speed dating! (U-Th)/He thermochronometry introduction and applications 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: (1) 4He = 8238U[e-λ238t-1] + 7235U[e-λ235t-1] + 6232Th[e-λ232t-1] + 147Sm[e-λ147t-1] 4 238 235 232 147 where He, U, U, Th, and Sm 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. He diffusion and closure temperature 0 hematite apatite Minerals are suitable for (U-Th)/He thermochronometry provided they yield goethite measurable U and Th (ppm), contain negligble 100 initial 4He, and retain 4He over geologic timescales. titanite calcite Loss of 4He from the crystal lattice typically occurs C) ° monazite by time and temperature-dependent diffusion. He 200 diffusion follows an Arrhenius relationship: rutile zircon (2) D/a2 = D /a2 e-Ea/RT garnet o where D is diffusivity at infinite temperature, E is the activiation energy, R is the 300 o a magnetite gas constant, and T is temperature. Temperature ( Temperature perovskite For most minerals (e.g., apatite, zircon, titanite), the crystal is the diffusion domain. Polycrystalline 400 aggregates such as hematite and goethite exhibit poly-domain He diffusion behavior where the individual baddeleyite 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 500 (U-Th)/He system in different minerals. 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. sample collection and aliquot selection 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. Alexis K. Ault, Utah State University, Logan, UT, [email protected] William R. Guenthner, University of Illinois at Urbana-Champaign, Urbana, IL, [email protected] Robert G. McDermott, Utah State University, Logan, UT, [email protected] complexities and opportunities (U-Th)/He laboratories (U-Th)/He dates typically have 2-8% analytical uncertainty (on Arizona State University, Noble Gas U-Th-Sm-He measurements) and mean dates yield 8-15% Geochronology and Geochemistry Laboratories (NG3L) standard deviation. Some sources of intrasample date dispersion Kip Hodges, [email protected] include: Caltech, Noble Gas Lab grain size variation: Larger crystals have larger diffusion domain Kenneth Farley, [email protected] lengthscales and higher T . Lehigh University, Noble-Gas Lab C Peter Zeitler, [email protected] U and Th zonation: Target crystals may be zoned with respect to parent isotopes, which impacts the accuracy of the F correction Stanford University, Noble Gas Laboratory T Marty Grove, [email protected] (made assuming homogeneous distribution), He concentration gradient, and spatial distribution of radiation damage damage University of Arizona, Arizona Radiogenic Helium Dating Lab and thus He mobility. Peter Reiners, [email protected] , Noble Gas fluid and mineral inclusions: Contribute “excess” and University of California Berkeley 4 Thermochronometry Lab “parentless” He, respectively. David Shuster, [email protected] fractured and broken grains: Fractured grains create fast University of Colorado Boulder, CU Thermochronology pathways for 4He diffusion. Broken grains modify the 4He Research and Instrumentation Laboratory (TRaIL) concentration profile and impact FT correction. Rebecca Flowers, [email protected] Grain size and zonation effects are manifest in slow-cooling University of Connecticut, Basin Analysis & thermal histories. These factors may be overcome or exploited Thermochronology Lab with careful grain selection and/or LA-ICPMS [U] and [Th] data. Julie Fosdick, [email protected] The primary control on He diffusion, T , and (U-Th)/He dates in University of Illinois at Urbana-Champaign, Helium Analysis C Laboratory (HAL) the apatite, zircon, and titanite systems is radiation damage William Guenthner, [email protected] accumulation from actinide decay. This effect is magnified in slow University of Michigan, Thermochronology Lab cooling scenarios and results in distinct correlations between Marin Clark, [email protected] and Nathan Niemi, individual date and effective U concentration (eU = [U] + [email protected] 0.235*[Th]). Date-eU correlations can be exploited to refine the University of California at Santa Cruz, thermal history and damage-diffusivity models should be used Jeremy Hourigan, [email protected] when simulating data (Flowers et al., 2009; Gautheron et al., 2009; Guenthner et al., 2013; Willet et al., 2017). University of Texas Austin, UT Chron Lab Daniel Stockli, [email protected] USGS, Argon and Helium Geochronology Lab Michael Cosca, [email protected] and Joseph Colgan, date and rates [email protected] Thermal histories (rates) derived from (U-Th)/He dates use Virginia Tech, Radiogenic Helium Laboratory damage-diffusivity models and available geologic and geo- and James Spotila, [email protected] thermochronologic constraints. Computer software (e.g., HeFTy, Ketcham, 2005; QTQt, Gallagher, 2012) incorporate the latest 4He/3He thermochronometry quantifies the spatial damage-diffusivity models. These histories are converted into 4 burial and unroofing histories assuming appropriate geothermal distribution of He within a crystal that is a function of the thermal history. Involves stepwise degassing of 4He and a gradients and surface temperatures. Succesful modeling requires 3 transparency in approach and inputs (e.g., Flowers et al., 2015). uniform He distribution (irradiation, proton bombardment). workflow and analytical procedures 1. grain selection and measurement: Phases examined and measured 3. Dissolution and spiking: Degassed aliquots in Nb 219 μm using a stereoscope. Hematite also packets retrived and dissolved. Different phases 151 μm imaged and grain size distribution require different dissolution regimens (e.g., apatite measured via SEM. Aliquots a1 a2 a3 a4 a5 a7 dissolved in HNO3 at 90 °C for 1 hr or zircon HF a6 a3 inserted into Nb tubes that are dissolution via Parr bomb). Addition of and loaded into a planchette. equilibration with spike (e.g., 233U-229Th-147Nd-42Ca 2. He outgassing and purification: 4He extracted via in-vaccum via heating for apatite and 233U-229Th-90Zr for zircon; Guenthner with Nd:YAG, CO2, or diode laser. et al., 2016). Heating schedules vary with target 4. U, Th (Sm) measurement: Analyses via ICP-MS mineral. For example, apatite heated to or QMS. ~950-1050 °C for 3 min. (no re-extract); 5. Concentration and date calculation: U, Th, He, zircon heated to ~1250 °C for 15 min. (Sm) concentrations from dimensional mass (grain followed by 1-2 re-extracts to purge measurements or from Ca, Zr measurements and grains of 4He. Hematite degassing T stoichiometry (Guenthner et al., 2016). Correct for and duration depend on grain size. long alpha particle stopping distances with an FT Extracted 4He gas spiked with 3He, correction (e.g., Farley et al., 1996) and calculate purified using cryogenic and gettering date via Eq. [1]. methods, and analyzed on a 6. Standards: In-run standards quadrupole mass spectrometer (QMS). such as Durango and Fish Analysis of known quantity of 4He Canyon Tuff are used to He extraction line and throughout run to monitor instrument monitor dissolution, chemistry, QMS at the HAL (UIUC) sensitivity drift. and isotope measurements..