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Measured He ages ranged from 61 to 8 Ma and 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.
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.