Helium extraction line for U-Th/He thermochronology

Helium Extraction Line Description

Quadrupole and He extraction line Helium determinations for (U-Th)/He analyses are made using a dedicated quadrupole mass spectrometer and gas handling system. The instrumentation and techniques we have developed for He measurements are based in most respects on those employed and successfully demonstrated in Ken Farley's laboratory at Caltech. We contracted Dr. Des Patterson to design and build a He extraction line to our specifications for outgassing accessory phases in conjunction with (U-Th)/He dating. We have a bakeable Balzers quadrupole mass spectrometer designed for UHV operation to determine 4He abundances. The instrument is fitted with electron multiplier detectors, providing it with a wide dynamic range. Similar to the Micromass 5400 extraction line, the He extraction line utilizes a custom double vacuum resistance-heated furnace. Furnace temperatures of >1500¡C are attainable with this design, sufficient for outgassing accessory phases. Typical furnace He blanks are <0.002 ncc. An all metal sample holder with a linear motion feedthrough allows multiple samples to be loaded for sequential analysis in the furnace. Helium outgassed from samples is purified in the all-metal extraction line previously pumped by a turbomolecular pump and a rotary backing pump. Gettering of active gasses is handled by small, standard SAES getters. Reservoirs containing a 3He spike and a 4He standard are attached to the line behind all metal pipettes. A temperature stabilized capacitance manometer was used to make accurate pressure measurements during the preparation of the spikes and the 4He pipette was cross-calibrated on Prof. Ken Farley's line at Caltech. The 4He pipette delivers 9.84 +0.08 ncc STP 4He per aliquot. A Janis cryogenic cold trap system is interchangeable between the He extraction line and the 5400 extraction line when required. Measurement of 4He is done by isotope dilution, using a 3He spike. 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, requiring considerable care when making high-precision isotope abundance measurements. We use Durango apatite as a mineral standard. Measurement of U and Th concentrations are currently being performed by solution ICPMS at CSIRO, Sydney, Australia. A 25 watt CO2 CW laser system is shared between the Micromass 5400 extraction line and the helium extraction line. The laser can be used for outgassing of single grains encapsulated in platinum tubules.

Custom extraction line dedicated to (U-Th)/He studies.   Return to the SU Thermochronology homepage Return to the SUNGIRL homepage