting pre-existing solids and contain iron oxide-bearing olivine?

"Impacts on icy planetesimals could have generated rapidly heated, relatively
high-pressure, water-rich vapor plumes containing high concentrations
of dust and droplets, environments favorable for formation of chondrules,"
Grossman said. Grossman and his UChicago co-author, research scientist
Alexei Fedkin, published their findings in the July issue of Geochimica
et Cosmochimica Acta.

Grossman and Fedkin worked out the mineralogical calculations, following
up earlier work done in collaboration with Fred Ciesla, associate professor
in geophysical sciences, and Steven Simon, senior scientist in geophysical
sciences. To verify the physics, Grossman is collaborating with Jay Melosh,
University Distinguished Professor of Earth & Atmospheric Sciences at
Purdue University, who will run additional computer simulations to see
if he can recreate chondrule-forming conditions in the aftermath of planetesimal
collisions.

"I think we can do it," Melosh said.

Longstanding objections

Grossman and Melosh are well-versed in the longstanding objections to
an impact origin for chondrules. "I've used many of those arguments myself,"
Melosh said.

Grossman re-evaluated the theory after Conel Alexander at the Carnegie
Institution of Washington and three of his colleagues supplied a missing
piece of the puzzle. They discovered a tiny pinch of sodium - a component
of ordinary table salt - in the cores of the olivine crystals embedded
within the chondrules.

When olivine crystallizes from a liquid of chondrule composition at temperatures
of approximately 2,000 degrees Kelvin (3,140 degrees Fahrenheit), most
sodium remains in the liquid if it doesn't evaporate entirely. But despite
the extreme volatility of sodium, enough of it stayed in the liquid to
be recorded in the olivine, a consequence of the evaporation suppression
exerted by either high pressure or high dust concentration. According
to Alexander and his colleagues, no more than 10 percent of the sodium
ever evaporated from the solidifying chondrules.

Grossman and his colleagues have calculated the conditions required to
prevent any greater degree of evaporation. They plotted their calculation
in terms of total pressure and dust enrichment in the solar nebula of
gas and dust from which some components of the chondrites formed. "You
can't do it in the solar nebula," Grossman explained. That's what led
him to planetesimal impacts. "That's where you get high dust enrichments.
That's where you can generate high pressures."

When the temperature of the solar nebula reached 1,800 degrees Kelvin
(2,780 degrees Fahrenheit), it was too hot for any solid material to condense.
By the time the cloud had cooled to 400 degrees Kelvin (260 degrees Fahrenheit),
however, most of it had condensed into solid particles. Grossman has devoted
most of his career to identifying the small percentage of substances that
materialized during the first 200 degrees of cooling: oxides of calcium,
aluminum and titanium, along with the silicates. His calculations predict
condensation of the same minerals that are found in meteorites.

Over the last decade, Grossman and his colleagues have written a slew
of papers exploring various scenarios for stabilizing iron oxide enough
that it would enter the silicates as they condensed at high temperatures,
none of which proved feasible as an explanation for chondrules. "We've
done everything that you can do," Grossman said.

This included adding hundreds or even thousands of times the concentrations
of water and dust that they had any reason to believe ever existed in
the early solar system. "This is cheating," Grossman admitted. It didn't
work anyway.

Instead, they added extra water and dust to the system and increased its
pressure to test a new idea that shock waves might form chondrules. If
shock waves of some unknown source had passed through the solar nebula,
they would have rapidly compressed and heated any solids in their path,
forming chondrules after the melted particles cooled off. Ciesla's simulations
showed that a shock wave can produce silicate liquid droplets if he increased
the pressure and the quantities of dust and water by these abnormally
if not impossibly high amounts, but the droplets would be different from
the chondrules actually found in meteorites today.

Cosmic shoving match

They differ in that actual chondrules contain no isotopic anomalies, whereas
the simulated shock-wave chondrules do. Isotopes are atoms of the same
element that have different masses from one another. The evaporation of
atoms of a given element from droplets drifting through the solar nebula
causes the production of isotopic anomalies, which are deviations from
the normal relative proportions of the element's isotopes. It's a cosmic
shoving match between dense gas and hot liquid. If the number of a given
type of atoms pushed out of the hot droplets equals the number of atoms
getting pushed in from the surrounding gas, no evaporation will result.
This prevents isotope anomalies from forming.

The olivine found in chondrules presents a problem. If a shock wave formed
the chondrules, then the olivine's isotopic composition would be concentrically
zoned, like tree rings. As the droplet cools, olivine crystallizes with
whatever isotopic composition existed in the liquid, starting at the center,
then moving out in concentric rings. But no one has yet found isotopically
zoned olivine crystals in chondrules.

Realistic-looking chondrules would result only if evaporation were suppressed
enough to eliminate the isotope anomalies. That, however, would require
higher pressure and dust concentrations that go beyond the range of Ciesla's
shock-wave simulations.

Providing some help was the discovery a few years ago that chondrules
are one or two million years younger than calcium-aluminum-rich inclusions
in meteorites. These inclusions are exactly the condensates that cosmochemical
calculations dictate would condense in the solar nebular cloud. That age
difference provides enough time after condensation for planetesimals to
form and start colliding before chondrules form, which then became part
of Fedkin and Grossman's radical scenario.

They now say that planetesimals consisting of metallic nickel-iron, magnesium
silicates and water ice condensed from the solar nebula, well ahead of
chondrule formation. Decaying radioactive elements inside the planetesimals
provided enough heat to melt the ice.

The water percolated through the planetesimals, interacted with the metal
and oxidized the iron. With further heating, either before or during planetesimal
collisions, the magnesium silicates re-formed, incorporating iron oxide
in the process. When the planetesimals then collided with each other,
generating the abnormally high pressures, liquid droplets containing iron
oxide sprayed out.

"That's where your first iron oxide comes from, not from what I've been
studying my whole career," Grossman said. He and his associates have now
reconstructed the recipe for producing chondrules. They come in two "flavors,"
depending on the pressures and dust compositions arising from the collision.

"I can retire now," he quipped.

------------
Citation: "Vapor saturation of sodium: Key to unlocking the origin of
chondrules," by Alexei V. Fedkin and Lawrence Grossman, Geochimica et
Cosmochimica Acta, Vol. 112, July 2013, pages 226-250.

Funding: National Aeronautics and Space Administration
Received on Mon 08 Jul 2013 07:06:49 PM PDT