Our laboratory is equiped with two noble gas mass spectrometers for carrying out 40Ar/39Ar age determinations. We have an Mass Analyser Products MAP 215-50 single-collector mass spectrometer and also a brand-new Thermo Scientific ARGUS VI multi-collector (installed March 2012) with five fixed Faraday detectors and one ion-counting CuBe electron multiplier.
The first instrument is a Mass Analyser Products (MAP) Model 215-50 mass spectrometer that is a single-collector 90° sector instrument with a Nier-type source. On the collector end it has a Johnston MM1-1SG electron multiplier and an electrostatic analyzer with adjustable collector slit for high resolution of masses in the Argon isotope range. This system is equipped with a Heine low-blank, double-vacuum resistance furnace and a first-generation Merchantek MIR10-TP integrated CO2 laser gas extraction system with an infrared pyrometer.
The MAP 215-50 is connected to an all-metal extraction system for 40Ar/39Ar age determinations. This is an ultra-clean, low volume (~1000 cc) gas cleanup line that uses one SAES ST-101 Zr-Al getter (450 °C) and two SAES ST-172 Zr-V-Fe getters (21 °C and 250 °C) to effectively remove reactive and interfering gasses in a two-stage clean up.
For gas extractions using the resistance furnace, irradiated samples, either whole rock mini-cores or Cu-wrapped mineral separates, are loaded into a sample manifold that feeds into a Ta/Nb-crucible with a Mo-liner. Temperatures are precisely controlled at the bottom of the crucible with a programmable power supply thermocouple system. For experiments using the laser system, irradiated samples, either crushed whole-rock, groundmass, mineral separates, single-grain or in-situ mineral veins are loaded into a Cu-planchette designed with a variety of pits/pans that hold <1 to 50 mg of material, which is then pumped to an ultra-high vacuum within a sample chamber fitted with a ZnS window that is transparent to the CO2 laser wavelength. Temperatures are monitored with an infrared pyrometer and controlled by varying the laser power percentage through a computer.
Ion beam currents are measured with the electron multiplier at m/e = 36, 37, 38, 39 and 40 and intervening (half-mass offset) baselines. Measurement times, peak/baseline voltages, data acquisition and storage are computer controlled using an 8.5 digit integrating multimeter. Peak heights typically decay during an analysis and the regressed peak heights vs. time generally follow first-order exponential fits. Mass discrimination is monitored using an air standard that can be aliquoted using an 0.1 cc air-pipette system (at ~3.5×10-12 moles of 40Ar per volume). All resulting ages are calculated using the ArArCALC v2.4 software package (Koppers, 2002).
The second instrument is an Thermo Scientific Model ARGUS VI multi-collector mass spectrometer with five fixed Faraday detectors (including amplifier circuits with 1012 Ohm resistors) and one ion-counting CuBe electron multiplier mounted next to the low mass 36 Faraday detector. This system is equipped with an 25 W Synrad CO2 laser with industrial scan head for carrying out gas extractions. The ARGUS VI can be operated in three different modes:
- Multi-collector Mode to simultaneously collect all masses m/e = 36, 37, 38, 39 and 40 on the 1012 Ohm Faraday collector array for samples providing sufficient amounts of gas for analyses
- Peak-switching Ion-counting Mode using the CuBe electron multiplier for high-precision analyses on (very) small gas fractions
- Combination Mode whereby all masses will be measured simultaneously in a multi-collector mode, but with mass m/e = 36 aimed on the CuBe electron multiplier and masses m/e = 37, 38, 39 and 40 on the adjacent Faraday cups
The latter configuration provides the advantages of running in a full multi-collector mode while measuring the lowest peak (on mass 36) on the highly sensitive electron multiplier. Even though the ARGUS VI has a fixed-position collector array, an electronic steering plate placed before every collector allows for nudging over the beams to fall exactly in the middle of all five collectors. In addition, in both multi-collector Faraday modes we would have the option to use 1011 Ohm resistors with a very low baseline noise of ~2×10-17 A or higher 1013 Ohm resistors that are more sensitive, when these come on the market.
The ARGUS VI is connected to an all-metal extraction system for 40Ar/39Ar age determinations. It has an overall lower volume (~500 cc) than our MAP 215-50 system but we use a similar set of getters for cleaning up the reactive gasses with one SAES ST-101 Zr-Al getter (450 °C) and two SAES ST-172 Zr-V-Fe getters (21 °C and 250 °C). Beside its lower volume, the design of the CO2 laser system is also different by using an industrial Synrad XY scan head for steering the laser beam during sample heating. This allows us to carry out the sample heating by setting up a beam raster pattern while keeping the sample housing stationary. Using this novel technique we can produce a laser beam that can move continuously up and down at speeds up to 300 in/s and that results in an even heating of the entire sample being analyzed, a prerequisite for carrying out first-rate incremental heating experiments. This laser system will be equipped with an air-cooled Synrad 25W CO2 laser that has a maximum laser power sufficient for the fusion of single crystals and flux monitors.
The custom-made extraction line will feature two laser chambers, an open port for later attachment of a low-volume resistance furnace with an automated sample carousel, three SAES getters, and two air pipette systems (one for air and one for an 38Ar spiked air standard). All resulting ages are calculated using the ArArCALC v2.5.2 software package (Koppers, 2002).