A transformative advance in Earth science is development of thermochronometry to quantify the thermal evolution of rocks through time. Low-temperature thermochronometry – specifically (U-Th)/He dating – is now a cornerstone of geoscience investigations documenting the timing and tempo of thermal processes, including mountain building, landscape evolution and erosion, fault slip, and mineralization.(U-Th)/He thermochronometry exploits the natural decay of isotopes of uranium (U), thorium (Th), and samarium (Sm), associated alpha particle (He) production, and temperature-sensitive He diffusion (closure temperature) through the crystal. Target phases include apatite, zircon, titanite, hematite, magnetite, goethite, perovskite, monazite, and others with collective closure temperatures spanning ~25-500 °C (see below). These tools are well suited to reconstruct the thermal imprint of processes operative in the upper ~10-15 km of Earth’s crust.4He nuclei, or α particles, are produced during radioactive α decay of 238U, 235U, 232Th, and 147Sm to stable isotopes of Pb and Nd. Time-dependent ingrowth of 4He is described by:

where 4He, 238U, 235U, 232Th, and 147Sm are the measured amounts of each isotope, λ238, λ235, λ232, and λ147 are the associated decay constants, and t is elapsed time. 4He concentration in a crystal reflects the balance between production by decay, which is modified by alpha ejection, and loss by diffusion, integrated over the time-temperature history of the crystal.

(1) 4He = 8238U[e-λ238t-1] + 7235U[e-λ235t-1] + 6232Th[e-λ232t-1] + 147Sm[e-λ147t-1]

Successful application of the (U-Th)/He method begins with careful sample selection and aliquot characterization.Targeting bedrock accessory phases requires 5-10 kg of unaltered, felsic rocks (granitoids, orthogneisses, paragneisses, sandstones). Mafic igneous rocks, volcanics, and fine-grained sedimentary rocks (limestone, shale) have low yield.Mineral seperation by crushing, seiving (<500 μm), density (water table, heavy liquids), and magnetic methods (Frantz).Accessory phases for (U-Th)/He dating selected using a stereoscope. For phases like apatite and zircon, ideal candidates for dating will be whole crystals free of cracks and imperfections, lack staining by grain boundary phases, and free of mineral and fluid inclusions (although mineral inclusions in zircon are acceptable).Aliqout characterization includes measuring grain dimensions (for alpha ejection correction (FT) and concentration calculation) and noting possible imperfections.Analysis of ~ 5 aliquots/sample, with each aliquot loaded into a separate Nb tube for U, Th, Sm, and He analysis.Grain size analysis of polycrystalline aliquots (hematite, goethite) via scanning electron microscopy to characterize aliquot TC range. Careful selection of aliquots of pure Fe-oxide that lack interstitial phases.

where Dois diffusivity at infinite temperature, Ea is the activiation energy, R is the gas constant, and T is temperature.

TC varies for each mineral and is dependent on factors such as parent isotope concentration and radiation damage accumulation, cooling rate, diffusion domain lengthscale (typically grain radius), and laboratory-derived diffusion kinetics. Polycrystalline aliquots may be characterized by a range of TC depending on the aliquot’s grain size distribution.

Minerals are suitable for (U-Th)/He thermochronometry provided they yield measurable U and Th (ppm), contain negligble initial 4He, and retain 4He over geologic timescales. Loss of 4He from the crystal lattice typically occurs by time and temperature-dependent diffusion. He diffusion follows an Arrhenius relationship:

(2) D/a2 = Do/a2 e-Ea/RT

For most minerals (e.g., apatite, zircon, titanite), the crystal is the diffusion domain. Polycrystalline aggregates such as hematite and goethite exhibit poly-domain He diffusion behavior where the individual crystals are the diffusion domains. A metric for describing the turning point of 4He loss versus retention is the closure temperature (TC). The figure on the left provides quantitative constraints on the TC for the (U-Th)/He system in different minerals.

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200

300

0

Tem

pera

ture

(°C

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monazite

zircon

apatite

magnetite

hematite

rutile

perovskite

goethite

garnet

titanite calcite

baddeleyite400

500

speed dating! (U-Th)/He thermochronometryintroduction and applications

He diffusion and closure temperature

sample collection and aliquot selection

Alexis K. Ault, Utah State University, Logan, UT, alexis.ault@usu.eduWilliam R. Guenthner, University of Illinois at Urbana-Champaign, Urbana, IL, wrg@illinois.eduRobert G. McDermott, Utah State University, Logan, UT, rgmcdermott@aggiemail.usu.edu

Arizona State University, Noble GasGeochronology and Geochemistry Laboratories (NG3L)Kip Hodges, kvhodges@asu.eduCaltech, Noble Gas LabKenneth Farley, farley@gps.caltech.eduLehigh University, Noble-Gas LabPeter Zeitler, peter.zeitler@lehigh.eduStanford University, Noble Gas LaboratoryMarty Grove, mjgrove@standford.eduUniversity of Arizona, Arizona Radiogenic Helium Dating LabPeter Reiners, preiners@email.arizona.eduUniversity of California Berkeley, Noble Gas Thermochronometry LabDavid Shuster, dshuster@berkeley.eduUniversity of Colorado Boulder, CU Thermochronology Research and Instrumentation Laboratory (TRaIL)Rebecca Flowers, rflowers@colorado.eduUniversity of Connecticut, Basin Analysis & Thermochronology LabJulie Fosdick, julie.fosdick@uconn.eduUniversity of Illinois at Urbana-Champaign, Helium Analysis Laboratory (HAL)William Guenthner, wrg@illinois.eduUniversity of Michigan, Thermochronology LabMarin Clark, marinkc@umich.edu and Nathan Niemi, naniemi@umich.eduUniversity of California at Santa Cruz,Jeremy Hourigan, hourigan@ucsc.eduUniversity of Texas Austin, UT Chron LabDaniel Stockli, stockli@jsg.utexas.eduUSGS, Argon and Helium Geochronology LabMichael Cosca, mcosca@usgs.gov and Joseph Colgan, jcolgan@usgs.govVirginia Tech, Radiogenic Helium LaboratoryJames Spotila, spotila@vt.edu

4He/3He thermochronometry quantifies the spatial distribution of 4He within a crystal that is a function of the thermal history. Involves stepwise degassing of 4He and a uniform 3He distribution (irradiation, proton bombardment).

complexities and opportunities (U-Th)/He laboratories

workflow and analytical procedures

date and rates

219 μm151 μm

1. grain selection and measurement:

(U-Th)/He dates typically have 2-8% analytical uncertainty (on U-Th-Sm-He measurements) and mean dates yield 8-15% standard deviation. Some sources of intrasample date dispersion include:

a1 a2 a3 a4 a5a6 a7

a3

Phases examined and measured using a stereoscope. Hematite also imaged and grain size distribution measured via SEM. Aliquots inserted into Nb tubes that are loaded into a planchette.

2. He outgassing and purification: 4He extracted via in-vaccum via heating with Nd:YAG, CO2, or diode laser.Heating schedules vary with target mineral. For example, apatite heated to ~950-1050 °C for 3 min. (no re-extract); zircon heated to ~1250 °C for 15 min. followed by 1-2 re-extracts to purge grains of 4He. Hematite degassing T and duration depend on grain size.Extracted 4He gas spiked with 3He, purified using cryogenic and gettering methods, and analyzed on a quadrupole mass spectrometer (QMS). Analysis of known quantity of 4He throughout run to monitor instrument sensitivity drift.

He extraction line andQMS at the HAL (UIUC)

3. Dissolution and spiking: Degassed aliquots in Nb packets retrived and dissolved. Different phases require different dissolution regimens (e.g., apatite dissolved in HNO3 at 90 °C for 1 hr or zircon HF dissolution via Parr bomb). Addition of and equilibration with spike (e.g., 233U-229Th-147Nd-42Ca for apatite and 233U-229Th-90Zr for zircon; Guenthner et al., 2016). 4. U, Th (Sm) measurement: Analyses via ICP-MS or QMS.

6. Standards: In-run standards such as Durango and Fish Canyon Tuff are used to monitor dissolution, chemistry, and isotope measurements.

5. Concentration and date calculation: U, Th, He, (Sm) concentrations from dimensional mass (grain measurements or from Ca, Zr measurements and stoichiometry (Guenthner et al., 2016). Correct for long alpha particle stopping distances with an FT correction (e.g., Farley et al., 1996) and calculate date via Eq. [1].

grain size variation: Larger crystals have larger diffusion domain lengthscales and higher TC.U and Th zonation: Target crystals may be zoned with respect to parent isotopes, which impacts the accuracy of the FT correction (made assuming homogeneous distribution), He concentration gradient, and spatial distribution of radiation damage damage and thus He mobility.fluid and mineral inclusions: Contribute “excess” and “parentless” 4He, respectively.fractured and broken grains: Fractured grains create fast pathways for 4He diffusion. Broken grains modify the 4He concentration profile and impact FT correction.

Grain size and zonation effects are manifest in slow-cooling thermal histories. These factors may be overcome or exploited with careful grain selection and/or LA-ICPMS [U] and [Th] data.The primary control on He diffusion, TC, and (U-Th)/He dates in the apatite, zircon, and titanite systems is radiation damage accumulation from actinide decay. This effect is magnified in slow cooling scenarios and results in distinct correlations between individual date and effective U concentration (eU = [U] + 0.235*[Th]). Date-eU correlations can be exploited to refine the thermal history and damage-diffusivity models should be used when simulating data (Flowers et al., 2009; Gautheron et al., 2009; Guenthner et al., 2013; Willet et al., 2017).

Thermal histories (rates) derived from (U-Th)/He dates use damage-diffusivity models and available geologic and geo- and thermochronologic constraints. Computer software (e.g., HeFTy, Ketcham, 2005; QTQt, Gallagher, 2012) incorporate the latest damage-diffusivity models. These histories are converted into burial and unroofing histories assuming appropriate geothermal gradients and surface temperatures. Succesful modeling requires transparency in approach and inputs (e.g., Flowers et al., 2015).