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my page - www.hostgator1centcoupon.info 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.

History and scientific impact

The SHRIMP originated in 1973 with a proposal by Prof. Bill Compston[1] 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.[2] Ion optic designer Steve Clement based the prototype instrument (now referred to as 'SHRIMP-I') on a design by Matsuda[3] which minimised aberrations in transmitting ions through the various sectors.[4] The instrument was built from 1975 and 1977 with testing and redesigning from 1978. The first successful geological applications occurred in 1980.[2]

The first major scientific impact was the discovery of Hadean (>4000 million year old) zircon grains at Mt. Narryer in Western Australia[5] and then later at the nearby Jack Hills.[6] These results and the SHRIMP analytical method itself were initially questioned[7][8] but subsequent conventional analysis confirmed the results.[9][10] SHRIMP-I also pioneered ion microprobe studies of titanium,[11] hafnium[12] and sulfur[13] 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.[14]

Fifteen SHRIMP instruments have now been installed around the world[15][16] 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[17][18] and further extending the age of zircons from the Jack Hills.[19] Other significant milestones include the first U/Pb ages for lunar zircon[20] and Martian apatite[21] dating. More recent uses include the determination of Ordovician sea surface temperature,[22] the timing of snowball Earth events[23] and development of stable isotope techniques.[24][25]

Design and operation

Template:Annotated image

Primary column

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.[26]

Sample chamber

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.[14] A Schwarzschild objective lens provides reflected-light direct microscopic viewing of the sample during analysis.[4][27]

Electrostatic analyzer

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[26] and an electrostatic quadrapole lens is used to reduce aberrations in transmitting the ions to the magnetic sector.[3]

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.

Detectors

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.

Vacuum system

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.[26]

Mass resolution and sensitivity

In normal operations, the SHRIMP achieves mass resolution of 5000 with sensitivity >20 counts/sec/ppm/nA for lead from zircon.[28][26]

Applications

Template:Expand section

Isotope dating

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.[29][30][31]

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
5 Hiroshima University Hiroshima II 1996
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
14 Geoscience Australia Canberra IIe 2008
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

References

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  1. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named AAS2005
  2. ↑ 2.0 2.1 Cite error: Invalid <ref> tag; no text was provided for refs named Foster2010
  3. ↑ 3.0 3.1 Cite error: Invalid <ref> tag; no text was provided for refs named Matsuda1974
  4. ↑ 4.0 4.1 Cite error: Invalid <ref> tag; no text was provided for refs named Clement1977
  5. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Froude1983
  6. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Compston1986
  7. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Moorbath1983
  8. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Schärer1985
  9. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Fanning1990
  10. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Amelin1998
  11. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Ireland1985
  12. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Kinny1991
  13. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Eldridge1987
  14. ↑ 14.0 14.1 Cite error: Invalid <ref> tag; no text was provided for refs named Ireland2008
  15. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named ASI2009
  16. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Stern2006
  17. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Bowring1999
  18. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Stern1998
  19. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Wilde2001
  20. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Compston1984
  21. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Terada2003
  22. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Trotter2008
  23. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Xu2009
  24. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Ickert2008
  25. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Heiss2010
  26. ↑ 26.0 26.1 26.2 26.3 Cite error: Invalid <ref> tag; no text was provided for refs named Stern1997
  27. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Riedl2001
  28. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Williams1998
  29. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Claoué-Long1995
  30. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Black2003a
  31. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Black2004
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