June 9, 1998 By David F. Salisbury
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?
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, email@example.com, and Joe Wooden, firstname.lastname@example.org for further information on what the SHRIMP can do and how it works.
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