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Tectonics and Geochronology

(U-Th)/He Laboratory


Mineral Separation &bull Sample Preparation &bull Laser He Extraction LineHe Diffusion LineClean Lab
ICP-MS FacilityLaboratory Procedures(U-Th)/He Age StandardsKU Laboratory Productivity
Recent Laboratory Visitors Prospective Graduate Students
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. For inquiries for collaborative research please contact Danny Stockli.

IGL Mineral Separation Facilities

The 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, 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.

(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. 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.

Automated (U-Th)/He Laser Extraction Lines

The KU (U-Th)/He laboratory houses two 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) Photonmachine 30AW Diode laser and 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 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. On extraction line #1, 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 44 samples. On laser extraction line #2, diode laser heating is controlled by an in-line two-color pyrometer. The 44 position planchet is fixed while the x-y-z position of the laser beam deliver is computer controlled.

The KU (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 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.


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.

(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 microliters/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.

KU (U-Th)/He Laboratory Procedures

This paragraph briefly summarizes standard laboratory procedures. Sample selection rigidly adheres to the following criteria to ensure top data quality - only euhedral grains, free of inclusions, and >70 µm in width (minimizing a-ejection correction). In our experience, the biggest hurdles to accurate and reproducible (U-Th)/He age determinations is the analysis of grains that are absolutely free of U- and Th-bearing inclusions. After careful microscopic grain selection/inspection (at 180x magnification), individual aliquots (3/sample) are photographed and measured for standard morphometric a-ejection age corrections. Subsequently, aliquots are wrapped in Pt foil tubes, laser heated for 5 minutes at 1070°C and analyzed for He, and reheated to ensure complete degassing. Zircon and all other silicate, oxide, and phosphate mineral phases are heated for 10 minutes at 1300°C and subsequently reheated until completely degassed (<1% He re-extract). After laser degassing, radiogenic He is analyzed in an all-metal, fully computer-automated UHV He extraction line that is equipped with precise volume aliquot systems for 3He isotope dilution and delivering of 4He standard gas, a cryogenic gas purification system capable of separating He from other gases by variable temperature release at 16-37K, and a Blazers Prisma QMS-200 quadrupole mass spectrometer for measuring 3He/4He ratios. Our helium isotope dilution procedure allows very low-blank and high-precision He measurements (~0.3-0.5%). The primary KU laser He extraction line is capable of analyzing up to 36 samples in a fully automated fashion. He gas standards and He extraction line blanks are routinely analyzed as part of unknown age analyses.
After complete degassing, aliquots are retrieved and dissolved for U, Th, and Sm determination. Dissolution procedures vary greatly and can be very time consuming (e.g., zircon or rutile). In the case of apatite, samples are spiked (230Th, 235U, 149Sm) and dissolved in 30% HNO3 (90°C for 1 hour). Zircon and rutile samples are unwrapped from Pt foil (PtAr inference with U) and dissolved using standard U-Pb double pressure-vessel digestion procedures (HF- HNO3 and HCl) for a total of 4 days. Titanite, monazite, and magnetite are dissolved in a HCl-HF mixture (150°C for 24 h). Rutile and magnetite undergo ion-exchange column chemistry procedures to eliminate interfering reactions with Ti and Fe. After dissolution, spiked aliquot solutions are analyzed for U, Th, and Sm using a VG PlasmaQuad IIXS Inductively Coupled Plasma Mass Spectrometer (ICP-MS), fitted with a micro-concentric nebulizer and an auto-sampling device. Samples with very low U and Th content are measured by ID-TIMS at KU.
Data reduction has been streamlined in light of the tremendous sample throughput (>2500 analyses/year) with all data (morphometric analyses to He and U, Th, and Sm analyses) compiled and reduced using in-house designed user-friendly visual-basic ExcelTM add-in macro software packages.

All analytical uncertainties are captured and properly propagated during multi-step and multi-instrument (U-Th-[Sm])/He analysis. Analytical uncertainties include easily quantifiable variables such as He measurement error (0.3-0.5%) and U, Th, and Sm measurement error (<1-2%). More difficult to assess are uncertainties related to a-ejection corrections due to inherit assumptions about U and Th distribution. As a result propagated analytical uncertainties (~3-4%, 2-s) generally underestimate aliquot reproducibility. Therefore, a commonly employed approach in (U-Th)/He dating is the assignment of a percentage error to an aliquot analysis based on the standard deviation of a population derived from standard samples; for example 6% for apatite and 8% for zircon. For the proposed work both propagated errors and reproducibility-based standard deviation errors will be reported.

KU (U-Th)/He Age Standard Results

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 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.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 KU (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 informal KU-developed, but independently dated intra-laboratory standards exist.

Summary Table of (U-Th)/He age standards most commonly analyzed at the KU (U-Th)/He laboratory to monitor procedural integrity and age reliability (see text). n denotes the number of aliquot analyses carried out at KU (U-Th)/He laboratory during the past 3 years.
Summary of KU (U-Th)/He age standards

Standard (Mineral) Rock type (U-Th)/He age [Ma] n RSD% 40Ar/39Ar age [Ma]
Durango (apatite) Rhyolite 31.9 ± 0.5 161 3.5 31.5
97MR22 (apatite) Tonalite 4.4 ± 0.1 225 6.5 4.5*
Silver Peak Tuff (apatite) Rhyolite 6.4 ± 0.2 45 8.7 6.5
Proprietary
Fish Canyon Tuff (zircon) Rhyolite 27.8 ± 0.8 162 7.5 28.2
Silver Peak Tuff (zircon) Rhyolite 6.4 ± 0.3 39 8.7 6.5
Taylor Creek (zircon) Rhyolite 28.8 ± 0.8 26 8.8 28.3
Wall Mountain Tuff (zircon) Rhyolite 36.2 ± 1.1 38 7.8 36.8
Proprietary
Fish Canyon Tuff (titanite) Rhyolite 28.7 ± 1.2 53 2.2 28.2
Silver Peak Tuff (titanite) Rhyolite 6.5 ± 0.4 29 4.3 6.5
Proprietary
Macusani (monazite) Rhyolite 24.2 ± 0.4 11 5.2 24.2
Proprietary
Sulliavan Buttes (rutile) Xenolith (latite) 23.1 ± 1.4 35 5.7 24.4
Sweet Grass (rutile) Xenolith (latite) 48.1 ± 3.9 13 8.7 50-54
Proprietary
*reference age is published (U-Th)/He age; sample used by several laboratories as
intra- and inter-laboratory age standard.

Duango Apatite Standard (2004)

KU (U-Th)/He aboratory Productivity


Recent Laboratory Visitors

The KU (U-Th)/He Laboratory host a significant number of faculty, graduate, and undergraduate visitors every year. Here is a list of recent visitors to the laboratory:

External Laboratory Visitors (2006-2009):

Amy Luther (NMT)
Manuel Sehrt (Uni Heidelberg) Vladimir Blanco (Ecopetrol)
Ivan Guerra (University of Lausanne
Dr. Eric Leonard (Colorado College)
Emily Parker (undergraduate, Colorado College)
Dr. Shari Kelley (New Mexico Tech)
Catherine Shirvell (MS student UCLA)(3 visits)
Yann Gavillot (MS student UCLA)(2 visits)
Dr. Benita Putlitz (University of Lausanne)
Alexandre Goy (MS student, University of Lausanne)
Jesse Moslof (MS student, UCLA)
Dr. Jeff Lee (Univ of Central Washington) Sergio Restrepo (Univ of Florida)
Jesse Davis (UNC)
Peter Druschke (UNLV)
Sharon Bywater (Univ of Wyoming)
John Trimble (Univ of Wyoming)
Ryan (Montana State)
Shari Kelley (NMT)
Holly Owens (MIT)

and a large number of collaborative analytical work....