Radiogenic Isotope Laboratory

Earth Surface Processes Team - Central Region

Radiogenic Isotope Laboratory - Denver

Radioactive Isotopes: U-Pb system

Uranium-Thorium-Lead (U-Th-Pb) Geochronology

Rocks and minerals, both terrestrial and extraterrestrial, which contain U and Th are suitable for age determinations. The decay of U and Th to stable isotopes of Pb is the foundation for several methods of dating using isotopic techniques. The usefulness of these techniques has been enhanced as improvements in instrumentation and sample pretreatment have allowed increased precision and accuracy of the age determinations. Samples as young as one million years and as old as Precambrian are capable of being dated by these techniques.

Examples of Applications:

age of emplacement of igneous rocks
age of metamorphism
thermochronology/P-T-t paths (in conjunction with other geochronometry)
sources of unconsolidated eolian sediments (paleo-climate research)
sources of sedimentary and metasedimentary rocks (tectonic studies)
age of ore deposits
age of deposition, mineralization of paleosols

U-series Geochronology

U-series dating is a very reliable for studying Quaternary events and processes. Quaternary dating also provides a time-scale for the global climate change record. U-series disequilibrium techniques are useful in studying a wide variety of surficial, hydrological, and environmental processes. For fossils and sedimentary carbonate and silica U-series methods provide high-precision techniques for dating Quaternary climate change and to rigorously test theories of climate change on orbital and shorter time-scales.

Examples of Applications:

time scale for geologic record of climate variability:
- dating speleothems - speleothems record valuable information for climate variability; age determinations on the speleothems provide a time reference for this variability
- dating coral reefs - yields information about the length and timing of interglacial periods in the climate record
groundwater circulation and mixing patterns:
- rock-water chemical interactions in groundwater and geothermal systems
- retardation and transport of radionuclides in groundwater
- constraining source and age of groundwater
- duration of hydrothermal activity
geomorphologic uses:
- rates of valley downcutting
- rates of areal denudation
- coastline uplift rates
implications for geological hazards:
- determine the frequency of earthquakes by determining ages on fracture fillings
uses in igneous geology:
- determining processes and times scales of magma formation and differentiation

Pb isotopes as tracers

Pb is known to be a toxic element that is commonly found in our everyday environment. Pb isotopes can be used to detect relative levels and sources of contamination in many common utilities, such as household water, streams, reservoirs, and wildlife, as it may relate to biological changes (e.g. changes in reproduction or death rates). The isotopic signature can also be used to help isolate and identify environmental polluters and in conjunction with other geochemical data can be used to assess the contributions from historical mining activities to the overall metal load in rivers. Pb isotopic composition data also assist in determining the pre-mining baseline metal levels in the river systems, thereby providing target values for remediation.

Examples of Applications:

delineation and origin of tectonostratigraphic terranes
sources of metals in ore deposits
sources of eolian sediments
sources and timing of pollution in tree rings
forensic analysis of biological material (blood, feathers, etc.)
amounts and sources of heavy metals in water and sediment
studies of oceanic ferromanganese nodules
degree of anthropogenic contamination of natural systems
petrogenetic studies of igneous rocks
sources of metals contamination in urban residential soils

U isotopes as tracers

Examples of Applications:

sources and paths of environmental contamination
determining groundwater flow paths

Information About:

Rb-Sr

Nd-Sm

More About the Radiogenic Isotope Laboratory

Radiogenic Isotope Laboratory Introduction

Radiogenic Isotopes

Thermal Ionization Mass Spectrometry

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