Shrimp

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The SHRIMP (Sensitive High-mass Resolution Ion Microprobe) is an ion microprobe instrument produced by Australian Scientific Instruments at the Australian National University in Canberra (Compston et al., 1984; Williams, 1998).

The SHRIMP allows microanalysis of minerals at a scale of ~30 μm and is therefore particularly well-suited for the dating of complex minerals, as often found in metamorphic terrains. The most common application is in U-Th-Pb geochronology, but SHRIMP can be used to measure other isotopic abundances.

For U-Th-Pb geochronology a beam of “primary” ions (O₂^-) is collimated and accelerated towards the target, and used to sputter “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 standard material (gem quality zircon of known age and isotopic composition), and determining a calibration factor following procedures described in detail by Claoué-Long et al. (1995)

References

Kennedy, A.K. and De Laeter, J.R., 1994. The performance characteristics of the WA SHRIMP II ion microprobe. US Geological survey circular, 1107: 166.
Claoué-Long, J., Compston, W., Roberts, J. and Fanning, C.M., 1995. Two Carboniferous ages: a comparison of SHRIMP zircon dating with conventional zircon ages and 40Ar/39Ar analysis. In: W.A. Berggren, D.V. Kent, M.-P. Aubry and J. Hardenbol (Editors), Geochronology, time scales and global stratigraphic correlation:. SEPM (Society of Sedimentary Petrology) Special Publication, pp. 3-21.
Claoué-Long, J., 1994. SHRIMP zircon data, pp. 33.
Williams, I.S., 1998. U-Th-Pb geochronology by ion microprobe. In: M.A. McKibben, W.C. Shanks III and W.I. Ridley (Editors), Applications of microanalytical techniques to understanding mineralizing processes. Reviews in Economic Geology, pp. 1-35.
Compston, W., 1999. Geological age by instrumental analysis: the 29th Hallimond Lecture. Mineralogical Magazine, 63(3): 297-311.
Compston, W., 2000. Interpretation of SHRIMP and isotope dilution zircon ages for the Palaeozoic time-scale: II. Silurian to Devonian. Mineralogical Magazine, 64(6): 1127-1146.
Compston, W., 2000. Interpretations of SHRIMP and isotope dilution zircon ages for the geological time-scale: I. The early Ordovician and late Cambrian. Mineralogical Magazine, 64(1): 43-57.
Compston, W., Williams, I.S. and Meyer, C., 1984. U-Pb geochronology of zircons from lunar breccia 73217 using a sensitive high mass-resolution ion microprobe. Journal of Geophysical Research, 89: 525-534.
Compston, W., Williams, I.S., Kirschvink, J.L., Zhang, Z., Ma,G., 1992. Zircon U-Pb ages for the Early Cambrian time scale. J. Geological Society of London, 149: 171-184.

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