Sensitive high-resolution ion microprobe

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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. Like other SIMS instruments, the SHRIMP microprobe bombards a sample under vacuum with a beam of primary ions that sputters secondary ions that are focused, filtered, and measured according to their energy and mass.

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. The most common application of the instrument is in uranium-thorium-lead geochronology, although the SHRIMP can be used to measure some other isotopic and elemental abundances.

History and scientific impact[edit]

The SHRIMP originated in 1973 with a proposal by Prof. Bill Compston,[1] trying 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] 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 were partially confirmed.[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 (already introduced in the market by a French company years before), 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[edit]

SHRIMP diagram.svg Magnetic sector Detector Electrostatic_Analyzer Sample chamber Primary column Metre
SHRIMP diagram.svg Magnetic sector Detector Electrostatic_Analyzer Sample chamber Primary column Metre
Schematic diagram of a SHRIMP instrument illustrating the ion beam path. After Figure 4, Williams, 1998.[26]

Primary column[edit]

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.[27]

Sample chamber[edit]

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][28]

Electrostatic analyzer[edit]

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

Magnetic sector[edit]

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[edit]

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[edit]

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.[27]

Mass resolution and sensitivity[edit]

In normal operations, the SHRIMP achieves mass resolution of 5000 with sensitivity >20 counts/sec/ppm/nA for lead from zircon.[26][27]

Applications[edit]

Isotope dating[edit]

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[edit]

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 Geoscience Australia Canberra IIe 2008
13 Korea Basic Science Institute Ochang IIe/mc 2009
14 University of São Paulo São Paulo II/mc 2010
15 University of Granada Granada IIe/mc 2011
16 Australian National University Canberra SI/mc 2012
17 Chinese Academy of Geological Sciences Beijing IIe/mc 2013
18 National Institute of Advanced Industrial Science and Technology Tsukuba IIe/amc 2013
19 Polish Geological Institute - National Research Institute Warsaw IIe/mc 2014
20 National Institute of Polar Research Tokyo IIe/amc 2014

References[edit]

  1. ^ Australian Academy of Sciences. "Interviews with Australian scientists: Professor Bill Compston". Retrieved 10 November 2010. 
  2. ^ a b Foster, J. (2010), "The construction and development of SHRIMP I: An historical outline.", Precambrian Research 183: 1–8, doi:10.1016/j.precamres.2010.07.016 
  3. ^ a b Matsuda, H. (1974), "Double focusing mass spectrometers of second order", International Journal of Mass Spectrometry and Ion Physics 14 (2): 219–233, doi:10.1016/0020-7381(74)80009-4 
  4. ^ a b Clement, S.W.J.; Compston, W.; Newstead, G. (1977). "Proceedings of the International Secondary Ion Mass Spectrometry Conference". Springer-Verlag. p. 12.  |chapter= ignored (help)
  5. ^ Froude, D.O.; Ireland, T.R.; Kinny, P.D.; Williams, I.S.; Compston, W.; Williams, I.R.; Myers, J.S. (1983), "Ion microprobe identification of 4,100-4,200 Myr-old terrestrial zircons.", Nature 304 (5927): 616–618, doi:10.1038/304616a0 
  6. ^ Compston, R.T.; Pidgeon (1986), "Jack Hills, evidence of more very old detrital zircons in Western Australia", Nature 321 (6072): 766–769, doi:10.1038/321766a0 
  7. ^ Moorbath, S. (1983), "The most ancient rocks?", Nature 304 (5927): 585–586, doi:10.1038/304585a0 
  8. ^ Schärer, C.J.; Allègre (1985), "Determination of the age of the Australian continent by single-grain zircon analysis of Mt Narryer metaquartzite", Nature 315 (6014): 52–55, doi:10.1038/315052a0 
  9. ^ Fanning, C.M.; McCulloch, M.T. (1990). "Third International Archean Symposium, Perth. Extended abstracts volume". pp. 15–17.  |chapter= ignored (help)
  10. ^ Amelin, Y.V. (1998), "Geochronology of the Jack Hills detrital zircons by precise U–Pb isotope dilution analysis of crystal fragments", Chemical Geology 146: 25–38, doi:10.1016/S0009-2541(97)00162-9 
  11. ^ Ireland, T.R.; Compston, W.; Heydegger, H.R. (1983), "Titanium isotopic anomalies in hibonites from the Murchison carbonaceous chondrite", Geochimica et Cosmochimica Acta 49: 1989–1993, doi:10.1016/0016-7037(85)90092-4 
  12. ^ Kinny, P.D.; Compston, W.; Williams, I.S. (1991), "A reconnaissance ion-probe study of hafnium isotopes in zircons", Geochimica et Cosmochimica Acta 55: 849–859, doi:10.1016/0016-7037(91)90346-7 
  13. ^ Eldridge, C.S.; Compston, W.; Williams, J.L.; Walshe (1987), "In-situ microanalysis for 34S/32S ratios using the ion microprobe SHRIMP", International Journal of Mass Spectrometry and Ion Processes 76: 65–83, doi:10.1016/0168-1176(87)85011-5 
  14. ^ a b Ireland, T.R.; Clement, S.; Compston, W.; Foster, J. J.; Holden, P.; Jenkins, B.; Lanc, P.; Schram, N.; Williams, I. S. (2008), "Development of SHRIMP", Australian Journal of Earth Sciences 55 (6): 937–954, doi:10.1080/08120090802097427 
  15. ^ "SHRIMP User Locations" (PDF). 2009. Retrieved 2010-08-13. 
  16. ^ Stern, R. (2006), "A time machine for Geoscience Australia", AusGeo News 81: 15–17 
  17. ^ 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 
  18. ^ 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 
  19. ^ 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 
  20. ^ 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 
  21. ^ Terada, K.; Monde, T.; Sano, Y. (Nov 2003), "Ion microprobe U-Th-Pb dating of phosphates in martian meteorite ALH 84001", Meteoritics & Planetary Science 38 (11): 1697–1703, doi:10.1111/j.1945-5100.2003.tb00009.x 
  22. ^ Trotter, J.A.; Williams, I.S.; Barnes, C.R.; Lécuyer, C.; Nicoll, R.S. (2008), "Did Cooling Oceans Trigger Ordovician Biodiversification? Evidence from Conodont Thermometry", Science 321 (5888): 550–554, Bibcode:2008Sci...321..550T, doi:10.1126/science.1155814, PMID 18653889 
  23. ^ Xu, Bei; Xiao, Shuhai; Zou, Haibo; Chen, Yan; Li, Zheng-Xiang; Song, Biao; Liu, Dunyi; Chuanming, Zhou; Xunlai, Yuan (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 
  24. ^ Ickert, R.B.; Hiess, J.; Williams, I.S.; Holden, P.; Ireland, T.R.; Jenkins, B.; Schram, N.; Foster, J. J.; Clement, S.W. (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 
  25. ^ Hiess, Joe; Bennett, Vickie; Nutman, Allen; Williams, Ian (2010), "Archaean fluid-assisted crustal cannibalism recorded by low δ18O and negative εHf(T) isotopic signatures of West Greenland granite zircon", Contributions to Mineralogy and Petrology, doi:10.1007/s00410-010-0578-z 
  26. ^ a b Williams, I.S. (1998), "U-Th-Pb geochronology by ion microprobe", In: McKibben, M.A.; Shanks III, W.C.; Ridley, W.I.; (Editors), "Applications of microanalytical techniques to understanding mineralizing processes", Reviews in Economic Geology Special Publication 7 : 1–35 
  27. ^ a b c d Stern, R.A. (1997), "The GSC Sensitive High Resolution Ion Microprobe (SHRIMP): analytical techniques of zircon U-Th-Pb age determinations and performance evaluation", Radiogenic Age and Isotopic Studies: Report 10 (F): 1–31 
  28. ^ Riedl, M. "Schwarzschild Objective". Retrieved 10 November 2010. 
  29. ^ Claoué-Long, J.; Compston, W.; Roberts, J.; Fanning, C.M. (1995), "Two Carboniferous ages: a comparison of SHRIMP zircon dating with conventional zircon ages and 40Ar/39Ar analysis", In: Berggren, W.A.; Kent, D.V.; Aubry, M.-P.; Hardenbol, J.; (Editors). Geochronology, time scales and global stratigraphic correlation. SEPM (Society for Sedimentary Petrology) Special Publication. : 3–21 
  30. ^ 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 
  31. ^ Black, Lance P.; Kamo, Sandra L.; Allen, Charlotte M.; Davis, Donald W.; Aleinikoff, John N.; Valley, John W.; Mundil, Roland; Campbell, Ian H.; Korsch, Russell J.; Williams, Ian S.; Foudoulis, Chris (2004), "Improved 206Pb/238U microprobe geochronology by the monitoring of a trace-element-related matrix effect; SHRIMP, ID-TIMS, ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards", Chemical Geology 205 (1-2), doi:10.1016/j.chemgeo.2004.01.003 

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