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- Isotope Chronology & Geochemistry
- A simple method for in-situ U-Th-He dating
- Thermochronometry | Open Energy Information
The given value was not understood. The thermochronometry method provides information on the thermal history of samples, the magnitude of thermal perturbation, and the longevity of the thermal structure which may reveal a geothermal resource. Thermochronometry could become a useful tool in exploration for blind geothermal systems as the absence of hydrothermal activity at the surface would not affect the results of a thermochronometry study.
Dating of major normal fault systems using thermochronology- An example from the Raft River detachment, Basin and Range, western United States. The Wassuk Range, Hawthorne, Nevada. Contributions to Mineralogy and Petrology. Retrieved from " https: Provide some quick feedback: Send Us an Email Close Submit. Thermochronometry Monaster And Coolbaugh, Thermochronometry At Coso Geothermal Area Fish Lake Valley Area. In-situ U-Th-He geochronology by laser ablation potentially offers the following advantages over conventional U-Th-He dating by whole grain degassing and dissolution.
First, it dramatically increases sample throughput. Measuring the U and Th content of zircon by isotope dilution requires dissolution in hydrofluoric acid at high temperature and pressure using a Parr bomb for up to 48 hours. Second, the process of in-situ measurements of U and Th content of grains yields U-Th-Pb ages as a by-product.
Polish and mount the crystals in Indium, which is a malleable material that does not break down in ultrahigh vacuum, in contrast with most epoxy resins or teflon. Ablate the sample with a short wavelength laser and measure the amount of helium released in moles. Measure the ablation pit volume with an interferometric microscope to calculate the helium concentration in moles per cc. Enter the U, Th, and He concentrations into the helium ingrowth equation to calculate an age.
The revised method presented in this paper greatly simplifies the second and fourth steps of the current method while completely removing the third.
Section 4 outlines a method to estimate relative differences in laser ablation rate without the need for absolute depth measurements, by tracking the signal strength of stoichiometric 29 Si. Finally, Section 7 applies the proposed method to three shards of gem-quality Sri Lanka zircons that had been previously dated with the conventional U-Th-He method. To calculate a helium age, it is not necessary to know the absolute concentrations of U, Th, and He. This insight forms the basis of the simplified method, which does not require knowledge of any absolute abundances or concentrations, but instead uses the raw mass spectrometer measurements.
In its simplest form, assuming identical laser ablation rates in the standard and the unknown, the method works as follows: Polish and mount two sets of grains in Indium: Ablate the grains and measure the raw helium signal in A, V, or Hz of the sample along with helium measurements of the age standard.
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In practice, the standard and the unknown are combined on a pairwise basis. To correct for instrument drift and plasma-induced fluctuations in the ICP-MS, the U and Th signals should be measured relative to stoichiometric 29 Si. Because any analytical uncertainty in the age standard propagates into the unknown age, the choice of standard is very important. In order to qualify as a good U-Th-He age standard, a sample must fulfill the following requirements.
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. 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.
Isotope Chronology & Geochemistry
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.
A simple method for in-situ U-Th-He dating
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.
Thermochronometry | Open Energy 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