Shuster et al.(2005)による〔『Weathering geochronology by (U-Th)/He dating of goethite』(659p)から〕

『針鉄鉱の(U-Th)/He年代決定法による風化作用の地質年代学』


Abstract
 Nine samples of supergene goethite (FeOOH) from Brazil and Australia were selected to test the suitability of this mineral for (U-Th)/He dating. Measured He ages ranged from 61 to 8 Ma ad were reproducible to better than a few percent despite very large variations in [U] and [Th]. In all samples with internal stratigraphy or independent age constraints, the He ages corroborated the expected relationships. These data demonstrate that internally consistent He ages can be obtained on goethite, but do not prove quantitative 4He retention. To assess possible diffusive He loss, stepped-heating experiments were performed on two goethite samples that were subjected to proton irradiation to produce a homogeneous distribution of spallogenic 3He. The 3He release pattern indicates the presence of at least two diffusion domains, one with high helium retentivity and the other with very low retentivity at Earth surface conditions. The low retentivity domain, which accounts for 〜5% of 3He, contains no natural 4He and may represent poorly crystalline or intergranular material which has lost all radiogenic 4He by diffusion in nature. Diffusive loss of 3He from the high retentivity domain is independent of the macroscopic dimensions of the analyzed polycrystalline aggregate, so probably represents diffusion from individual micrometer-size goethite crystals. The 4He/3He evolution during the incremental heating experiments shows that the high retentivity domain has retained 90%-95% of its radiogenic helium. This degree of retentivity is in excellent agreement with that independently predicted from the helium diffusion coefficients extrapolated to Earth surface temperature and held for the appropriate duration. Considering both the high and low retentivity domains, these data indicate that one of the samples retained 90% of its radiogenic 4He over 47.5 Ma and the other retained 86% over 12.3 Ma. Thus while diffusive-loss corrections to supergene goethite He ages are required, these initial results indicate that the corrections are not extremely large and can be rigorously quantified using the proton-irradiation 4He/3He method.』

1. Introduction
2. Samples
3. Methods
 3.1. He dating
 3.2. Step-heating diffusion experiments
4. Helium dating−Results
 4.1. Comparing He ages with Ar ages
5. Helium dating−Discussion
6. Stepwise degassing experiments−Results
 6.1. Proton-induced 3He diffusion parameters
 6.2. Radiogenic 4He diffusion coefficients
 6.3. Ratio evolution diagrams
 6.4. Helium retentivity
7. Stepwise degassing experiments−Discussion
 7.1. Two-domain model
 7.2. Quantifying deficit gas fractions
 7.3. He age corrections
 7.4. Internal consistency and mean temperature
 7.5. Physical significance of diffusion domains
 7.6. General extrapolation of goethite diffusion parameters
8. Conclusions
Acknowledgments
References


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