『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