- A simple method for in-situ U-Th-He dating
- A simple method for in-situ U-Th-He dating
- A simple method for in-situ U-Th-He dating
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This model generates positive or negative date-eU correlations depending on the extent of damage in each grain and the date-eU sample's time-temperature history. User Name Password Sign In. 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.
There is also renewed interest in He dating of other U- and Th-bearing minerals both for dating mineral formation and for thermochronometry. For example, Lippolt and coworkers have undertaken detailed studies of He diffusion and dating of various phases, most notably hematite formed in hydrothermal systems Lippolt and Weigel ; Wernicke and Lippolt ; Lippolt et al.
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- (U-Th)/He Dating: Techniques, Calibrations, and Applications.
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- A simple method for in-situ U-Th-He dating.
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Much of this paper focuses on apatite, because the He behavior and requisite analytical techniques are better established for this phase than for other target minerals, such as zircon and titanite. Similarly, much of this paper concerns He diffusivity behavior required for thermochronometric applications, yet recent work is also considering applications to direct dating, for example, of young tephras Farley et al. A Caltech Library Service. More information and software credits. 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.
A simple method for in-situ 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. 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.
A simple method for in-situ U-Th-He dating
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.
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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.
A simple method for in-situ U-Th-He dating
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. 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.
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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