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Helium diffusion parameters of hematite from a single-diffusion-domain crystal

Farley, K. A. (2018) Helium diffusion parameters of hematite from a single-diffusion-domain crystal. Geochimica et Cosmochimica Acta, 231 . pp. 117-129. ISSN 0016-7037. doi:10.1016/j.gca.2018.04.005.

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This contribution reports new parameters for helium diffusion in hematite useful for interpretation of cosmogenic ^3He and radiogenic ^4He chronometry. Fragments of a coarse, euhedral single crystal of hematite from Minas Gerais, Brazil were subjected to bulk step-heating helium diffusion experiments after proton irradiation to make a uniform distribution of ^3He. Aliquots of three different grain sizes ranging from ∼300 to ∼700 μm in equivalent-sphere radius yielded helium diffusion activation energies E_a∼170 kJ/mol, very similar to previous estimates for E_a in hematite. Uniquely in this specimen, diffusivity varies with the dimensions of the analyzed fragments in precisely the fashion expected if the diffusion domain corresponds to the physical grain. This contrasts with previous studies that concluded that the analyzed hematites consist of polycrystalline aggregates in which helium migration is governed by the size distribution of the constituent crystallites. These new data permit a direct estimate of the helium diffusivity at infinite temperature for hematite of ln(D_o) = -0.66 ± 0.35 cm^2/sec. The major implication of the new diffusion parameters is that hematite is very retentive of helium even at very small crystal sizes. For example, a 20 nm radius hematite crystal, at the smallest end of the size range so far described in dated polycrystalline hematite specimens, will retain more than 99% of its ingrown He over 1 Myr at 30°C, and more than 90% over 100 Myr. Under most conditions, hematite is close to quantitatively helium-retentive on the Earth's surface, simplifying radiogenic and cosmogenic helium dating of this phase. In a system cooling at 10°C/Myr, the 20 nm hematite crystal has a He closure temperature of ∼70°C, similar to a typical ∼100 μm apatite crystal. Helium is likely held tightly in hematite owing to its dense hexagonal closest packing structure and absence of migration-enhancing channels. The isostructural minerals corundum and sapphire are likely to be similarly helium retentive.

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Farley, K. A.0000-0002-7846-7546
Additional Information:© 2018 Elsevier Ltd. Received 17 December 2017, Accepted 5 April 2018, Available online 12 April 2018. This work was supported by the National Science Foundation, Grant EAR-1144500. Fruitful discussions with Paulo Vasconcelos and helpful reviews by Alexis Ault, Oscar Lovera, and Marissa Tremblay are gratefully acknowledged.
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Official Citation:K.A. Farley, Helium diffusion parameters of hematite from a single-diffusion-domain crystal, Geochimica et Cosmochimica Acta, Volume 231, 2018, Pages 117-129, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:85816
Deposited By: Tony Diaz
Deposited On:13 Apr 2018 16:15
Last Modified:15 Nov 2021 20:32

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