Microprobe dating

John de Laeter of Curtin University (Perth, Western Australia), led to the project in 1989 to build a commercial version of the instrument, the SHRIMP-II, in association with ANUTECH, the Australian National University's commercial arm.Refined ion optic designs in the mid-1990s prompted development and construction of the SHRIMP-RG (Reverse Geometry) with improved mass resolution.Typical ion beam density on the sample is ~10 p A/µm The primary beam is 45° incident to the plane of the sample surface with secondary ions extracted at 90° and accelerated at 10 k V.Three quadrupole lenses focus the secondary ions onto a source slit and the design aims to maximise transmission of ions rather than preserving an ion image unlike other ion probe designs.Essentially, the path of a less massive ion will have a greater curvature through the magnetic field than the path of a more massive ion.Thus, altering the current in the electromagnet focuses a particular mass species at the detector.Therefore, direct sampling techniques with high spatial resolution are required, in order to study these tiny zones individually, without damaging the textures and zonations.The advantage of monazite geochronology is the ability to relate monazite compositions with geological processes.

It is a powerful tool in studying the complex history of metamorphic rocks particularly, as well as igneous, sedimentary and hydrothermal rocks.The SHRIMP is primarily used for geological and geochemical applications.It can measure the isotopic and elemental abundances in minerals at a 30 micrometre-scale and is therefore well-suited for the analysis of complex minerals, as often found in metamorphic terrains, some igneous rocks, and for relatively rapid analysis of statistical valid sets of detrital minerals from sedimentary rocks.Turbomolecular pumps evacuate the entire beam path of the SHRIMP to maximise transmission and reduce contamination.The sample chamber also employs a cryopump to trap contaminants, especially water.The dating uses the radioactive processes in monazite as a clock.The uniqueness of monazite geochronology comes from the high thermal resistance of monazite, which allows age information to be retained during the geological history.Since the sputtering yield differs between ion species and relative sputtering yield increases or decreases with time depending on the ion species (due to increasing crater depth, charging effects and other factors), the measured relative isotopic abundances do not relate to the real relative isotopic abundances in the target.Corrections are determined by analysing unknowns and reference material (matrix-matched material of known isotopic composition), and determining an analytical-session specific calibration factor.The ions pass through a collector slit in the focal plane of the magnetic sector and the collector assembly can be moved along an axis to optimise the focus of a given isotopic species.In typical U-Pb zircon analysis, a single secondary electron multiplier is used for ion counting.

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