USGS-GD-Scientific Capabilities - U-PB SYSTEM Technique

 

U-PB SYSTEM

Technique

 

U-series dating is the most reliable method for dating Quaternary sedimentary carbonate and silica, and fossils. Quaternary dating also provides a record of climate change and geologically recent changes in environment. U-Pb geochronology of zircon, baddelyite, and monazite is used for determining the age of emplacement of igneous rocks of all compositions, ranging in age from Tertiary to Early Archean. U-Pb ages of metamorphic minerals, such as zircon, sphene and monazite are used to date thermal events, including terrestrial meteoritic impacts, and to determine thermochronologic histories of terranes. U-Pb ages of zircon in sediments are used to determine the provenance of the sediments. Strontium-isotope stratigraphy is a method that uses extremely precise Sr-isotope measurements of marine fossils in conjunction with the known Sr isotopic composition of sea water 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.

U-Pb geochronology of igneous rocks:

  • Age of emplacement of igneous rocks.
  • Can often provide both age of emplacement and age of metamorphism.
  • Uses Zircon or other high-U mineral (monazite, allanite, rutile)
  • Thermal history (sphene, apatite), particularly in conjunction with 40 Ar-39Ar ages.
  • Age range: Archean (3.8 billion years) to early Tertiary (last 65 million years), possibly mid Teritary.
  • Provenance ages: can be used to determine the source of material in wind-blown or stream-deposited sediments by determining the age of sediment-contained zircons.

    • U-Pb geochronology of opal

  • Age of deposition of opal.
  • Brand new technique: to as young as 0.1 Ma.
  • Requires pure (essentially Pb-free), but moderately uraniferous opal.

    • U-series dating

  • Age of deposition using ratios of intermediate daughters, 234U, 230Th.
  • Age range: several thousand to approximately 500,000 years, with greater precision from higher U content and younger age sample. Extremely young material can be dated given sufficient 234U (hundreds of years).
  • Most commonly applied to carbonates, opal, or other high-U, low-Th mineral.
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    URL http://geology.cr.usgs.gov/capabilities/gronemtrac/geochron/uthpb/tech.html