Radiogenic Isotopes and the Eastern Mineral Resources Program of the U.S. Geological Survey

Radiogenic isotope geology relies on a small group of naturally occurring but unstable elements that naturally produce energy and disintegrate or decay to more stable atoms of different elements. Because this phenomenon occurs at a known rate, is unaffected by most physical and chemical processes, and takes place over periods of time comparable to the age of the Earth, these elements can be used in radiometric dating or geochronology.

Radioactive Decay and Age Determinations

Differences in the number of protons in the nucleus are used to distinguish elements so that the total number of protons represents the element's unique atomic number. The sum of the masses of protons and neutrons in the nucleus represents the atomic weight of an atom. Although all atoms of a given element have the same atomic number, atoms of a given element may contain different numbers of neutrons in the nucleus; such atoms of a given element that differ only as a function of the number of neutrons in the nucleus are known as isotopes. Lead (symbol Pb; atomic number 82) has four naturally occurring isotopes of different masses (²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb). Only the isotope of Pb with mass ²⁰⁴Pb is stable; ²⁰⁶Pb and ²⁰⁷Pb, on the other hand, are daughter products resulting from the radioactive decay of uranium (U), and ²⁰⁸Pb is a product from thorium (Th) decay. As a result of decay, the heavier isotopes of lead will increase in abundance compared to ²⁰⁴Pb.

The half-lives of these decays -- the time necessary for the original number of atoms of a radioactive element in a rock to be reduced by half -- are relatively well known and range from hundreds of millions to billions of years. Small amounts of radio- active elements (for example, potassium, K; rubidium, Rb; samarium, Sm; lutetium, Lu; rhenium, Re; uranium, U; and thorium; Th) used for age determinations are found in common minerals, rocks, soils, sediments, and fluids. Geologists are able to read Nature's radioactive clocks as follows:

• by extracting and purifying the radioactive parent and daughter from the relevant rock or mineral,

• by precisely measuring variations in the masses of the parent and daughter isotopes using machines known as mass spectrometers (figs. 1, 2), and

• by combining the isotope abundances with the known rates of decay to calculate an age.

Figure 1. Features of a mass spectrometer. A mass analysis is obtained by volatilizing the element placed in the thermal ion source of the machine, which is kept at high vacuum. The vapor is then ionized and accelerated through a magnetic field produced by an electromagnet. The ion beam travels through the analyzer tube and then goes to a collector, where it is measured.

Figure 2. Automated, nine-collector Finnigan-MAT 262 mass spectrometer equipped with ion-counting capabilities. This spectrometer is used by the U.S. Geological Survey in Reston, Va., for research. Photograph by David Usher.

As the physical laws governing radioactive decay are universally obeyed, radiogenic isotope dating techniques represent a quantitative method to reliably establish the age of materials in Nature, applicable even to the oldest objects on the Earth, the Moon, meteorites, and asteroids and, potentially, in the rest of the universe.

Isotopes used for age determinations include radiogenic Sr (strontium produced by decay of radioactive rubidium), Nd (neodymium from decay of samarium), and Os (osmium from decay of rhenium); application of all radiogenic systems requires knowledge of the appropriate half-lives and chemical analyses to detect the relative abundances of parents and daughters. Isotopic dating is useful for all types of rocks having a closed system -- geologists assume that the age is an estimate of the time elapsed since the radioactive parent became part of the rock or mineral and that no parent elements or daughter products have been added or lost.

Isotopic Tracer Geology

• changes in temperature and pressure during transport and accumulation,

• different rates of chemical reactions, and

• the presence of different chemical species available in the environment.

The behavior of heavy isotopes means, for example, that chemical reactions and processes involving the element Pb will not discriminate among the naturally occurring isotopes of this element on the basis of atomic mass differences (²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb). As a result of these special attributes, radiogenic isotopes are routinely used to study the architecture and evolution of the continents and oceanic rocks, to learn about unexposed material in the interior of the Earth that feeds volcanoes, and to establish the evolution of seawater through geological time.

In the Eastern Mineral Resources Program of the U.S. Geological Survey (USGS), radiogenic isotope tracers have been used traditionally as follows:

• to establish the identity of reservoirs supplying metals of economic interest,

• to provide information on the origin of rocks hosting ore deposits in different geologic environments,

• to provide insights into the genesis and evolution of mineralizing systems in order to explain a region's metal endowment,

• to recognize fluids involved with the formation of ore deposits, and

• to catalog economic ore deposits having similar isotopic compositions to facilitate recognition of differences between economic and subeconomic deposits in the same region.

New avenues of USGS research have also been initiated that will use radiogenic isotopic tracers to evaluate processes leading to the availability of metals from mineral deposits to the surface environment and to gage the effect and toxicity of mine areas on their immediate surroundings.

Innovative applications of heavy-isotope tracers have flourished in fields as varied as public health and environmental research. A health application is to identify from among possible reservoirs the one responsible for lead pollution in children. Environmental research can trace transport pathways and assess the relative environmental impact of human activities; for example, by determining the origins of industrial wastes contributed to complex natural environments.

Analytical Capabilities in Reston, Va.

Carolina Slate Belt Gold Deposits

U-Th-Pb, Nd-Sm, and Rb-Sr dating is being done on sulfides, silicate minerals, and rocks in our Reston labs, and U-Th-Pb zircon dating is being done as part of a collaborative study between the USGS and VPI (Virginia Polytechnic Institute and State University). New isotopic data have shown that the gold deposits are Early to Middle Cambrian (about 560 million years old) and have different ranges in Pb, Sr, and Nd isotopic compositions. All of the major gold deposits (Haile, Brewer, Ridgeway, and Barite Hill) in the slate belt show evidence of gold mobilization after the original formation of the deposits. Another fundamental observation as a result of the radiogenic isotope studies, critical for successful exploration for gold deposits in the Eastern United States, is that the gold mines have distinctive isotopic signatures that uniquely identify the rocks involved with their genesis.

Pb and Nd isotopic compositions demonstrate that the gold deposits are underlain by root rocks (basements) that are not found west of the Carolina slate belt but that occur only as part of a trend that stretches to the northeast and southwest for hundreds of miles. This presence of basement rocks is a feature of considerable importance that will lead to more precise regional exploration and assessment of the endowment of precious metals in the Carolina slate belt and adjacent regions. Ongoing research will improve the understanding of the factors controlling the distribution of the gold deposits in the Eastern United States by documenting their characteristic geologic, geochemical, and geophysical features and will facilitate comparisons of the slate belt with deposits in adjacent regions.

Base- and Precious-Metal Deposits in New England

The USGS work is currently focused on the Bald Mountain massive sulfide deposit in northern Maine and consists of U-Th-Pb zircon dating of key units, in addition to Pb-Nd-Sr isotopic studies of volcanic rocks hosting the ore deposits. Isotopic tracer studies on sulfide minerals seek to establish constraints on the possible sources of the metals and on the contributions of different types of mineralizing fluids.

Cooperative Studies, International Studies, and Other Outreach Activities

The radiogenic isotope laboratory in Reston, Va., is also involved with outreach, technology transfer efforts, and training of guest investigators from within the USGS and from Europe and Latin America. Current collaborative work includes studies on the metallogeny of alkalic rocks from Italy and on precious-metal deposits in Mexico.

Metallogeny and Basement Terranes

Isotopic variations of Pb, Nd, and Sr have been used to determine the internal variations within large rock masses in New England and to identify zones associated with anomalously high contents of economically important elements (tin, molybdenum, and tungsten). Isotopic and geochemical studies indicated that rocks associated with copper-molybdenum mineralization in New England originated from crustal blocks and sources that were fundamentally distinct from sources of rocks containing tin-tungsten and rare-earth-elements-uranium mineralization. More importantly, Pb and Nd isotopic compositions have been used in recent studies to uniquely characterize the types of root rocks (basements) underlying the crustal blocks that make up the Appalachian Mountains. Recognition of such key isotopic differences in unexposed rocks from deep within the Earth and associated with the mechanisms of mountain building provides powerful insights into the evolution of continental land masses.

References on Radiogenic Isotope Research

General References

Koppel, V., and Grunenfelder, M., 1979, Isotope geochemistry of lead, in Jager, E., and Hunziker, J.C., eds., Lectures in isotope geology; Berlin, Springer-Verlag, p. 132-153.

Selected Articles of Interest

Van Metre, P.C., and Callender, E., 1996, Water-quality trends in White Rock Creek Basin from 1912-1994 identified using sediment cores from White Rock Lake reservoir, Dallas, Texas: Journal of Paleolimnology, v. 10, p. 1-11.