University of Kansas - Department of Geology
Tectonics and Geochronology

The following sections describe in detail the infrastructure and instrumentation of the (U-Th)/He laboratory and associated facilities at the University of Kansas needed for (U-Th)/He geo- and thermochronology. The (U-Th)/He laboratory was fully financed through start-up funds to Stockli by the KU Geology Department and the College of Liberal Arts and Sciences and was constructed by Stockli.

1. IGL Mineral Separation Facilities
The shared KU Geology rock crushing and grinding facilities include a jaw crusher, disc grinder, ball mill, and various dry and wet sieving equipment. The IGL mineral separation laboratory is optimized for the separation of apatite, zircon, monazite, titanite etc. and includes sample washing and drying facilities, a new Pyramid table (similar to Wilfley table), sample drying ovens, two large fume hoods with heavy liquids apparatuses, two Frantz magnetic separators, centrifuges, ultrasonic cleaners, and binocular and petrographic microscopes for final sample inspection. The mineral separation facilities employ several undergraduate students.

2. (U-Th)/He Laboratory Picking and Grain Measurement Facilities
Mineral grains are handpicked and screened for inclusions using a Nikon SMZ-U stereomicroscope with a rotating stage (Fig. 1). The stereomicroscope has both transmitted (polarized) and reflected light capabilities. Prior to loading samples into Pt sleeves, all grains are digitally photographed using a Nikon digital ColorView® camera and all digital pictures are archived. AnalySIS® imaging software is used to morphometrically measure each grain before loading into Pt tubes. These morphometric values are subsequently imported into a LabView routine to calculate the alpha-ejection correction. The (U-Th)/He picking and grain measurement facilities were funded through departmental startup funds to Stockli.

3. Automated (U-Th)/He Laser Extraction and QMS Line
The KU (U-Th)/He laboratory houses a state-of-the-art, all metal, ultra-high vacuum noble gas extraction and purification line for measuring 4He (Fig. 2). The quadrupole He mass spectrometry system consists of the following principle components: 1) a U.S. Laser continuous-mode Nd-YAG laser for total fusion He laser extraction, ideal for single-crystal work (see House et al., 2000), 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 dated crystals while allowing for recovery of the crystals for U and Th measurement in the same aliquot (see sections 5 and 6).

Modeled after K. Farley’s system at Caltech, the KU 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. The laser heating of samples is controlled through a feedback loop using a video capture card allowing for continuous computerized adjustment of the laser output power for even heating. The laser sample planchet sits on a computer-controlled Newport X-Y stage and holds up to 36 samples.

4. Automated He Extraction and QMS Line for He Diffusion Experiments
The KU (U-Th)/He laboratory also houses a second 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.

5. He Diffusion Experiment Apparatus
As discussed above, 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 KU (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.

Most of our experimental efforts to date have concentrated on investigating the diffusion characteristics of monazite and rutile. 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.

5. Clean Lab for Dissolution and U, Th, Sm and REE Chemistry
Shared with the TIMS facility, we also have a three-unit clean-room suite. The clean lab is equipped with ceiling-mounted laminar-flow units for room air, laminar-flow workstations for column chemistry, and filtered-air evaporation boxes for drying samples. We use this facility extensively for sample dissolution following He degassing and sample spiking with various mixed 230Th-235U-149Sm-REE spike for ICP-MS based isotope dilution analysis of U, Th, Sm, and REEs. The clean lab also is where we mix and store all sample standard and spike solutions and acid wash all pipette tips and teflon beakers and vials. In addition, HF and microwave dissolution and LiB2 fluxing are carried out in an adjacent separate digestion facility. The laboratory is set up for HF-HNO3 and HCl pressure vessel digestion of silicate and oxide mineral samples. Any large mineral samples (>100 mg) or samples with high concentrations of Ti, Fe, and Ca are subjected to ion-exchange column chemistry to prevent sensitivity suppression and to ensure stable signals during ICP-MS or TIMS analysis.

6. (U-Th)/He laboratory ICP-MS Facility
Following He degassing, samples are analyzed for U, Th, Sm and selected REE at the (U-Th)/He laboratory using a dedicated Fisons/VG PlasmaQuad II Inductively Coupled Plasma Mass Spectrometer (ICP-MS). The ICP-MS is a very fast sequential mass analyzer that extracts positively charged ions from a plasma powered by a 27 MHz RF generator. It is equipped with a three-channel peristaltic pump (sample introduction, spray chamber drain, and autosampler-probe continuous wash), and a Gilson 222XL autosampler with full X-Y-Z capabilities. Stockli has custom-tailored the autosampler and fitted the instrument with a new CETAC micrconcentric nebulizer with 50?l/minute sample uptake to accommodate small-volume samples for U, Th, Sm, and REE analyses for (U-Th)/He dating. The precision and sensitivity of the instrument allow the performance of isotopic analyses to better than 1% RSD. Samples with low U, Th, and Sm concentrations (<0.1 ppm) are analyzed by TIMS (KU) or MC-ICP-MS (DTM, Washington D.C.) after undergoing appropriate column chemistry clean-up procedures. A nearly identical Fisons/VG PlasmaQuad II ICP-MS is located in the KU Plasma Analytical Laboratory and is equipped with a with a Merchantek LUV266X laser-ablation microprobe. The laser is quadrupled to 266 nm in order to operate in the ultraviolet. Computer control of the laser energy intensity and the size of the laser beam allow exquisite manipulation of the laser energy density on the sample surface. Typical diameters of pits drilled by the laser are 30 to 100 microns. The LA-ICP-MS capabilities will allow us to carry out chemical characterizations of problematic samples and depth-profiling studies to investigate U and Th zoning.

7. KU (U-Th)/He Age Standard Results
Although the (U-Th)/He technique is calibrated against first principles, we analyze well-characterized primary and secondary age standards to ensure data quality, machine performance and inter-laboratory consistency. The KU (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 (Fig. 4).