Giant SHRIMP to target
big questions
BY FRANCES COLE
Picture the solar nebula,
that hot cloud of gas and dust that collapsed to form our
solar system. Where did the gas and dust molecules come
from? And what was the sequence of events that
transformed them from a swirling amorphous blob into the
well-organized planets and atmospheres that we know
today?
Related
Information:
A brand new $2.5 million,
12-ton instrument called the SHRIMP arrived at Stanford
this past April and is poised to answer these and other
fundamental questions about the origins of our Earth and
solar system. The SHRIMP is not a time machine, and it's
not an overgrown crustacean. It's a Sensitive High
Resolution Ion MicroProbe,
arguably the most coveted instrument of its type in the
world, equaled only by its twin at the Australian
National University in Canberra, where both machines were
designed and built.
This is the machine whose
predecessors determined the ages of the oldest minerals
on Earth in 1983, the oldest rocks on Earth (3.96 billion
years) in 1989, and the oldest minerals in the solar
system (4.56 billion years) in 1992.
The SHRIMP was purchased
jointly by the Stanford School of Earth Sciences and the
U.S. Geological Survey as a result of an agreement signed
in 1989. Geological and environmental sciences Professor
Gary Ernst, who was dean of earth sciences at the time,
saw the SHRIMP as a remarkable opportunity to attract
collaborative world-class geochemical research to
Stanford and to enhance ties with the U.S. Geological
Survey.
The SHRIMP is located in
the basement of the Green Earth Sciences building and
operates under the direction of Trevor Ireland, assistant
professor in geological and environmental sciences, who
came to Stanford from the Australian National University,
and Joe Wooden of the U.S. Geological Survey. Brad Ito,
also of the USGS, plays a critical role as full-time
electronics technician. Mike McWilliams, associate
professor of geophysics and geological and environmental
sciences at Stanford, and Charlie Bacon of the USGS
contribute to planning and coordinating the SHRIMP's busy
research schedule.
Earth and planetary
scientists already are lining up to get bits of their
favorite rocks into the new SHRIMP, because this machine
is not only shiny-new, super-fast and highly precise,
it's also very easy to use. Give it a tiny grain of
Earth, Mars, interstellar dust or other solid material,
and the SHRIMP can divine the exact chemical constituents
of the sample down to minuscule differences in atomic
mass within 15 minutes. Four sample analyses per hour,
30-some analyses per day that's enough information to
satisfy a data junkie's habit indefinitely.
Rocks and minerals that
Stanford scientists are preparing for the SHRIMP include
bits of stardust from very old meteorites, minerals from
far-traveled sedimentary basins in western Canada, and
samples from deep crustal rocks coughed up by volcanoes
in the Bering Strait region, near the border between
Alaska and Russia.
The creators of the new
SHRIMP assert that it is endowed with a combined
sensitivity and mass resolution that far surpasses that
of any previous ion probe. This instrument has the
sensitivity to detect very small concentrations of atoms,
down to a few parts per billion. And its mass resolution
is 40,000, meaning that it can distinguish between atoms
that differ in mass by as little as one part in 40,000.
That's analogous to discriminating between a 20-ton whale
(that's 40,000 pounds) and a 20-ton whale who just ate a
pound of plankton (that's 40,001 pounds).
Several earlier ion
probes, including the SHRIMP's predecessors, SHRIMP I and
SHRIMP II, and its closest competitors, the French CAMECA
probes, have comparable sensitivity ratings, but much
lower mass resolution, on the order of 5000 for the
CAMECA probes and 10,000 for the earlier SHRIMP models.
Here's how the SHRIMP
works. It shoots the sample, usually an individual
mineral grain from a rock or meteorite, with high-energy
oxygen ions fired at speeds of 350 kilometers per second
or nearly 800,000 miles per hour. The oxygen ions are
focused into a very fine beam about the width of a single
strand of human hair. The ions have a negative electrical
charge, and when they hit the sample they kick off
positively charged ions and leave impact craters like
tiny potholes on its surface.
This process, called
"sputtering," liberates ions from the sample
and sends them traveling down a tube into a curved magnet
about 1 meter long. The magnet separates the ions
according to their mass and energy, so that lighter and
slower ions hug the inside lane, whereas heavier and
faster ones are accelerated to the outer lanes.
The ions exit the magnet
in a broad beam, then enter an electrostatic compensator,
which re-organizes them according to mass only, removing
the effects of energy differences between ions of the
same mass. The result, on the exit end of the
electrostatic compensator, is a spectrum of ions
perfectly organized in order of increasing mass from
hydrogen, with an atomic mass of one, up to uranium, with
an atomic mass of 238. The scientist can inspect the part
of the mass spectrum of interest, at the collector, and
ascertain the exact proportions of chemical elements
sputtered out of the sample.
So how do these sputtered
ions lead to the age of a rock, or better yet, the
origins of the solar system? By way of radiometric dating
and isotopic fingerprinting. In both cases, the key is
the isotopes atoms of the same element that have
slight differences in mass. Radiometric dating uses
certain isotopes of uranium and thorium which over time
turn to lead by radioactive decay. By measuring relative
abundances of the original isotopes and their decay
products, it is possible to calculate the age in millions
of years of a very old rock.
Isotopic fingerprinting is
applied mainly to extraterrestrial samples, usually from
meteorites. The presence of particular isotopes can be
used to link samples of unknown origins to a probable
source inside or outside the solar system. For example,
scientists believe certain meteorites came from Mars
because they have that unusual mix of hydrogen isotopes
that is peculiar to Mars. The SHRIMP is well equipped for
both these types of isotopic studies, because it has the
resolution to measure and compare ions with very small
mass differences and the sensitivity to obtain good
results from very small samples, typically a limiting
factor in extraterrestrial research.
Although the more
conventional applications for the SHRIMP are in
radiometric dating, its greatest potential may lie in
isotopic studies of the early solar system. Until
recently, all of the gas and dust particles in the solar
nebula were thought to have been thoroughly heated and
mixed that is, totally homogenized prior to that
momentous collapse that led to agglomeration of the sun
and orbiting planets. This explains why most bodies in
our solar system show broadly similar isotopic trends.
However, recent isotopic
studies, some of which were conducted on earlier models
of the SHRIMP by Trevor Ireland and his colleagues at the
Australian National University, have shaken the
long-standing homogenization theories. Some star dust
particles embedded in early-formed meteorites contain
highly anomalous isotope concentrations when compared
with normal abundances for the Earth, sun and normal
meteorites. How these bits of dust escaped homogenization
is not clear, but their pristine chemistry makes them an
important link to possible interstellar sources for the
stuff in the solar nebula. Some particles may have
drifted in on prevailing interstellar winds. Others may
have been catastrophically blown into our solar nebula by
the explosion of a neighboring star. Could such an
explosion have triggered the collapse of the solar
nebula? This is a scientific frontier rife with new
questions and rapidly changing theories, and the new
SHRIMP promises to feed this debate with much-needed
isotopic data.
Scientists at Stanford and
the U.S. Geological Survey also have plans for more
down-to-Earth applications for the SHRIMP. Kathy
Degraaff, Stanford doctoral student and recipient of the
U.S. Geological Survey fellowship award, will delve into
a hot controversy over the geographic origins of a big
chunk of western Canada. It has been proposed that most
of what we call British Columbia is a recent arrival from
a position near Baja California. By analyzing mineral
grains in sedimentary rocks from British Columbia and
comparing their isotopic signatures with possible source
terrains up and down the western part of North America,
DeGraaff hopes to see where these rocks may have
originated if they are indeed immigrants from Mexico.
Stanford geological and
environmental sciences Professor Elizabeth Miller and
doctoral student Jeremy Hourigan, along with Russian
colleagues Slava Akinin and Julia Apt from the
Northeastern Interdisciplinary Scientific Research
Institute in Magadan, plan to use the SHRIMP for
radiometric dating of rocks in the Bering Strait region
between Alaska and Russia. Over the past 30 million
years, volcanoes in the Bering Strait region have been
coughing up fragments of rock thought to have originated
deep in the continental crust at depths of 10 kilometers
or more. Miller and her colleagues hope that age-dating
these crustal blocks will help them to develop better
models for the history of tectonic stretching and
crystallization of the Earth's crust in this poorly
understood region.
Stay tuned for new
developments with the SHRIMP by visiting the web site http://shrimprg.stanford.edu, which includes a time-lapse tour
of the SHRIMP's installation and assembly. You can also
e-mail SHRIMP gurus Trevor Ireland, tri@pangea.stanford.edu, and Joe Wooden, jwooden@mojave.wr.usgs.gov for further information on what
the SHRIMP can do and how it works. SR
Frances Cole is a
recent M.S. graduate in geological and environmental
sciences at Stanford.
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