Geochronology facilities

• Minerals separation laboratories and procedures
• Geochronology services
• Mineral separation procedures
• Sample preparation for mass spectrometer analysis
• Mass Spectrometer facilities
• SHRIMP movie
• Related links

Mineral separation laboratories and procedures

Geoscience Australia houses a dedicated mineral separation laboratory, which includes specialised down draft fume cabinets to accommodate use of heavy liquids.

• Geochronology services
• Mineral separation procedures
• Sample preparation for mass spectrometer analysis

Geochronology services

Mineral separation work undertaken is within the National Geoscience Agreement (more information on the NGA can be found in the current year's Work Program). Mineral types most frequently separated are zircon, monazite and potassium-rich rock forming minerals including k-feldspar and micas. Other mineral types that are also separated include titanite, rutiles, apatite, sulphides, tungstates and gold.

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Mineral separation procedures

Rock samples collected for geochronological analysis from outcrop or drill core can range in size from 100 g to 30 kg in weight. Once they are submitted to the mineral separation laboratory, the following steps are undertaken to separate the minerals required for analysis:

1. Sample crushing Splitters, jaw crushers, ring mills and disc mills are used to reduce the sample to a rock flour in two dedicated crushing rooms.
2. Wilfley table method Wilfley tables are used as a quick, reliable and consistent method for de-sliming bulk rock flours and reducing sample composition to minerals of interest.
3. Heavy liquids Using the known variations in specific gravity between different minerals, heavy liquids are used to further concentrate the minerals required for analysis. In the Geoscience Australia laboratories, two types of heavy liquids used, tetrabromoethane density 2.96 (TBE) and diidomethane density 3.3 (DIM), which require specialised operating environments.
4. Barrier magnetic separation Arguably the most essential method for bulk heavy mineral concentration, variable current induced magnetic fields interact with mineral grains passing through at set slope and tilt to further separate the sample. Surface slicks, smears coatings, cracks, inclusions and internal fractures may be discriminated in order to select the highest quality grains for analysis.
5. Mineral concentrates & determination Final pure mineral concentrates are achieved through hand-picking individual grains using a binocular microscope. The sample is now ready to be prepared for mass spectrometer analysis.

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Sample preparation for mass spectrometer analysis

1. Sensitive high resolution ion mircoprobe (SHRIMP) mounts Mineral grains are placed in rows, cast into epoxy resin and polished. The mount is then photographed in plane and transmitted light, and also for cathode luminescence in order to identify compositional and textural variations within grains. The mount is then gold-coated and ready to be loaded into the SHRIMP for analysis.

2. Ar/Ar methods

Mineral concentrates (commonly micas or k-feldspar grains) are packed into an Al (aluminium) foil parcels and placed into irradiation canister. Irradiated samples are then repackaged into Sn (tin) foil parcels which are dropped into a furnace ready for step heat mass spectrometer analysis.

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Mass Spectrometer facilities

Geoscience Australia has installed a new Sensitive High Resolution Ion MicroProbe IIe (SHRIMP IIe) at the end of 2007 to enable in-house analysis of mineral phases such as zircon and monazite. Along with existing resources and experience, this facility will allow the group to have detailed management of the 'outcrop-to-publication' analytical cycle that will ensure that the highest quality data is provided to research projects. The in-house facility will also enable the development of new analytical methods, expanding Geoscience Australia's capability to address increasingly complex geoscience issues.

The Geoscience Australia Geochronology Laboratory also has a strong network of relationships with other geochronology laboratories world-wide that provide access to a range of analytical methods such as Ar-Ar, TIMS U-Pb, etc.

SHRIMP movie

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Related links

• Geochronology home page
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