Crustal Imaging and Characterization Team

Radiogenic Isotopes - TIMS

Task Objectives

Isotope research techniques are exceptionally powerful tools in essential baseline regional and local mapping studies as well as resource, hazard, environmental, surficial, and global change studies. Isotopic data may also be used to help understand biological, ecological and hydrological processes. The Radiogenic Isotope Facility in Denver provides both geochronologic and isotopic tracer studies applied to both natural and artificial systems. The Radiogenic Isotope Facility provides high quality isotopic data with rapid throughput. The staff of the facility keeps abreast of new developments in the discipline and investigates new applications of isotopic techniques to geologic problems. The scientific staff interprets the analytical results and is an integral part of the programmatic research of the USGS. In all endeavors every possible effort is made to minimize down time and improve running capability.

Statement of Work

The Radiogenic Isotope Facility is a multi-user, multi-funded research laboratory that utilizes state-of-the-art isotopic techniques to solve geologic problems. Isotope research techniques are exceptionally powerful tools to essential baseline regional and local mapping studies as well as resource, hazard, environmental, surficial processes, and global change studies. Isotopic data may also be used to help understand biological, ecological and hydrological processes. The Radiogenic Isotope Facility in Denver provides both geochronological and isotopic tracer studies applied to both natural and artificial systems.

For fossils and sedimentary carbonate and silica U-series methods provide some of the most high-precision techniques for dating Quaternary climate change. U-Pb geochronology is primarily used for zircon separates from silicic igneous rocks to determine the numerical age of emplacement of the rocks and may be used for samples ranging in age from Mesozoic to Early Proterozoic. Zircon U-Pb ages in young sediments may also be used to trace the source of the sediments. Strontium-isotope stratigraphy is a method that uses extremely precise Sr-isotope measurements of marine fossils in conjunction with the changing Sr-isotopic composition of seawater through time to provide an age for the fossil. This technique can also be used to determine ages for other geologic processes, such as dolomitization.

Radiogenic isotopic tracers (Pb, Sr, Nd, Sr, U) can be used to provide information regarding geologic processes such as magma evolution, sources of magmas and sediments, eolian dust transport, sources of metals (human-caused or natural in various ecosystems), and baseline data for environmentally pristine ecosystems. Radiogenic isotopes can also be used to trace groundwater flow paths and as indicators of mixing of waters from discrete aquifers.

Highlights & Key Findings

Human activities, such as mining, application of chemical fertilizer and manure, diversion of storm-water runoff, and disposal of wastewater, can contribute to nutrient loads in surface water and shallow ground water, with resulting impacts on natural ecosystems. Pb concentrations and isotopic signatures in eider blood, sediment and lead shot were used to determine the source(s)of lead poisoning in several species of eiders in Alaska. Sr- and U-isotopic data from water samples assisted in tracing groundwater flow paths. Pb-isotopic data in conjunction with other geochemical data were used to assess the contributions from historical mining activities to the overall metal load in rivers. The data also assisted in determining the pre-mining baseline metal levels in the river system, thereby providing target values for remediation. Isotopic data improved the understanding of the life cycle of metals and nutrients in sedimentary basins which will improve assessments of (1) mineral and energy resources, (2) environmental consequences of mining and oil petroleum, and (3) their biologic and environmental effects.

Determining the effect of dust on the atmosphere has implications for climate change. Pb-isotopic compositions of dust records of the last glacial period provided information concerning the sources of the dust, which will assist in testing climate models. U-series dating on corals from Puerto Rico assisted in evaluating tectonic models for this island. Climate and land use have influenced surficial geologic processes; Pb- and Sr-isotopic data were used to trace sources of eolian dust and to aid in understanding the sensitivity of soil surfaces in deserts. Srisotopic compositions recorded in lake sediments were studied to provide information on past hydrologic conditions; Sr and U-isotopic compositions of water, in conjuction with other geochemical data assisted in understanding hydrology. U-Pb analyses of zircon were used to date various plutons; the data will be used in conjunction with 40Ar-39Ar data to establish cooling curves for the plutons and to determine the age and number of mineralizing events. The age and origin of volcanogenic massive sulfide deposits were investigated using Pb-Sr-Nd isotopic techniques.

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