Ar-Ar Geochronology

 

What it tells us?

⁴⁰Ar/³⁹Ar Geochronology is a geochronometer that can obtain a history of different K-rich grains that represent different events within a sample. This geochronometer is diverse in its ability: it can obtain information on events that are thousands of years old to samples as old as the Earth; even samples with low potassium (K) content can be analysed which includes a large number of different minerals, we focus on K-feldspar and micas. Closure temperatures are calculated on each domain within a crystal and this information aids in the plotting of the history of different structures, different events, different fluid alteration zones and so forth.

Description of Analytical Method

⁴⁰Ar/³⁹Ar Geochronology is a geochronometer that is based on K-decay. A significant amount of information is produced during analysis which aids in the interpretation of data.

Grains need to be extracted from the rock, the structure or grains that are to be analysed need to be cut from the rock and mineral separation needs to be done as appropriate for that sample.

The final aliquot of grains to be measured is hand-picked under a binnocular microscope, to ensure data quality. These grains are then packed and sent to be irradiated to produce ³⁹Ar.On their return they are analysed in a multicollector mass spectrometer where isotope abundances are ‘counted’ and all necessary standards and other processes are performed.

Raw data is then loaded into software for processing and interpretation: Noble for data reduction (adapted by ANU), and eArgon for graphical presentation and analysis.

Type of Sample Material/Media

⁴⁰Ar/³⁹Ar geochronology needs minerals that contain potassium. The percentage that can be successfully measured can be as low as 0.1% as occurs in basalts and potentially in amphiboles. However, micas and K-feldspars are the preferred mineral which have between ~7 and 11 % K-content. Grains can be metamorphic, volcanic, granitic, alteration of different types.

MinEx Research

⁴⁰Ar/³⁹Ar Geochronology has been successfully used in Program 3 related research, particularly in collaboration with the state Geological Surveys through the National Drilling Initiatives. This includes the:

• East Tennant NDI
• Delamerian NDI 

 

MinEx CRC Researcher Contact Details