description of the noble-gas lab
40Ar/39Ar Dating
Facilities for 40Ar/39Ar dating at Lehigh include automated furnace and laser extraction systems connected on-line to a VG 3600 noble-gas mass spectrometer outfitted with faraday and electron-multiplier detection systems. The furnace line is built around a Modifications Ltd. double-vacuum resistance furnace (a modified Staudacher design) pumped by a 60 l/s Balzers turbomolecular pump; this pump also provides intermediate roughing during bakeout of the extraction line. The extraction line itself is pumped by a 30 l/s Varian StarCell ion pump, and sample cleanup is provided by two SAES internally heated getter pumps and optionally by a cold finger cooled to LN2 temperature. All valves essential for gas transfer and pumpout have air-operated drivers and can be operated either manually or under computer control. Furnace temperature is monitored by a W-Re thermocouple connected to a closed-loop SCR power controller. Temperature precision is on the order of ± 1 C, and independent calibrations using a secondary thermocouple at the sample position have demonstrated that temperature gradients across the crucible base are generally less than 2-3 C for empty or nearly empty crucibles. Blanks of <1.5x10-15 moles of 40Ar at 1200 C are routinely achieved in the furnace line after several days of pumping time.
The laser extraction line is equipped with a Merchantek dual UV/CO2 laser system. The CO2 laser produces a pulsed or continuous 10.6 µm beam with output power continuously variable up to 35 W. The beam can be focused to a spot size of ~80 µm and couples well with silicate minerals routinely used for 40Ar/39Ar dating, allowing total fusion and step-wise outgassing of single crystals and multiple grain aggregates. The UV laser generates a pulsed 266 nm beam with up to 10 mJ output power at a repetition rate of 20 Hz. Spot sizes of <10 µm and minimal heating effects outside the target area allow the UV laser to be used for in situ dating of grains in small slabs and thin sections and for intragrain age mapping studies. Up to 200 single-grain samples can be loaded into the sample chamber, which is maneuvered under the beam by a Newport 3-axis servomotor micropositioning system operable under either computer or manual control. Gas cleanup in the laser line is accomplished by SAES getters and for larger of hydrous samples, a cold finger held at LN2 temperature to trap H2O and other condensable gases. Blanks in the laser line are typically <4x10-16 moles of 40Ar several days after reloading and pump-down of the sample chamber.
The VG 3600 mass spectrometer is routinely operated at 4.5 kV accelerating potential and 400 mA trap current. Under these conditions the instrument has a background for 36Ar of 4x10-14 cc STP, and on-line to the furnace extraction system has an effective sensitivity of 1.2x10-15 moles/mV using the faraday detector. Argon analyses are currently performed almost exclusively using the electron multiplier detection system, with the multiplier gain set to achieve an effective sensitivity of ~6x10-17 moles/mV for the furnace line and ~1.5x10-17 moles/mV for the laser line. Mass discrimination is determined by analyzing purified atmospheric argon admitted to the furnace extraction line from a pipette system. Regular monitoring of the discrimination using the electron multiplier typically yields 40Ar/36Ar values for atmospheric Ar of ~288 ± 0.5%, with less than 1% long-term variation. The mass spectrometer and extraction lines are integrated into an automated system under the control of a Macintosh computer running a custom LabVIEW program developed at Lehigh by Bruce Idleman. Both laser and furnace extractions can be performed under computer control at the rate of two to three analyses per hour with no user intervention other than to load new samples.
Irradiations are carried out in the McMaster reactor. We currently use ANU biotite GA1550 as our primary flux monitor, with Fish Canyon sanidine and MMHb-1 hornblende used as secondary standards. Interferences from Ca and K are monitored by analyzing irradiated optical grade CaF2 and vacuum fused K2SO4 included in every irradiation package.
U-Th/He Dating
The Lehigh U-Th/He lab measures 4He and 3He using a Balzers bakeable quadrupole mass spectrometer designed for UHV operation. The instrument is fitted with both Faraday and electron multiplier detectors, providing it with a wide dynamic range. Under typical operating conditions using the multiplier the system has an effective sensitivity of about 1000 amps/mole. For He extractions, we use a standard double-vacuum resistance furnace. An all-metal sample dropping mechanism designed around a linear-motion feedthrough permits multiple samples to be loaded for sequential analysis in the resistance furnace. Helium evolved from heated samples is purified in an all-metal extraction line pumped by an ion pump during routine operation as well as a 70 l/s turbomolecular pump and a small rotary backing pump during bakeout. Gettering of active gasses is handled by a SAES getters in the extraction line; a smaller SAES getter in the mass spectrometer volume is used to minimize hydrogen peaks. Reservoirs containing a 3He spike and a 4He/3He standard are attached to the line behind all-metal pipettes. Two temperature-stabilized capacitance manometers provide the precise, accurate pressure measurements needed during spike preparation. The extraction line has negligible helium blank of 1 x 10-16 moles or less 4He.
We measure 4He by isotope dilution, using a 3He spike. This approach has several advantages. First, it eliminates concern about the linearity of response to He partial pressure that can complicate manometric measurements made by peak-height comparison. Second, it eliminates the problem of helium ionization suppression caused by high concentrations of neon in the sample gas. Finally, abundance measurements based on the ratio of two isotopes in a spiked sample are less prone to small variations in the mass calibration of the mass spectrometer than those made by single peak-height comparison. This issue is a particular concern with quadrupoles, which tend to have non-ideal (i.e., pointed and asymmetric) peak shapes. In addition to the 3He spike used for routine isotope-dilution measurements, we have prepared a mixed 4He/3He standard to permit us to determine and monitor mass discrimination. This also provides a backup means of sample measurement, should the spike analysis be compromised. We have found that long term, spike and manometric determinations agree to within 1%. Over periods spanning the analysis of sample batches, we find the 4He/3He ratio for the standard to be precise to within 0.3%, with a value of about 0.800 (the true 4He/3He ratio of our standard is 1.000). For our current spike preparation, the size of a typical spike is 2.4 x 10-13 moles.
We are currently developing facilities to measure U and Th concentrations by isotope-dilution ICP-MS. In the meantime, we are obtaining measurements courtesy of Dr. Peter Reiners at Yale University, who dissolves, spikes, and analyses samples that we send following helium extraction. U and Th are measured on the same aliquots of apatite and zircon that have been used for He determinations, eliminating any uncertainties contributed by weighing errors and sample heterogeneity. We run all samples in duplicate to assess effective precision and effects such as U and Th zoning. Usually we run single grains except for very young apatite samples. For both apatites and zircons, we pick unbroken, symmetric grains without visible inclusions under a high-power dissecting microscope. Samples are digitally photographed in order to record their sizes and shapes for determination of alpha-correction factors. Apatites are placed in small Pt tubes whose ends are crimped. Zircons are placed in very small handmade Nb-foil packets. In order to identify samples, apatite packets are placed in re-usable Pt-foil jackets and zircon packets are placed in Nb-foil jackets.
Samples are weighed and then loaded into the sample-dropping mechanism, which can hold 16 packets. After a brief overnight bake at 70 to 100 C and high-temperature outgassing of the empty crucible to lower hydrogen pressures, samples are outgassed sequentially. In our experience, heating to 950 C for 10 minutes is sufficient to outgas apatites, but heating to at least 1350 C for 60 minutes is required to consistently and completely outgas zircons to blank levels. Following heating, sample gas is allowed to circulate over the main getter while the crucible cools and the spike is added, and then the spiked gas is admitted to the mass spectrometer by expansion. We measure 4He and 3He and monitor H2; measured peaks are extrapolated to time zero using robust regression techniques, although in our experience there is negligible beam pumping or memory in the quadrupole. Once all samples have been run, the system is vented, the samples are removed from the crucible, and they are weighed to identify them. The unwrapped sample packets are then sent for U and Th analysis, where the apatites are dissolved in nitric acid (leaving the Pt packet untouched) and the zircons plus Nb packet are digested in hydrofluoric acid.
Supporting Facilities
Sample-preparation facilities at Lehigh include a jaw crusher, disk pulverizer, Wilfley table, Franz magnetic separator, and a facilities for heavy-liquid separations using sodium polytungstate and methylene iodide (when necessary). A full suite of software tools is available in the geochronology lab to handle data reduction and analysis on the full range of materials analyzed in 40Ar/39Ar dating. In particular, in the area of K-feldspar thermochronology, we have developed a numerical inversion that can be used to extract thermal histories from age spectra using the constrained random search method. For typical samples, this software currently runs in well less than an hour even on computers as slow as ~400 Mhz.
Additional supporting equipment important for geochronology and thermochronology is available in the Department of Earth and Environmental Sciences at Lehigh, including facilities for rock crushing, sawing, sieving, and thin section preparation. Basic facilities are available for fission-track dating, including an Olympus research-grade petrographic microscope, photomicrography system, and a digitizing pad with drawing tube for track-length measurements. Excellent analytical equipment is available for sample characterization, including an automated Phillips x-ray diffractometer, an ARL ICP spectrometer for the analysis of major and trace elements, and a stable-isotope laboratory housing a Finnigan MAT 252 isotope ratio mass spectrometer and extraction lines for the analysis of C, O, H, and N in carbonates and O in silicates. A world-class laboratory for electron microscopy is maintained by the Department of Materials Science and Engineering, providing facilities for SEM, TEM, STEM, and electron microprobe analysis, and the Chemistry Department houses an ESCA facility.
