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

Abstract

This contribution reports new parameters for helium diffusion in hematite useful for interpretation of cosmogenic 3 He and radiogenic 4 He 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 3 He. Aliquots of three different grain sizes ranging from ~300 to ~700 μm in equivalent-sphere radius yielded helium diffusion activation energies Ea~170 kJ/mol, very similar to previous estimates for Ea 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(Do) =-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 o 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 o C/Myr, the 20 nm hematite crystal has a He closure temperature of ~70 o 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.

Innovations in (U–Th)/He, Fission Track, and Trapped Charge Thermochronometry with Applications to Earthquakes, Weathering, Surface‐Mantle Connections, and the Growth and Decay of Mountains

“…Hematite He thermochronometry has been successfully applied to constrain deformation, hydrothermal, and diagenesis histories (Ault, Reiners, et al, ; Ault et al, ; Calzolari et al, ; Evenson et al, ; Farley & Flowers, ; Farley & McKeon, ; McDermott et al, ; Moser et al, ; Wernicke & Lippolt, , ). Hematite occurs as polycrystalline aggregates and exhibits polydomain He diffusion behavior (Evenson et al, ; Farley, ; Farley & Flowers, ). Individual crystallites are the He diffusion domains and the hematite He T c , which is ~25–250 °C (10 °C/Myr cooling rate), increases with increasing domain (grain) size (Balout et al, ; Evenson et al, ; Farley, ; Farley & Flowers, ; Jensen et al, ).…”

“…Hematite occurs as polycrystalline aggregates and exhibits polydomain He diffusion behavior (Evenson et al, ; Farley, ; Farley & Flowers, ). Individual crystallites are the He diffusion domains and the hematite He T c , which is ~25–250 °C (10 °C/Myr cooling rate), increases with increasing domain (grain) size (Balout et al, ; Evenson et al, ; Farley, ; Farley & Flowers, ; Jensen et al, ). Goethite He has a T c ~25–60 °C with possible He loss at surface conditions and is used to date supergene alteration and deep weathering profiles (Deng et al, ; Heim et al, ; Miller et al, ; Monteiro et al, ; Riffel et al, ; Shuster, Vasconcelos, et al, ; Shuster et al, ; Vasconcelos et al, ).…”

“…Recent and ongoing development of 4 He/ 3 He thermochronometry permits recovery of aliquot‐specific diffusion kinetics and a sample's detailed thermal history (Farley, ; Farley & Flowers, ; Fox, McKeon, & Shuster, ; Shuster et al, ; Shuster & Farley, , ; Shuster, Vasconcelos, et al, ). This approach quantifies the distribution of 4 He in a single crystal or polycrystalline aggregate.…”

“…Zircon 4 He/ 3 He thermochronometry has been applied to the fast‐cooled Fish Canyon Tuff zircon standard and zircon crystals from the Sierra Nevada batholith in the vicinity of the Little Devil's Postpile intrusion (Tripathy‐Lang et al, ). This tool has also been used to constrain the diffusion kinetics and thermal history of hematite (Evenson et al, ; Farley, ; Farley & Flowers, ; Farley & McKeon, ), goethite (Allard et al, ; Deng et al, ; Shuster, Vasconcelos, et al, ; Vasconcelos et al, ), and Mn oxides (Garcia et al, ).…”

“…For example, 4 He/ 3 He thermochronometry allows us to recover thermal history information from a single crystal or aliquot (e.g., Farley & McKeon, ; Flowers & Farley, ; Garcia et al, ; Schildgen et al, ; Tripathy‐Lang et al, ; Valla et al, ). Advances in our understanding of He diffusion in hematite, magnetite, goethite, and Mn oxides (e.g., Balout et al, ; Farley, ; Garcia et al, ), new aliquot selection approaches (e.g., Jensen et al, ), and integration of nanotextural observations with oxide thermochronometry (e.g., Ault et al, ; Ault, Reiners, et al, ; McDermott et al, ; Moser et al, ) enable robust application of these thermochronometry tools to hydrothermal systems, faults, and surficial deposits in recent and deep time.…”