The Sensitive High Resolution Ion Micro Probe (SHRIMP) is a high precision Secondary Ion Mass Spectrometer (SIMS). Ion microprobes make in situ isotopic and chemical ‘surface’ analysis of solid targets by bombarding the sample with an ion beam with a diameter of several microns typically employing Kohler focussing. The high mass resolution of SHRIMP is achieved by the use of double-focussing mass spectrometer (simultaneous energy and mass refocussing) with a very large turning radius of Magnet and Electrostatic Analyser. SHRIMP has many applications: zircon dating in copper-uranium-gold-silver deposits, uranium-lead dating of sulphur in the sulphide minerals that form metal ores, the isotopic composition of sulphur in the giant base metal ore bodies.
With the addition of an optional cesium gun rather than the standard duoplasmatron, the SHRIMP excels in the analysis of oxygen and other stable isotopes. Analysis of non-conductive samples is assisted with the optional electron gun for charge neutralisation.
The SHRIMP V is the next commercial version of the SHRIMP SI ion microprobe built at the ANU by Professor Trevor Ireland, for both positive mode geochronology and negative mode stable isotope analysis.
Primary Ion Beam
- Independent control of primary and extracted beam energies and polarities.
- 45 degree angle primary beam incident angle for efficient Sputtering of sample.
- Hollow cathode duoplasmatron ion source designed at Cambridge University (UK) especially for ion probe applications – dual polarity – exceptionally bright and stable.
- Option of Cesium gun for analysis of oxygen and electron gun for source neutralisation.
- Wien filter for mass filtering of primary ion beam – adjustable resolution – may be turned off to maximize sputtering rates.
- Kohler focussing provides radially ion flux on the primary beam.
- Beam diameter variable from 5µm to 30µm.
- Beam diameter and brightness adjusted independently.
- Sharply defined, flat-bottomed pits.
- Standard 25mm diameter sample mounts – Thin section holder or large diameter mega-mount options.
- Fully automated sample transfer.
- Total computer control of sample stage movements – Multiple sample coordinates can be stored and revisited.
- Visual optics permit viewing of sample stage during analysis – Variable magnification – Colour CCD camera/video monitor.
- Up to three samples at one time can be stored in the vacuum lock In addition, two more samples can be placed in the specimen chamber, allowing for the easy comparison of a standard with an unknown.
Seconday Ion Beam
- 90 degree angle extraction of the secondary beam to minimise instrumental discrimination.
- Large aperture eliminates sample-to-sample memory Low field gradient extraction minimizes inter-element discrimination.
- Triple quadrupole lens matching of secondary beam for maximum transmission.
- Simultaneous collection of secondary and mass analysed beams for maximum precision of analysis.
- Large (1272 mm) radius electrostatic analyser.
- Rotatable source slit, width continuously variable from 5µm to 150µm.
- Isotopic mapping of samples accomplished by rastering sample beneath primary beam.
- Elegant, simple integrated ion lens system minimizes beam abberations and simplifies operation.
- High mass dispersion achieved with large (1000mm) radius sector electromagnet.
- Very stable, high speed laminated electromagnet controlled by multiple Hall Effect probes
- Resolution > 5000 (1% definition) with flat-tops for 80µm source slit and 100µm collector slit.
- Sensitivity better than 18cps/ppm/206Pb/nA 02- under above conditions
- Rotatable collector slit, width continuously variable from ~5µm to 300µm.
- Ion counting with robust, high gain, high speed electron multiplier, or
- Ion current measurement via Faraday cup and electrometer.
- Advanced, low-noise electrometers, with computer-configurable gain and settling time.
- Five channels of Faraday cups or electron multipliers.
- Fully reconfigurable under vacuum; no lost time in re-pumping.
- Most variables adjustable under computer control (slit size, head spacing, focus).
- PC platform standard.
- Entire vacuum system under microprocessor control.
- Highly intuitive, easy-to-use graphically orientated operation of entire machine.
- Remote operation via the web, allowing monitoring or real-time control.
- Optional Automated operation for unattended operation and sample pre-screening.
- Examining stellar nucleosynthesis.
- Calibrating the Palaeozoic time-scale.
- Dating of the Earth’s oldest crust.
- Examining the oldest zircons in the solar system.
- Measuring trace elements in diamond inclusions.
- Investigating Ti isotopic ratios in meteorites.
- Determining Pb isotopic composition of lunar granites.
- Paleoclimatology from oxygen isotope ratios in fossil apatite.
SHRIMP zircon dating has led to improved understanding, for example, of the timing and origins of mineralisation of the giant Olympic Dam copper-uranium-gold-silver deposit in South Australia, gold-copper deposits in Tennant Creek area of the Northern Territory, the gold and nickel deposits of Western Australia and Canada. The versitility of SHRIMP in fields of other than uranium-lead dating has been illustrated graphically by probing the composition of sulphur at the micro scale in the sulphide minerals that form metal ores. Vital new understanding of the origin of mineral deposits around the world has resulted.The isotopic composition of sulphur in the giant base metal ore bodies which supply most of the world’s copper, zinc, lead and silver is sensitive to whether that sulphur is derived from sediments or from hot fluids originating deep in the Earth. Knowing the source of the sulphur for each ore body helps in determining why metals were deposited and directs the exploration strategy adopted in the search for new ore bodies.
Exploration of oil is a technically sophisticated and fiercely competitive international business. The cost of trial wells, often dry is huge, and millions of dollars are saved if location of oil can be predicted more accurately prior to drilling. The same applies to finding mineral deposits such as gold, copper and nickel ores buried deep underground. The exploration industry therefore relies increasingly on modelling how, where, and when these commodities form and become trapped in rock structures, so as to target drilling in a risk-efficient pattern.
Geologists learn about oil and mineral formation by close study of the crystals that form rocks. One key mineral is zircon, a crystal just a few thousandths of a millimeter in size that contains trace amounts of uranium (typically one hundred parts per million). The time of formation of rocks and mineral deposits can be measured in millions of years from the progress of the uraniums natural and regular radioactive decay to form lead. This gives vital information on when rocks crystallised, when mineral deposits where emplaced, and when traps suitable for oil were being formed.
Zircons have several concentric zones of growth, rather like tree rings, indicating they have crystallised in stages over periods of many millions of years. Conventional methods of dating zircons involve chemically dissolving the crystals in a beaker, and placing the solution in ag formed.
The science of dating therefore urgently needed a new type of mass spectrometer capable of probing, and dating, the individual zones of crystals. SHRIMP II was developed in response to this need.
In oil and gas exploration, a clear need exists for accurate calculation of rates of basin substance, sedimentation and sea level change and their correlation with crystal movements. In sediments which carry no fossils to aid dating, such calculations have carried unacceptable levels of error. SHRIMP zircon dating is bring a new level of accuracy to the study of the process that form the sedimentary basins.