Sensitive high-resolution ion microprobe
The Sensitive High Resolution Ion Microprobe (SHRIMP) is a large-diameter, double-focusing secondary ion mass spectrometer (SIMS) sector instrument produced by Australian Scientific Instruments in Canberra, Australia. The SHRIMP microprobe focuses a primary beam of ions on a sample sputtering secondary ions which are focussed, filtered and measured according to their energy and mass.
The SHRIMP is primarily used for geological and geochemical applications. It can rapidly measure the isotopic and elemental abundances in minerals at a 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. The most common application of the instrument is in uranium-thorium-lead geochronology, although the SHRIMP can be used to measure other isotopic and elemental abundances.
- 1 History and scientific impact
- 2 Design and operation
- 3 Applications
- 4 SHRIMP instruments around the world
- 5 References
- 6 External links
History and scientific impact
The SHRIMP originated in 1973 with a proposal by Prof. Bill Compston to build an ion microprobe at the Research School of Earth Sciences of the Australian National University that exceeded the sensitivity and resolution of ion probes available at the time in order to analyse individual mineral grains. Ion optic designer Steve Clement based the prototype instrument (now referred to as 'SHRIMP-I') on a design by Matsuda which minimised aberrations in transmitting ions through the various sectors. The instrument was built from 1975 and 1977 with testing and redesigning from 1978. The first successful geological applications occurred in 1980.
The first major scientific impact was the discovery of Hadean (>4000 million year old) zircon grains at Mt. Narryer in Western Australia and then later at the nearby Jack Hills. These results and the SHRIMP analytical method itself were initially questioned but subsequent conventional analysis confirmed the results. SHRIMP-I also pioneered ion microprobe studies of titanium, hafnium and sulfur isotopic systems.
Growing interest from commercial companies and other academic research groups, notably Prof. 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. Further advances in design have also led to multiple ion collection systems, negative-ion stable isotope measurements and on-going work in developing a dedicated instrument for light stable isotopes.
Fifteen SHRIMP instruments have now been installed around the world and SHRIMP results have been reported in more than 2000 peer reviewed scientific papers. SHRIMP is an important tool for understanding early Earth history having analysed some of the oldest terrestrial material including the Acasta Gneiss and further extending the age of zircons from the Jack Hills. Other significant milestones include the first U/Pb ages for lunar zircon and Martian apatite dating. More recent uses include the determination of Ordovician sea surface temperature, the timing of snowball Earth events and development of stable isotope techniques.
Design and operation
In a typical U-Pb geochronology analytical mode, a beam of (O2)1- primary ions are produced from a high-purity oxygen gas discharge in the hollow Ni cathode of a duoplasmatron. The ions are extracted from the plasma and accelerated at 10 kV. The primary column uses Köhler illumination to produce a uniform ion density across the target spot. The spot diameter can vary from ~5 µm to over 30 µm as required. Typical ion beam density on the sample is ~10 pA/µm2 and an analysis of 15–20 minutes creates an ablation pit of less than 1 µm.
The primary beam is 45° incident to the plane of the sample surface with secondary ions extracted at 90° and accelerated at 10 kV. Three quadrapole 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. A Schwarzschild objective lens provides reflected-light direct microscopic viewing of the sample during analysis.
The secondary ions are filtered and focussed according to their kinetic energy by a 1272 mm radius 90° electrostatic sector. A mechanically-operated slit provides fine-tuning of the energy spectrum transmitted into the magnetic sector and an electrostatic quadrapole lens is used to reduce aberrations in transmitting the ions to the magnetic sector.
The electromagnet has a 1000 mm radius through 72.5° to focus the secondary ions according to their mass/charge ratio according to the principles of the Lorentz force. 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.
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.
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. Typical pressures inside the SHRIMP are between ~7 x 10−9 mbar in the detector and ~1 x 10−6 mbar in the primary column.
Mass resolution and sensitivity
|This section requires expansion. (December 2010)|
For U-Th-Pb geochronology a beam of primary ions (O2)1- are accelerated and collimated towards the target where it sputters “secondary” ions from the sample. These secondary ions are accelerated along the instrument where the various isotopes of uranium, lead and thorium are measured successively, along with reference peaks for Zr2O+, ThO+ and UO+. 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.
SHRIMP instruments around the world
|Instrument number||Institution||Location||SHRIMP model||Year of commissioning|
|1||Australian National University||Canberra||I||1980 (retired 2011)|
|2||Australian National University||Canberra||II/mc||1992|
|3||Curtin University of Technology||Perth||II||1993|
|4||Geological Survey of Canada||Ottawa||II||1995|
|6||Australian National University||Canberra||RG||1998|
|7||USGS & Stanford University||Stanford||RG||1998|
|8||National Institute of Polar Research||Tokyo||II||1999|
|9||Chinese Academy of Geological Sciences||Beijing||II||2001|
|10||All Russian Geological Research Institute||St. Petersburg||II/mc||2003|
|11||Curtin University of Technology||Perth||II/mc||2003|
|12||University of São Paulo||São Paulo||II/mc||2010|
|13||Chinese Academy of Geological Sciences||Beijing||IIe/mc||2013|
|15||Korea Basic Science Institute||Ochang||IIe/mc||2009|
|16||University of Granada||Granada||IIe/mc||2011|
|17||Australian National University||Canberra||SI/mc||2012|
|18||National Institute of Advanced Industrial Science and Technology||Tsukuba||IIe/mc||2013|
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- Bowring, S.A.; Williams, I.S. (1999), "Priscoan (4.00-4.03 Ga) orthogneisses from northwestern Canada", Contributions to Mineralogy and Petrology 134: 3–16, Bibcode:1999CoMP..134....3B, doi:10.1007/s004100050465
- Stern, R.A.; Bleeker, W. (1998), "Age of the world's oldest rocks refined using Canada's SHRIMP. the Acasta gneiss complex, Northwest Territories, Canada", Geoscience Canada 25: 27–31
- Wilde, S.A.; Valley, J.W.; Peck, C.M.; Graham (2001), "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago", Nature 409 (6817): 175–178, doi:10.1038/35051550, PMID 11196637
- Compston, W.; Williams, I.S.; Meyer, C. (Feb 1984), "U-Pb geochronology of zircons form lunar Breccia 73217 using a sensitive high mass-resolution ion microprobe", Journal of Geophysical Research, Supplement 89: B525–B534, ISSN 0148-0227
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- Xu, Bei; Xiao, Shuhai; Zou, Haibo; Chen, Yan; Li, Zheng-Xiang; Song, Biao; Liu, Dunyi; Chuanming, Zhou et al. (2009), "SHRIMP zircon U–Pb age constraints on Neoproterozoic Quruqtagh diamictites in NW China", Precambrian Research 168: 247–258, doi:10.1016/j.precamres.2008.10.008
- Ickert, R.B.; Hiess, J.; Williams, I.S.; Holden, P.; Ireland, T.R.; Jenkins, B.; Schram, N.; Foster, J. J. et al. (2008), "Determining high precision, in situ, oxygen isotope ratios with a SHRIMP II: Analyses of MPI-DING silicate-glass reference materials and zircon from contrasting granites", Chemical Geology 257: 114–128, doi:10.1016/j.chemgeo.2008.08.024
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- Black, Lance P.; Kamo, Sandra L.; Allen, Charlotte M.; Aleinikoff, John N.; Davis, Donald W.; Korsch, Russell J.; Foudoulis, Chris (2003), "TEMORA 1; a new zircon standard for Phanerozoic U-Pb geochronology", Chemical Geology 200 (1-2), doi:10.1016/S0009-2541(03)00165-7
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