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- U-Pb dating of zircon by laser ablation ICP-MS: recent improvements and new insights
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- Sensitive high-resolution ion microprobe - Wikipedia
Detailed examination of target grains using a range of microscopy methods and applying reasonable but higher uncertainties to pit volumes where features were noted, will minimize the effect of these uncertainties. The implications of Fig.
In addition, sediment provenance and landscape evolution studies 9,22,25,44 that require large numbers of double dates for legitimate statistical treatment and identification of significant age populations will benefit from this powerful tool. Received 13th March , Accepted 1st June The volume of the raised rim surrounding the laser pit was excluded for volume analysis, with the volume calculated from the 3D topography of the pit below the surface level of the sample.
Confocal laser scanning microscopy.
Accurate determination of laser pit dimensions for volume measurement, including diameter and depth, was achieved using a confocal laser scanning microscope LSM , Carl Zeiss located in the School of Earth Sciences, University of Melbourne. Laser light from the CLM strikes the sample at the confocal point of the objective lens. The individual optical sections were then combined to build a three-dimensional image stack. CLM analysis was performed over a AFM utilizes a sharp mechanical probe attached to a flexible cantilever to scan over the ablated pits creating a 3-dimensional topography image from which an apparent volume can be measured.
The volume of each pit was extracted directly from the topography image Fig. However, during routine in situ dating, additional uncertainty is introduced by the roughness of the polished zircon surface measured to range from 5 to 10 nm in amplitude.
U-Pb dating of zircon by laser ablation ICP-MS: recent improvements and new insights
Accordingly, a cumulative uncertainty of 2. For in situ dating, it is critical that instrumentation is capable of measuring low 4 He contents, on the order of pico cubic centimeters. Sensitivity tests were conducted on Mud Tank zircon, which has relatively low U and Th concentrations 10 ppm and 6 ppm, respectively and a U—Pb age of Ma. For younger zircon with relatively higher U and Th concentrations e. The fact that the structures were not present in all ablation pits suggests that they do not result from a reduction in ablation efficiency due to a temporal fluctuation of the incident laser energy, as reported previously.
Although it is beyond the scope of this study to investigate in detail the processes leading to the formation of these structures, we believe that their development is related to nonlinear growth of surface irregularities, initiated by the scattering of light out of the irradiated laser beam by the microfracture surface.
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A BSE image of two zircon grains showing pits ablated using the same operating conditions. Note the structures formed within two of the ablation pits. The features appear to be related to the presence of microfractures within the zircon grains, which are only visible under transmitted light B. C Confocal laser microscope image and D cross section profile corresponding to the ablation pit in C showing that the structures formed within the ablation pits are cone-shaped.
This is consistent with a nonlinear growth of surface irregularities during laser irradiation. U, Th, Sm and Pb measurements.
Our preferred protocol for U, Th and Sm analysis is to ablate directly over the 4 He pits. Zircon standards B and RB The pairwise dating approach requires the adoption of standard reference materials of similar matrix to the mineral being age dated. After testing a number of potential zircon candidates, it was determined that Sri Lanka zircon B fulfills the key attributes of an effective standard reference mineral because: For this reason, B is preferred as the primary standard and was utilized in pairwise dating.
Neither of the pits for the two outliers one of B yielding a too young He age and one RB yielding a too young U—Pb age showed any unusual features.
Sensitive high-resolution ion microprobe - Wikipedia
Table 3 Input required for online pairwise resochronometer. Uncertainty on AFM pit volume was 2. B 35 59 4 1.
Prior to application of pairwise age calculation, standard analyses falling outside the range obtained using traditional methods should be closely examined and discarded if debris or structures that cause pit volume inaccuracies are detected. Pairwise in situ ages reproduce age distribution patterns identified by traditional analysis. A linear scale was used for U—Pb data as there was no significant difference between lamproite and country rock ages.
Ellendale pipe 1 a. Ellendale pipe 2 a. Ellendale pipe 3 a.
Ellendale pipe 4 a. Ellendale pipe 5 a. Laser induced time-dependent elemental fractionation is corrected using the intercept method, whereas instrument drift, mass bias and elemental fractionation caused by ionisation differences are corrected by applying external standardisation using the matrix-matched reference zircon GJ The isobaric interference of Hg on Pb is reduced by using a gold trap enabling the application of an accurate common Pb correction.
A single analysis takes 65 s and the automated analysis of a measurement sequence ensures optimal efficiency. The performance of the established procedure was assessed by analysing zircon ranging in age from Mesoproterozoic to Miocene. Concordia ages obtained from multiple analyses on the various well-characterised zircons are all within uncertainty of the accepted values, demonstrating the high accuracy of the method. Show menu Hide menu. Search Search in European Journal of Mineralogy: