U-th/he dating

The combination of large mm-scale grain size, which enables multiple laser spots to be placed on the same crystal, and inter-sample compositional variability makes the Sri Lanka zircons ideally suited to test the precision and accuracy of the simplified method. In this study, we have used sample G as a standard, and samples RB and B as unknowns.

In addition, the in-situ dating method was also applied to zircons from the Fish Canyon Tuff These compositionally zoned samples were used to illustrate the pseudo-depth profile technique proposed in Section 4. They were then extracted from the teflon and pressed into strips of Indium foil with the polished side facing upwards. Helium analyses were done at the Open University. The extracted gas was cleaned for three minutes using three SAES AP getters to remove active gases before analysing 4 He using a multiplier collector on a MAP noble gas mass spectrometer.

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A mixed Ar-He flux of 1. Data reduction was done by taking the average ratios of the raw signals in counts per second , and analytical uncertainties are reported as the standard errors of those averages. This section discusses the different columns of this table from left to right. Extended data files for all three Sri Lanka zircon shards, as well as a detailed description of the data reduction protocols are provided in the Supplementary Information. These values cluster tightly around a mean value of 0. This confirms that G does not exhibit significant compositional zoning, thus fulfilling an important requirement for its use as a U-Th-He age standard.

Only RB shows a significant disagreement, possibly indicating the presence of compositional zoning between different shards of this crystal. Black line shows the 1: All these values are significantly less than one reflecting the lower actinide concentrations of samples B and RB compared to G At this point, the He signal has not yet been corrected for possible variations in the laser drill rate. Precise depth measurements of the ablation pits with a Zygo NewView white-light microscope-based interferometer at Imperial College London indicate small but significant variability of the laser drill rates between samples.

The average pit depths in the three Sri Lanka zircons used in this study are It is interesting to note that the greatest pit depths are found in sample G, which also has the highest U and Th content.


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The eighth column contains the drill rate proxy fSi which shows a reasonable linear correlation with fD for the three Sri Lanka zircons Figure 3. The individual values of the drill rate proxy are significantly more scattered than the pit depths, but their averages scale proportionally. The black line shows the best linear fit with zero intercept.

Weighted mean values are shown next to the axes. The first, shown in the seventh column of Table 1 , was calculated using the measured pit depths i. The second, shown in the last column, was calculated using the drill rate proxy fHe divided by fSi. This tight clustering confirms, first, the potential of in-situ U-Th-He geochronology to produce data that are just as precise as conventional whole-grain degassing and dissolution and, second, the suitability of Sri Lanka zircon as a well-behaved age standard.

A simple method for in-situ U-Th-He dating

Black ellipses show the error-weighted geometric mean compositions. MSWD values refer to the compositions and not the ages. Unfortunately, the relatively low sensitivity of the MAP noble gas mass spectrometer used for the He measurements, combined with the young ages Therefore, the two depth-profiles shown in Figure 5 are provided for illustrative purposes only.

They do confirm, however, that the analysed FCT and TR zircons are compositionally inhomogeneous, causing non-flat pseudo-depth profiles. Vertical axes show age in Ma. Instead of measuring absolute concentrations of U, Th, and He, as is customary in conventional U-Th-He geochronology, the new method uses an independently dated age standard, against which the samples of interest are compared. Three shards of gem-quality Sri Lanka zircon were successfully dated, demonstrating the effectiveness of the pairwise dating method as a means of determining accurate and precise U-Th-He ages in just a fraction of the time required to perform a conventional U-Th-He age measurement.

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The use of independently dated calibration standards is not a new idea in thermochronology. An important area of future progress is the further development of the drill rate proxy as a means of removing the need to measure pit depths by interferometric microscopy. Helium diffusion in natural zircon: Services Email this article to a colleague Alert me when this article is cited Alert me if a correction is posted Similar articles in this journal Similar articles in Web of Science Download to citation manager.

Google Scholar Articles by Guenthner, W. Articles by Giester, G. First, it should be a large gem-quality crystal, ensuring relatively uniform ablation behaviour, while saving the user the trouble of polishing and mounting large numbers of crystals in Indium. Second, it should lack major compositional zoning and have relatively uniform U and Th concentrations.

Third, it must not show any significant core-to-rim depletion in helium content due to diffusive loss during cooling. Sri Lanka zircon fulfills all these requirements and will be used to illustrate the effectiveness of the proposed method at the end of this paper Section 7. Conceptual diagram of the pairwise dating method. Age calculation involves the following steps: Apply these scaling factors to the normalised U, Th, and He content of the standard and plug the resulting products ellipse in the U-Th-He age equation to obtain the U-Th-He age of the unknown sample.

In other words, the use of Equation 1 does not require U-Th-Pb concordance of the sample. Violation of this assumption may result in systematic errors.

Caltech Noble Gas Lab Research Activities: (U-Th)/He dating

At this point we should note that it is far easier to measure the depth of an ablation pit than it is to measure its volume. This is because the walls of pits produced by excimer lasers are often so steep that not enough light is available to produce an interferometric depth estimate of the ablation pit edges. Performing depth measurements adds another analytical step and partly defeats the purpose of the pairwise dating method.

The idea behind the drill rate proxy is that the beam intensity of a stoichiometric nuclide, such as 29 Si should increase proportionally with the rate of laser ablation. If this is correct, then the average ratio fSi of the time-resolved 29 Si spectrum of the unknown sample over that of the standard should equal the ratio of the ablation pit depths fD.

It then suffices to divide the normalised helium signal by fSi to account for the differential drill rates. These problems are not any easier to solve with the in-situ dating method than they are with the conventional U-Th-He method. In-situ dating, however, does allow zoning effects to be detected and quantified in ways that are not possible by conventional whole grain degassing and dissolution.

These pseudo depth profiles are very useful for detecting compositional zoning. If a pseudo depth profile is not flat, then the best age is obtained from the shallow parts of the U-Th ablation pit, which are closest to where the He was measured from. This section provides the analytical details of the instrumental setup used in this proof of concept study. The combination of large mm-scale grain size, which enables multiple laser spots to be placed on the same crystal, and inter-sample compositional variability makes the Sri Lanka zircons ideally suited to test the precision and accuracy of the simplified method.

In this study, we have used sample G as a standard, and samples RB and B as unknowns. In addition, the in-situ dating method was also applied to zircons from the Fish Canyon Tuff These compositionally zoned samples were used to illustrate the pseudo-depth profile technique proposed in Section 4. They were then extracted from the teflon and pressed into strips of Indium foil with the polished side facing upwards.

Helium analyses were done at the Open University. The extracted gas was cleaned for three minutes using three SAES AP getters to remove active gases before analysing 4 He using a multiplier collector on a MAP noble gas mass spectrometer. A mixed Ar-He flux of 1. Data reduction was done by taking the average ratios of the raw signals in counts per second , and analytical uncertainties are reported as the standard errors of those averages.

This section discusses the different columns of this table from left to right. Extended data files for all three Sri Lanka zircon shards, as well as a detailed description of the data reduction protocols are provided in the Supplementary Information. These values cluster tightly around a mean value of 0.

This confirms that G does not exhibit significant compositional zoning, thus fulfilling an important requirement for its use as a U-Th-He age standard. Only RB shows a significant disagreement, possibly indicating the presence of compositional zoning between different shards of this crystal. Black line shows the 1: All these values are significantly less than one reflecting the lower actinide concentrations of samples B and RB compared to G At this point, the He signal has not yet been corrected for possible variations in the laser drill rate.