UTChron Picking and Grain Measurement Facilities
Mineral grains are handpicked and screened for inclusions using 4 customized Nikon SMZ-U/100 stereomicroscopes with a rotating stages. The stereomicroscopes have both transmitted (polarized) and reflected light capabilities. Prior to loading samples into Pt sleeves, all grains are digitally photographed using a Nikon cameras and all digital pictures are archived. Custom-made LabVIEW code is used for imaging and morphometrically characterizing each grain before loading into Pt tubes. LabView code is also used to calculate the alpha-ejection correction and upload these data and metadata to a customized database. The (U-Th)/He picking and grain measurement facilities are also used for U-Pb (LA-ICP-MS and TIMS) and other geochronologic sample preparation.
Automated QMS He Extraction Line for (U-Th)/He Dating
The UTChron (U-Th)/He laboratory houses three state-of-the-art, all metal, ultra-high vacuum noble gas extraction and purification lines for measuring 4He. The quadrupole He mass spectrometry systems consist of the following principle components: 1) One Photonmachine 30W Diode Laser and two Photonmachine 75W Diode lasers and a U.S. Laser continuous-mode Nd-YAG lasers for total fusion He laser extraction, ideal for single-crystal work (see House et al., 2000), 2) two separate all-metal extraction lines equipped with computer-controlled pneumatic Nupro valves and pumped by a combination of ion, turbo and rough pumps, 3) precise volume aliquot systems for spiking sample gas with 3He for isotope dilution, 4) precise volume aliquot systems for delivering a 4He standard with separate depletion tank systems to monitor 4He tank depletion, 5) gas purification system consisting of two SAES NP10 getters and two Janis cryogenic trap capable of separating He from other gases by variable temperature release at 16-37K, and 6) two Blazers Prisma QMS-200 quadrupole mass spectrometer for measuring 3He/4He ratios. Our helium isotope dilution procedure allows very low-blank (< 1 femtomole) and high-precision (<1-2%) measurements of 4He in dated crystals while allowing for recovery of the crystals for U and Th measurement in the same aliquot (see sections 5 and 6).
The UT extraction line components and valves are fully interfaced with a central computer and are fully automated using LabView software. The 3He spiking, cryogenic trap temperature cycling, and mass spectrometric analysis of samples, gas standards, and blanks are fully computer controlled. On extraction line #1 and #2, heat and gas extraction is accomplished through PhontonMachine diode laser heating systems controlled by an in-line two-color pyrometer. The 44 position planchets are fixed while the x-y-z position of the laser beam deliver, laser heating temperature and duration are computer controlled.
Helix SFT Sector Noble Gass Mass-Spec Line for 4He/3He Analysis
The UTChron (U-Th)/He laboratory also houses a Thermo Helix SFT magnetic sector mass-spectrometer with dedicated UHV extraction and purification line for 4He/3He and low 4He yield samples. For He extraction the stainless-steel line is equipped with (1) a diode laser for laser total gas and step-heating gas extraction, (2) light-bulb furnace for low-temperature step heating, and (3) a PhotonMachine Analyte G.2 Excimer Laser Ablation system for in-situ laser-ablation He dating. The Analyte G.2 is shared between the QMS and SFT Sector MS. This state-of-the-art instrumentation is being used for zircon and apatite 4He/3He thermochronometry (Brennan et al., 2020), analysis of very young low He-yield volcanic samples, low radiogenic samples (e.g., young magnetite), He laser ablation measurements, and 3He cosmogenic nuclide measurements (Glown et al., 2019)
Automated He Extraction Line for Diffusion Experiments
The UT (U-Th)/He laboratory also houses a third state-of-the-art, all metal, ultra-high vacuum noble gas extraction and purification line for measuring 4He dedicated to He diffusion experimental work and step-heating He extraction, but also has the potential for in-vacuum dissolution (acid digestion) degassing. The second quadrupole He mass spectrometry system consists of the following principle components: 1) four automated He diffusion experiment apparatuses (see details below), 2) an all-metal extraction line equipped with computer-controlled pneumatic Nupro valves and pumped by a combination of ion, turbo and rough pumps, 3) a precise volume aliquot system for spiking sample gas with 3He for isotope dilution, 4) a precise volume aliquot system for delivering a 4He standard with a separate system to monitor 4He tank depletion, 5) a gas purification system consisting of two SAES NP10 getters and a Janis cryogenic trap capable of separating He from other gases by variable temperature release at 16-37K, and 6) a Blazers Prisma QMS-200 quadrupole mass spectrometer for measuring 3He/4He ratios. Our helium isotope dilution procedure allows very low-blank (< 1 femtomole) and high-precision (<1-2%) measurements of 4He in analyzed crystals.
He Diffusion Experimental Line
As it is essential to accurately quantify the helium diffusion characteristics of mineral phases in order to use them for thermochronological purposes. Diffusion properties such as activation energy and diffusivities (Do/a2) can be constrained by measuring fractional helium release in controlled step-heating experiments, which have traditionally been carried out using resistance furnaces. The UT (U-Th)/He laboratory is equipped with four specially designed diffusion cells to perform detailed in-vacuo step-heating experiments (Farley et al., 1999). Inside the diffusion cell the Cu-foil wrapped sample is suspended by a K or J-type thermocouple and is heated by a 120V halogen bulb projected through a sapphire window. Precise temperature control is accomplished through a feedback mechanism involving a Watlow thermal controller and a phase-angle-fired Eurotherm power supply. Estimated temperature stability during each step is better than ±1°C. The Watlow thermal controllers are interfaced with the lab computer, allowing the helium diffusion experiments to be executed in a fully automated mode using LabView software.
Most of our experimental efforts to date have concentrated on investigating the diffusion characteristics of a variety of new mineral phases, such as monazite, rutile, magnetite, fluorite, perovskite, or garnet. Extensive work on monazite has demonstrated a closure temperature of between 190 and 250°C (assuming a cooling rate of 10°C/m.y.) that is significantly dependent on composition and slightly dependent on grain size. The Arrhenius plot in Figure 3b shows an example of a monazite diffusion experiment using multiple heating cycles between 400 and 650°C.
Although the (U-Th)/He dating technique is calibrated against first principles and does not require standardization, it is invaluable to have mineral standards to monitor procedural performance and to use as benchmarks by which to judge the quality of results and for inter-laboratory comparison.
The UT (U-Th)/He laboratory regularly analyzes a variety of recognized, inter-laboratory, and intra-laboratory standards, such as Durango apatite (~31.5 Ma; Young et al., 1969; McDowell and Keizer, 1977, Farley, 2000), Fish Canyon Tuff apatite and titanite (~27.9 Ma; see summary in Villeneuve et al., 2000), and 97MR22, a well-characterized plutonic sample from British Columbia (4.5 Ma; Farley et al., 2001). Our measured ages for these age standards are in excellent agreement with published values, giving us confidence in the quality of our data and the performance of our multi-step laboratory procedures.
Over the years the active (U-Th)/He laboratories have used a series of quickly-cooled standards that fall into three categories, (1) regular age standards dated by other analytical techniques, such as 40Ar/39Ar dating (e.g., Fish Canyon Tuff), (2) semi-formal standards used by the leading (U-Th)/He laboratories as inter-laboratory standards (e.g., 97MR22 distributed by Caltech), and (3) intra-laboratory standards that have been independently dated and/or have proven to yield extremely well-behaved and reproducible age data. All three types of standards are in use at the UT (U-Th)/He laboratory.
Co-analyzed standard data will be included in both the analytical report as well as the electronic database (see below). The summary table below documents all commonly used age standards dated in our laboratory over the past three years, listing mean (U-Th)/He age (2-s), number of analyses (n), the relative standard deviation of the population, and the accepted 40Ar/39Ar age. For monazite, rutile, and magnetite only informally developed by Stockli’s group/laboratory, but independently dated intra-laboratory standards exist.