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28-512: The sensitive high-resolution ion microprobe (also sensitive high mass-resolution ion microprobe or SHRIMP ) is a large-diameter, double-focusing secondary ion mass spectrometer (SIMS) sector instrument that was produced by Australian Scientific Instruments in Canberra, Australia and now has been taken over by Chinese company Dunyi Technology Development Co. (DTDC) in Beijing. Similar to

56-492: A cryopump to trap contaminants, especially water. Typical pressures inside the SHRIMP are between ~7 x 10 mbar in the detector and ~1 x 10 mbar in the primary column (with oxygen duoplasmatron source). In normal operations, the SHRIMP achieves mass resolution of 5000 with sensitivity >20 counts/sec/ppm/nA for lead from zircon. For U-Th-Pb geochronology a beam of primary ions (O 2 ) are accelerated and collimated towards

84-467: 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

112-415: 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

140-408: A network of radiation damaged areas. Fission tracks and micro-cracks within the crystal will further extend this radiation damage network. These fission tracks act as conduits deep within the crystal, providing a method of transport to facilitate the leaching of lead isotopes from the zircon crystal. Under conditions where no lead loss or gain from the outside environment has occurred, the age of

168-413: A series of alpha and beta decays, in which U and its daughter nuclides undergo a total of eight alpha and six beta decays, whereas U and its daughters only experience seven alpha and four beta decays. The existence of two 'parallel' uranium–lead decay routes ( U to Pb and U to Pb) leads to multiple feasible dating techniques within the overall U–Pb system. The term U–Pb dating normally implies

196-449: A series of zircon samples has lost different amounts of lead, the samples generate a discordant line. The upper intercept of the concordia and the discordia line will reflect the original age of formation, while the lower intercept will reflect the age of the event that led to open system behavior and therefore the lead loss; although there has been some disagreement regarding the meaning of the lower intercept ages. Undamaged zircon retains

224-452: A typical U-Pb geochronology analytical mode, a beam of (O 2 ) 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

252-586: Is in uranium-thorium-lead geochronology , although the SHRIMP can be used to measure some other isotope ratio measurements (e.g., δLi or δB) and trace element abundances. The SHRIMP originated in 1973 with a proposal by Prof. Bill Compston , 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

280-569: The oldest terrestrial material including the Acasta Gneiss and further extending the age of zircons from the Jack Hills and the oldest impact crater on the planet. 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. In

308-465: The IMS 1270-1280-1300 large-geometry ion microprobes produced by CAMECA , Gennevilliers, France and 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

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336-566: The coupled use of both decay schemes in the 'concordia diagram' (see below). However, use of a single decay scheme (usually U to Pb) leads to the U–Pb isochron dating method, analogous to the rubidium–strontium dating method. Finally, ages can also be determined from the U–Pb system by analysis of Pb isotope ratios alone. This is termed the lead–lead dating method. Clair Cameron Patterson , an American geochemist who pioneered studies of uranium–lead radiometric dating methods, used it to obtain one of

364-486: The details below. Request from 172.68.168.237 via cp1104 cp1104, Varnish XID 202036354 Upstream caches: cp1104 int Error: 429, Too Many Requests at Thu, 28 Nov 2024 07:41:36 GMT Uranium-lead dating The method is usually applied to zircon . This mineral incorporates uranium and thorium atoms into its crystal structure , but strongly rejects lead when forming. As a result, newly-formed zircon crystals will contain no lead, meaning that any lead found in

392-751: The earliest estimates of the age of the Earth in 1956 to be 4.550Gy ± 70My; a figure that has remained largely unchallenged since. Although zircon (ZrSiO 4 ) is most commonly used, other minerals such as monazite (see: monazite geochronology ), titanite , and baddeleyite can also be used. Where crystals such as zircon with uranium and thorium inclusions cannot be obtained, uranium–lead dating techniques have also been applied to other minerals such as calcite / aragonite and other carbonate minerals . These types of minerals often produce lower-precision ages than igneous and metamorphic minerals traditionally used for age dating, but are more commonly available in

420-444: The geologic record. During the alpha decay steps, the zircon crystal experiences radiation damage, associated with each alpha decay. This damage is most concentrated around the parent isotope (U and Th), expelling the daughter isotope (Pb) from its original position in the zircon lattice. In areas with a high concentration of the parent isotope, damage to the crystal lattice is quite extensive, and will often interconnect to form

448-406: The isotopic and elemental abundances in minerals at a 10 to 30 μm-diameter scale and with a depth resolution of 1–5 μm. Thus, SIMS method is 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

476-580: The lead generated by radioactive decay of uranium and thorium up to very high temperatures (about 900 °C), though accumulated radiation damage within zones of very high uranium can lower this temperature substantially. Zircon is very chemically inert and resistant to mechanical weathering – a mixed blessing for geochronologists, as zones or even whole crystals can survive melting of their parent rock with their original uranium–lead age intact. Thus, zircon crystals with prolonged and complicated histories can contain zones of dramatically different ages (usually with

504-417: 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. 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

532-537: 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. Secondary ion mass spectrometry Too Many Requests If you report this error to the Wikimedia System Administrators, please include

560-448: The mineral is radiogenic . Since the exact rate at which uranium decays into lead is known, the current ratio of lead to uranium in a sample of the mineral can be used to reliably determine its age. The method relies on two separate decay chains , the uranium series from U to Pb, with a half-life of 4.47 billion years and the actinium series from U to Pb, with a half-life of 710 million years. Uranium decays to lead via

588-468: The notation can be ignored.) These are said to yield concordant ages ( t from each equation 1 and 2). It is these concordant ages, plotted over a series of time intervals, that result in the concordant line. Loss (leakage) of lead from the sample will result in a discrepancy in the ages determined by each decay scheme. This effect is referred to as discordance and is demonstrated in Figure ;1. If

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616-611: 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

644-411: 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 quadrupole lens is used to reduce aberrations in transmitting the ions to the magnetic sector. The electromagnet has

672-512: The sample is ~10 pA/μm 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 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. A Schwarzschild objective lens provides reflected-light direct microscopic viewing of

700-479: 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 Zr 2 O, 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),

728-433: The time in order to analyse individual mineral grains. 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

756-451: The zircon can be calculated by assuming exponential decay of uranium. That is where This gives which can be written as The more commonly used decay chains of Uranium and Lead gives the following equations: (The notation Pb ∗ {\displaystyle {\text{Pb}}^{*}} , sometimes used in this context, refers to radiogenic lead. For zircon, the original lead content can be assumed to be zero, and

784-695: 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 were partially confirmed. 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

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