High-Resolution Ion-Microprobe Zircon Geochonology: Principles and Techniques
Peter R. Johnson
Since 2000, the Saudi Geological Survey has been engaged in a small ongoing program of high-resolution ion-microprobe U-Pb zircon geochronology of the Precambrian rocks of the Arabian Shield. The sample medium for dating comprises individual zircon grains extracted by crushing and separation from selected plutonic and volcanic rocks; the method is isotopic analysis utilizing “sensitive high-resolution ionmicroprobe” (SHRIMP) instruments at the Stanford University-United States Geological Survey Microprobe Laboratory, California and the West Australian Consortium, Curtin University, West Australia. The combination of medium and analytic method yield results that are mostly accurate, precise, and reliable. They are accurate in the sense that they are analytically sound – the error can be determined within a small range of 1-5 million years. The results are precise in the sense that the microprobe technique gives the operator great control on which part(s) of a given zircon grain is analyzed. The results are reliable in the sense that they can be trusted as true ages and support meaningful geologic interpretation. It is important to remember that useful geochronology depends on obtaining results that are analytically correct and are geologically sensible. Since the mid-1950s, isotope geochronology has been done on rocks from the Arabian Shield, using K-Ar, Rb-Sr, and U-Pb methods. However, as commented by WGM(1992, p. 1) “Results from these earlier programs are now virtually obsolete. Many problems related to excessive lead loss, assumed lower intercepts, inherited zircons, and excessive lead background in the analyses have reduced the accuracy of many results. As a result, regional correlations are both limited and suspect until additional precise direct dating by the U-Pb method is carried out”. Because SHRIMP instruments now easily provide precise direct dating, SGS has started its new program of geochronology.
Johnson, P.R., 2003, High-resolution ion-microprobe zircon geochonology: Principles and techniques: Saudi Geological Survey Data-File Report SGS-DF-2003-3, 10 p., 8 figs., 1 table.