neighboring mass isotopes present in high abundance is the goal of a new generation of mass spectrometers now being developed. These high-dynamic-range isotope-ratio mass spectrometers offer the promise of being able to make direct measurements of rare short-lived radioisotopes such as carbon-14, helium-3, beryllium-10, and thorium-230. As currently measured by decay-counting techniques, the study of these isotopes at natural abundances requires long counting times (days to months) and large samples. The improvement, by several orders of magnitude, in sensitivity offered by the new generation of mass spectrometers will allow for greatly improved precision in dating geological processes that occur on the time scale of a few hundred to several million years. The advent of accelerator mass spectrometry (AMS), based on high-voltage accelerators, offers ultra-high dynamic range and the capability of measuring isotopic ratios as small as 10–16. AMS has had an important and exciting impact on the earth sciences. It has enhanced the utility of a great variety of cosmogonic radioisotopes in geological studies and has opened up entire fields to quantitative study, such as the age and circulation patterns of groundwater, the rate of physical and chemical erosion in surface and near-surface crustal environments, and the role of subducted sediments in island-arc volcanism. As one example, AMS is providing an understanding of the evolution of island arcs and the role of marine sediments in that evolution through the measurement of beryllium-10 in island-arc volcanic rocks. The unique capabilities of AMS have also allowed extension of the carbon-isotope dating technique to smaller and older samples. Many exciting applications involving other cosmogonic isotopes have been identified and need to be developed systematically.

  • High-Spatial-Resolution Analysis. The primary tool for measuring major and minor elements in individual mineral grains is the electron microprobe. Continuing improvements in these instruments have increased their spatial resolution (now at the micrometer level), speed of analysis, and ease of operation. The performance capabilities of the instrument are very well suited to a wide variety of studies as well as to more general applications in materials sciences, chemistry, engineering, and life sciences. However, their role in modern earth science research is so important that at most universities electron microprobes are based in earth science departments.

High spatial resolution for trace elements and isotopes has been sought primarily through the so-called ion microprobe or the secondary ionization mass spectrometer (SIMS). The super-high-resolution ion microprobe, or SHRIMP, developed at the Australian National University, is a very successful version of a SIMS instrument that emphasizes mass resolution. Successful SIMS applications include reliable radiometric determinations on a single mineral grain or even age zonations within grains, isotopic tracing in meteorites of nucleosynthesis in the solar system, and measurement of trace element distributions between minerals. High-spatial-resolution trace element analysis is useful in studies of magma formation and differentiation, in kinetic studies of metamorphic reactions, and in laboratory studies of distribution coefficients and diffusion kinetics. Another important application of the ion microprobe is determination of the abundances and isotopic compositions of trace elements in fluid inclusions of minerals, which provide information on the pressure, temperature, and composition of the fluids that form ore deposits.

High-Pressure, High-Temperature Technology

It is now possible to investigate material properties under the extreme conditions in the Earth's interior. Pressure, temperature, and chemical variables can be controlled in ways that simulate conditions even in the core. Indeed, earth scientists have pioneered techniques for achieving the highest sustained pressure and temperature conditions attainable in the laboratory. Furthermore, simultaneous developments in synchrotron radiation and geochemical analysis techniques will permit molecular structure, bulk density, elastic and viscous properties, phase transitions, and diffusion rates to be directly determined while the material is subject to the extreme conditions of the deep interior.

High-pressure, high-temperature research on samples larger than those that can be studied in diamond anvils is now carried out largely in Japan; there are a few such laboratory facilities in the United States. Large volumes (cubic millimeters or larger) are especially critical for investigating phase transitions in polyphase systems, deformation mechanisms, and interfacial or grain-boundary phenomena. Pressures up to 300 kb (30 GPa) and temperatures in excess of 2000°C can now be obtained with existing devices, making them ideal for studying the structure and processes of the mantle.

New technology is needed to reach pressures



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement