Thermal ionisation radiogenic isotope mass spectrometry (TIMS)

CERG routinely utilises thermal ionisation mass spectrometry, to characterise isotopic composition of rocks and minerals. The isotopic systems that we employ are Sm-Nd, Rb-Sr and U-Pb.

Two thermal ionisation mass spectrometers, a Finnegan 261 double collector and a Finnegan 262 multi-collector, are located within the Mawson Laboratories and operated by John Foden and David Bruce. The Finnigan MAT 261 thermal ionization mass spectrometer is equipped with two fixed collectors. It is used for the measurement of elemental concentrations in the Rb-Sr, Sm-Nd and U-Th-Pb systems. The Finnigan MAT 262 mass spectrometer is equipped with seven movable faraday collectors, which allows for both static and multidynamic mode data. In addition, the mass spectrometer includes an ion-counting channel, which allows for the measurement of signal currents down into the 10 -18 amp range. The machine is ideal for Rb-Sr, Sm-Nd and U-Pb isotope analyses. The mass spectrometer has a thirteen-sample turret with both manual and automated running capabilities. Additionally, the mass spectrometer areas house a filament degassing machine and laminar flow bench for sample loading.

As radiogenic elements generally only exist in ppb to ppm levels. Radiogenic isotope geochemistry requires the measurement of microgram (10-6 ) quantities of an element. Thus, the chemical processing of earth materials requires a very clean environment. The Radiogenic Isotope Geochemistry Lab consists of two parts: a clean lab for sample preparation and thermal ionization mass spectrometer areas to house the necessary analytical equipment.

When measuring such small quantities, any sample contamination from atmospheric dust can present a problem. We have a 60m 2 lab with the general air supply filtered to a 95% level. Because CERG concentrates on Precambrian geology and crustal evolution, most of the sample dissolution takes place in sealed Teflon bombs, to ensure complete dissolution of refractory phases. The bombs are put in stainless steel jackets in an oven at 160 ° C.

The chemical separations and sample evaporation are undertaken in fumehoods or filtered laminar-flow work stations. In addition, there is an adjacent balance room for sample weighing and spiking.

A deionising water purification system is used to produce very clean water. Acid reagents are purified in either a pure silica glass stills or sub-boiling Teflon stills.

Enriched isotope spikes and standard solutions are available in this lab for isotope composition and isotope dilution analysis of U-series isotopes, Rb-Sr, and Sm-Nd. Samples for Sr isotopic analysis may be purified using either EIChroM SR-spec resin or conventional cation resin. HDEHP columns are available for REE chemistry; and U/Th/Pb chemistry is performed on anion resin columns.

The labs are supported by a sample preparation facility, which includes short-term sample storage, a jaw crusher, a disc mill pulverizer, shatterboxes for sample pulverization, a Wilfley table, ceramic mortal and pestle, sieves, heavy liquids and Franz magnetic separators for mineral separation.