Geochronology

Geochronology is the science of dating the time sequence of events in the history of the Earth and of determining the temporal rates of geological processes.
By measuring the amount of radiogenic isotopes produced by radioactive decay from a “mother” isotope with a known half-life, geologists can establish the “absolute age”, i.e, the age in absolute units of time (Ma, ka, etc.) for the parent material. A number of radioactive isotopes from different elements, for instance uranium, thorium, rhenium, samarium, lutetium, rubidium, potassium, carbon are used for this purpose, and depending on the rate of decay, are used for dating geological periods of strongly different age.Today the Earth Scientist can rely on a wide variety of geochronological tools or methods to allow the “absolute” dating of crystalline rocks and sediments. Techniques exist to date all geological materials, from billions of years in age to historical records. For instance: Uranium-lead (U-Pb) and Uranium-thorium-lead (U-Th-Pb) geochronology of zircon, monazite, and xenotime is used for determining the age of emplacement of igneous rocks of all compositions and ages. U-Th-Pb ages of metamorphic minerals, such as zircon, sphene and monazite are used to date high temperature thermal events, including terrestrial meteoritic impacts, and to determine thermochronologic histories of terranes. Samarium-Neodymium (Sm-Nd) ages on garnet-clinopyroxene (and amphibole) are most useful to constrain high-pressure events reflecting eclogitisation of oceanic and continental crust. Rubidium-Strontium (Rb-Sr), Argon-Argon (Ar-Ar), Fission track, and Uranium-Thorium-Helium (U/Th)-He mineral ages are all suited to date the cooling of a rock unit subsequent to the thermal maximum of regional metamorphism. Samarium-Neodymium (Sm-Nd), Lutetium-Hafnium (Lu-Hf), Rubidium-Strontium (Rb-Sr), and Rhenium-Osmium (Re-Os) isotope data help to distinguish different mantle and crustal reservoirs and allow the reconstruction of the evolution of these reservoirs. 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. Tephrochronology dates volcanic ash and tuff, with a range of the last 15 million years. Radiocarbon dates material with organic carbon, and has a maximum range of 100,000 years. Thermoluminescence and optically-stimulated luminescence provides ages within a range of 100-1,000,000 years. 210Pb has a short half-life, and dates material from the past 150-200 years. U-series dating is the most reliable method for dating Neogene sedimentary carbonate, silica, and fossil material for the last 2 million years. This then can provide a record of climate change and geologically recent changes in environment.The geochronology group in Vienna has a strong focus on providing geochronological data on orogenic processes. From the large number of different geochronological methods the following are currently used in our laboratory:

Rb-Sr dating and Sr isotope geochemistry
Our work is concentrated on material from the Alps, the Himalayan-Tibet orogenic belt and the wider Mediterranean area.

Sm-Nd dating and Nd isotope geochemistry
The Sm-Nd method is a vital isotope analytical tool to approach both petrogenetic as well as geochronological questions.

U-Pb and Th-Pb dating
U-Pb dating of accessory minerals by conventional techniques and in-situ U-Th-Pb dating by laser-ablation MC-ICP-MS.


Laboratories and Instrumentation
The Geochronology Division of the Department of Lithospheric Research is equipped with all the necessary facilities to allow state-of-the-art geochronological and isotope geochemical research.