### Abstract

Accurate thermochronologic interpretation of zircon (U-Th)/He dates requires a realistic and practically useful understanding of He diffusion kinetics in natural zircon, ideally across the range of variation that characterize typically dated specimens. Here we present a series of date and diffusion measurements that document the importance of alpha dose, which we interpret to be correlated with accumulated radiation damage, on He diffusivity. This effect is manifest in both date-effective uranium (eU) correlations among zircon grains from single hand samples and in diffusion experiments on pairs of crystallographically oriented slabs of zircon with alpha doses ranging from ∼10^{16} to 1019 α/g. We interpret these results as due to two contrasting effects of radiation damage in zircon, both of which have much larger effects on He diffusivity and thermal sensitivity of the zircon (U-Th)/He system than crystallographic anisotropy. Between 1.2×1016 α/g and 1.4×1018α/g, the frequency factor, D0, measured in the c-axis parallel direction decreases by roughly four orders of magnitude, causing He diffusivity to decrease dramatically (for example by three orders of magnitude at temperatures between 140 and 220 °C). Above ∼2×1018α/g, however, activation energy decreases by a factor of roughly two, and diffusivity increases by about nine orders of magnitude by 8.2×1018α/g. We interpret these two trends with a model that describes the increasing tortuosity of diffusion pathways with progressive damage accumulation, which in turn causes decreases in He diffusivity at low damage. At high damage, increasing diffusivity results from damage zone interconnection and consequential shrinking of the effective diffusion domain size. Our model predicts that the bulk zircon (U-Th)/He closure temperature (Tc) increases from about 140 to 220 °C between alpha doses of 1016 to 1018 /g, followed by a dramatic decrease in Tc above this dose. Linking this parameterization to one describing damage annealing as a function of time and temperature, we can model the coevolution of damage, He diffusivity, and (U-Th)/He date of zircon. This model generates positive or negative date-eU correlations depending on the extent of damage in each grain and the date-eU sample's time-temperature history.

Original language | English (US) |
---|---|

Pages (from-to) | 145-198 |

Number of pages | 54 |

Journal | American Journal of Science |

Volume | 313 |

Issue number | 3 |

DOIs | |

State | Published - Mar 1 2013 |

### Fingerprint

### Keywords

- He diffusion
- Radiation damage
- Thermochronology
- Zircon

### ASJC Scopus subject areas

- Earth and Planetary Sciences(all)

### Cite this

*American Journal of Science*,

*313*(3), 145-198. https://doi.org/10.2475/03.2013.01

**Helium diffusion in natural zircon : radiation damage, anisotropy, and the interpretation of zircon (U-TH)/He thermochronology.** / Guenthner, William R.; Reiners, Peter W.; Ketcham, Richard A.; Nasdala, Lutz; Giester, Gerald.

Research output: Contribution to journal › Article

*American Journal of Science*, vol. 313, no. 3, pp. 145-198. https://doi.org/10.2475/03.2013.01

}

TY - JOUR

T1 - Helium diffusion in natural zircon

T2 - radiation damage, anisotropy, and the interpretation of zircon (U-TH)/He thermochronology

AU - Guenthner, William R.

AU - Reiners, Peter W.

AU - Ketcham, Richard A.

AU - Nasdala, Lutz

AU - Giester, Gerald

PY - 2013/3/1

Y1 - 2013/3/1

N2 - Accurate thermochronologic interpretation of zircon (U-Th)/He dates requires a realistic and practically useful understanding of He diffusion kinetics in natural zircon, ideally across the range of variation that characterize typically dated specimens. Here we present a series of date and diffusion measurements that document the importance of alpha dose, which we interpret to be correlated with accumulated radiation damage, on He diffusivity. This effect is manifest in both date-effective uranium (eU) correlations among zircon grains from single hand samples and in diffusion experiments on pairs of crystallographically oriented slabs of zircon with alpha doses ranging from ∼1016 to 1019 α/g. We interpret these results as due to two contrasting effects of radiation damage in zircon, both of which have much larger effects on He diffusivity and thermal sensitivity of the zircon (U-Th)/He system than crystallographic anisotropy. Between 1.2×1016 α/g and 1.4×1018α/g, the frequency factor, D0, measured in the c-axis parallel direction decreases by roughly four orders of magnitude, causing He diffusivity to decrease dramatically (for example by three orders of magnitude at temperatures between 140 and 220 °C). Above ∼2×1018α/g, however, activation energy decreases by a factor of roughly two, and diffusivity increases by about nine orders of magnitude by 8.2×1018α/g. We interpret these two trends with a model that describes the increasing tortuosity of diffusion pathways with progressive damage accumulation, which in turn causes decreases in He diffusivity at low damage. At high damage, increasing diffusivity results from damage zone interconnection and consequential shrinking of the effective diffusion domain size. Our model predicts that the bulk zircon (U-Th)/He closure temperature (Tc) increases from about 140 to 220 °C between alpha doses of 1016 to 1018 /g, followed by a dramatic decrease in Tc above this dose. Linking this parameterization to one describing damage annealing as a function of time and temperature, we can model the coevolution of damage, He diffusivity, and (U-Th)/He date of zircon. This model generates positive or negative date-eU correlations depending on the extent of damage in each grain and the date-eU sample's time-temperature history.

AB - Accurate thermochronologic interpretation of zircon (U-Th)/He dates requires a realistic and practically useful understanding of He diffusion kinetics in natural zircon, ideally across the range of variation that characterize typically dated specimens. Here we present a series of date and diffusion measurements that document the importance of alpha dose, which we interpret to be correlated with accumulated radiation damage, on He diffusivity. This effect is manifest in both date-effective uranium (eU) correlations among zircon grains from single hand samples and in diffusion experiments on pairs of crystallographically oriented slabs of zircon with alpha doses ranging from ∼1016 to 1019 α/g. We interpret these results as due to two contrasting effects of radiation damage in zircon, both of which have much larger effects on He diffusivity and thermal sensitivity of the zircon (U-Th)/He system than crystallographic anisotropy. Between 1.2×1016 α/g and 1.4×1018α/g, the frequency factor, D0, measured in the c-axis parallel direction decreases by roughly four orders of magnitude, causing He diffusivity to decrease dramatically (for example by three orders of magnitude at temperatures between 140 and 220 °C). Above ∼2×1018α/g, however, activation energy decreases by a factor of roughly two, and diffusivity increases by about nine orders of magnitude by 8.2×1018α/g. We interpret these two trends with a model that describes the increasing tortuosity of diffusion pathways with progressive damage accumulation, which in turn causes decreases in He diffusivity at low damage. At high damage, increasing diffusivity results from damage zone interconnection and consequential shrinking of the effective diffusion domain size. Our model predicts that the bulk zircon (U-Th)/He closure temperature (Tc) increases from about 140 to 220 °C between alpha doses of 1016 to 1018 /g, followed by a dramatic decrease in Tc above this dose. Linking this parameterization to one describing damage annealing as a function of time and temperature, we can model the coevolution of damage, He diffusivity, and (U-Th)/He date of zircon. This model generates positive or negative date-eU correlations depending on the extent of damage in each grain and the date-eU sample's time-temperature history.

KW - He diffusion

KW - Radiation damage

KW - Thermochronology

KW - Zircon

UR - http://www.scopus.com/inward/record.url?scp=84881315206&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=84881315206&partnerID=8YFLogxK

U2 - 10.2475/03.2013.01

DO - 10.2475/03.2013.01

M3 - Article

AN - SCOPUS:84881315206

VL - 313

SP - 145

EP - 198

JO - American Journal of Science

JF - American Journal of Science

SN - 0002-9599

IS - 3

ER -