[meteorite-list] Silicate Stardust in Meteorites
Ron Baalke
baalke at zagami.jpl.nasa.gov
Wed Jun 2 13:33:33 EDT 2004
http://www.psrd.hawaii.edu/June04/silicatesMeteorites.html
Silicate Stardust in Meteorites
Planetary Science Research Discoveries
June 1, 2004
--- Silicates are the most abundant solids
in disks around growing stars, but they
have not been found in even the most
primitive meteorite--until now.
Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology
One of the most exciting discoveries in cosmochemistry during the past 15
years is the presence of presolar grains in meteorites. They are identified
by the unusual abundances of isotopes of oxygen, silicon, and other
elements. Presolar grains, also called stardust, are exotic compounds such
as diamond, graphite, aluminum oxide, and silicon carbide. Why are there no
silicates? Spectroscopic observations of young stars show that silicates are
abundant. This means that silicates are abundant in molecular clouds like
the one in which the solar system formed. Cosmochemists wondered why do we
not find silicates in the most primitive extraterrestrial materials:
interplanetary dust particles (IDPs) and primitive chondrites. These
materials are the least altered since they formed and if any preserved
presolar silicate grains, IDPs and chondrites would. Were they all destroyed
as the solar system formed? Or was it that we were looking for stardust in
all the wrong places?
As we reported previously [see PSRD article A New Type of Stardust], Scott
Messenger and colleagues have found silicates in IDPs. Now, researchers
report finding presolar silicate grains in primitive chondritic meteorites.
Ann Nguyen and Ernst Zinner (Washington University in St. Louis) and
Kazuhide Nagashima and Hisayoshi Yurimoto (Tokyo Institute of Technology),
with Alexander Krot (University of Hawai`i) used advanced instrumentation to
image the isotopic compositions of small regions of the Acfer 094
carbonaceous chondrite and found several silicate grains with isotopically
anomalous oxygen isotopes, a clear indicator of presolar origin. Nagashima
and his colleagues also investigated the primitive CR2 carbonaceous
chondrite Northwest Africa 530, finding presolar grains in it as well. The
grains will shed (star)light on the histories of the stars in which they
formed. The relative abundances of presolar silicates in different types of
meteorites will help cosmochemists understand the processes of heating and
chemical reaction that took place in the cloud of gas and dust in which the
Sun and planets formed. The significance of this work is discussed in a
lucid editorial by Sara Russell (Natural History Museum, London.)
References:
Nguyen, A. N. and Zinner, E. (2004) Discovery of ancient silicate
stardust in a meteorite. Science, v. 303, p. 1496-1499.
Nagashima, K., Krot, A. N., and Yurimoto, H. (2004) Stardust silicates
from primitive meteorites. Nature, v. 428, p. 921-924.
Russell, S. S. (2004) Stars in stones. Nature, v. 428, p. 903-904.
--------------------------------------------------
Abundant Silicates
Telescopic observations of the disks around young stars and materials
surrounding other stars indicate an abundance of both amorphous and
crystalline silicates dust grains. Crystalline versions are magnesium-rich
olivine and pyroxene. This is not particularly surprising. The Earth and
other inner planets are composed mostly of silicates. Meteorites and the
asteroids they come from are mostly silicates. The tiny grains composing
interplanetary dust particles (IDPs) are mostly silicates. Comets have been
called dirty snowballs--the dirt is silicate. Silicates abound.
[star AB Aurigae disk]
Photograph taken by the Hubble Space Telescope of a swirling disk
of gas and dust surrounding the young star AB Aurigae. The black
bars are part of the coronographic system used by Hubble.
Spectroscopic observations show that silicate compounds are
abundant in disks like this one. (High resolution version will open
in a new window.)
If silicates are so common, why have all the presolar grains we find in
meteorites been oxides, carbides, nitrides, and carbon compounds? One
explanation is that until recently presolar grains were found by using
strong acids to dissolve 99.9% of the meteorite, leaving a residue of
acid-resistant grains. Silicates are not resistant to acids, so were lost
during the process. Another reason is that silicates are abundant, but
presolar silicates make up only a tiny fraction of all the silicates. In
contrast, all silicon carbide grains are presolar. It would be better to
simply measure the isotopic compositions of tiny grains in meteorites and
IDPs until we found silicates with anomalous isotopic compositions, or
concluded that there are none. But this would require measuring thousands of
grains, many only a micrometer or less across, an impossible task--until
technological advances made the search possible.
--------------------------------------------------
Technological Innovations
The ideal way to find small presolar survivors of solar system formation is
to be able to image a sample using a special light that causes presolar
grains to light up, as if they were using microscopic flares to help us
locate them. That is more or less what advances in instrumentation allow us
to do. Ann Nguyen and Ernst Zinner used the latest type of ion microprobe,
the NanoSIMS at Washington University in St. Louis. SIMS stands for
secondary ion mass spectrometry, and the "Nano" refers to the tiny size of
the ion beam used to sputter atoms from a sample. The beam size can be as
small as 30 nanometers in diameter, so the abundances of isotopes can be
measured in individual submicron grains. The NanoSIMS is manufactured by
Cameca Instruments, Inc. (Paris, France), but almost all instruments
purchased by cosmochemists are modified substantially to make them even more
capable.
Besides having a tiny beam size and excellent sensitivity, the NanoSIMS can
measure up to five isotopes simultaneously, allowing precise measurements on
the identical spot. Most important for the hunt for presolar silicates is
the ability to raster the beam over a region packed with hundreds or
thousands of grains, allowing automatic measurements of a large number of
grains in a relatively short time. By analyzing oxygen isotopes (16O, 17O,
and 18O), the rare presolar grains will be conspicuous in a sea of grains
with normal solar system oxygen isotopic composition, like supernovae in the
normal starry sky.
[NanoSIMS at Wash U]
This is a photograph of the Cameca NanoSIMS at Washington
University in St. Louis, the first such instrument in the world.
The major parts are labeled. This state-of-the-art instrument makes
it possible to measure the isotopic compositions of extremely small
grains in extraterrestrial materials. Ann Nguyen and Ernst Zinner
used this instrument to discover presolar silicate grains in the
meteorite Acfer 094.
Kazu Nagashima and Yoshi Yurimoto used a different type of ion microprobe,
the Cameca IMS 1270. This instrument has great sensitivity, but it does not
have the required nanometer beam size (grains must be about 10 micrometers
or larger). To get around this problem, they and their colleagues at TiTech
devised a whole new ion detection system, building a solid-state detector
called SCAPS. Working backwards, APS refers to "active pixel sensor." The C
stands for CMOS, or complementary metal oxide semiconductor; in short, a
transistor. The S stands for stacked, referring to the arrangement of metal
layers for ion-irradiation stacked on the APS. The device allows for
sensitive measurements and no distortions in the signal across the detector,
and can measure the isotopic compositions of oxygen and other elements in
grains as small as about 1 micrometer, but can detect presolar grains as
small as about 100 nanometers.
[Isotope microscope system at TiTech]
Isotope microscope system consisting of the SCAPS and IMS 1270
ion microprobe. The SCAPS detector is located on the
image-focusing plane inside the vacuum of the ion microprobe.
--------------------------------------------------
The Right Extraterrestrial Materials
Formation of the solar system involved heating of the cloud of gas and dust
surrounding the nascent Sun. This caused some silicate grains to vaporize
and others to react chemically. As planetesimals formed, they heated up by
decay of short-lived isotopes such as 26Al or possibly by impact, causing
additional chemical reactions including those driven by water. Thus, much of
the record of presolar silicates has been destroyed. To find them we must
search in the least altered materials we can find.
Scott Messenger and his colleagues at Washington University in St. Louis
(including Ernst Zinner) searched for presolar grains in IDPs (see PSRD
article A New Type of Stardust). Ann Nguyen and Zinner, and Kazu Nagashima
and his colleagues obtained samples of one of the most primitive
carbonaceous chondrites, Acfer 094. This meteorite was found in the Sahara
desert, an excellent place for preservation since the meteorite arrived on
Earth. More importantly, Ansgar Greshake (Wilhelms-Universität, Münster,
Germany) showed through detailed electron microscopy that the meteorite is
exceptionally primitive and has little or no aqueous alteration. Its
fine-grained matrix contains a substantial amount of amorphous silicate
material that contains tiny (few hundred nanometers) grains of olivine,
low-Ca pyroxene, and metallic Fe-Ni. The meteorite contains the highest
content of presolar SiC (obtained by the whole-rock dissolution method),
leading Greshake to suggest that some of the silicates may also be presolar.
[Element abundances in Acfer 094]
Map of Acfer 094 made from element abundances determined with an
electron microprobe. The presolar grains were found in the
fine-grained matrix (colored brown). Ca,Al-rich inclusions are
colored blue. Chondrules and amoeboid olivine aggregates are
colored red. The Acfer 094 meteorite is one of the most primitive
carbonaceous chondrites known, making it a logical target for the
search for presolar silicates.
Kazu Nagashima and his colleagues studied Acfer 094, too, but also searched
for presolar silicates in Northwest Africa (NWA) 530. This meteorite is also
a very primitive carbonaceous chondrite that experienced some very mild
aqueous alteration, but no heating.
--------------------------------------------------
Silicate Stardust
The new technologies and well-chosen primitive samples have led to the
unambiguous discovery of presolar silicate grains in chondritic meteorites.
They show up on images of the 17O/16O ratio and 18O/16O ratio, and other
measurements prove that the grains are silicates.
[O-isotopes of grain in Acfer 094]
Images of the isotopic composition of a small region (only 100
square micrometers) in Acfer 094. The scale on the right is in
deviations from solar oxygen isotopic composition in parts per
thousand. Most of the material has typical solar system
compositions, averaging a zero deviation from solar. The grain
identified with the red circle, however, stands out by being
enriched in 17O and depleted in 18O compared to solar values; it is
a presolar grain. Chemical analyses show that it is a silicate (see
graph below).
--------------------------------------------------
[X-ray counts vs energy for Acer 094 grain]
Scanning electron microscope (SEM) image (insert) of the presolar
grain shown above, with the spectrum of x-rays emitted from the
particle due to electron bombardment in the scanning electron
microscope (SEM). The tall peaks for oxygen and silicon show that
it is a silicate. The presence of magnesium and calcium indicate
that it is a pyroxene. Carbon is a contaminate in the SEM; gold is
from the substrate the sample was placed on.
Nagashima and his colleagues surveyed a total area of 44,100 square microns
in each meteorite, and Nguyen and Zinner surveyed 5750 square microns in
Acfer 094. In total they found 15 grains that are clearly different from
average solar material in their oxygen isotopic compositions. For Acfer,
this translates into an abundance of 30 to 40 parts per million, which is
much more than the abundance of non-silicate presolar grains. Silicon
carbide is the most abundant (except for nanodiamonds, whose presolar origin
is somewhat disputed), at 14 parts per million. So, not only did our
silicate searchers find presolar silicate grains, they found that silicates
are more abundant than oxides, carbides, and nitrides.
[Presolar olivine grain in Acfer 094] [Presolar grains in Acfer 094]
LEFT: Backscattered electron image of a presolar olivine grain in
the Acfer 094 chondrite. The SCAPS detector in the Cameca IMS 1270
allows in situ isotope imaging of such small grains. (Courtesy of
Kazu Nagashima.) RIGHT: Oxygen isotopic compositions of nine
anomalous (hence presolar) grains in Acfer 094 from Nguyen and
Zinner. The large cloud of points centered at the intersection of
the dashed lines is where most components in chondrites and IDPs
plots--it is average solar system material. The numbered grains are
significantly different in composition from ordinary solar system,
and their positions on the diagram suggest origins in different
types of stellar environments.
Continued searches for presolar grains in meteorites will undoubtedly lead
to additional tiny samples. Their isotopic compositions will allow
cosmochemists and astrophysicists to test models of star formation and
evolution. Variation in abundances in different types of meteorites and
subtle elemental differences among the populations of presolar grains in
different meteorites will help us understand the extent of heating and
chemical reaction during formation of the solar system.
None of this would be possible without the great leaps forward in analytical
instrumentation. There is an essential synergism between cosmochemistry and
instrument development. One drives the other. As our understanding of
extraterrestrial materials improves, we ask sophisticated questions that
require better analytical capabilities to answer. Cosmochemists and other
scientists and engineers develop new tools, such as the SCAPS detector,
allowing the questions to be addressed. Of course, this leads to other
questions. It is an exciting cycle of discovery and instrument innovation,
punctuated by an influx of new extraterrestrial materials--Apollo samples 25
years ago, Antarctic meteorites, recognition of meteorites from Mars,
collection of IDPs in the stratosphere, return of the samples from the
Stardust misson.
--------------------------------------------------
ADDITIONAL RESOURCES
Nagashima, K., Krot, A. N., and Yurimoto, H. (2004) Stardust silicates
from primitive meteorites. Nature, v. 428, p. 921-924.
Nguyen, A. N. and Zinner, E. (2004) Discovery of ancient silicate
stardust in a meteorite. Science, v. 303, p. 1496-1499.
Russell, S. S. (2004) Stars in stones. Nature, v. 428, p. 903-904.
Taylor, G. J. (2003) A new type of stardust. Planetary Science Research
Discoveries. http://www.psrd.hawaii.edu/Aug03/stardust.html
Additional Microimaging and SCAPS Development References:
Nagashima, K., Kunihiro, T., Takayanagi, I., Nakamura, J., Kosaka,
K. and Yurimoto, H. (2001) Output characteristics of stacked
CMOS-type active pixel sensor for charged particles. Surf. Interf.
Anal., v. 31, p. 131-137.
Takayanagi, I., Nakamura, J., Fossum, E.R., Nagashima, K.,
Kunihiro, T., Yurimoto, H. (2003) Dark current reduction in
stacked-type CMOS-APS for charged particle imaging. IEEE Trans.
Electron Dev., v. 50, p. 70-76.
Yurimoto, H., Nagashima, K. and Kunihiro, T. (2003) High precision
isotope micro-imaging of materials. Appl. Surf. Sci., 203-204, p.
793-797.
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