The noble gases (or "rare gases") helium, neon, argon, krypton, and xenon are chemically inert. Due to their volatile nature they have a strong tendency to partition into gas or fluid phases and can be used as tracers for the origin and the transport of fluids.
In rocks they are typically present in very low concentrations of ~10-9 to 10-6 cm3 STP/g (He, Ar; 1 cm3 STP is equivalent to 2.7x 1019 atoms) and ~10-13 to 10-10 cm3 STP/g (Ne, Kr, Xe). Therefore their concentrations and isotopic compositions may be modified to a measurable extent by nuclear processes such as radioactive decay or natural nuclear reactions. They can thus be used as dating tools (e.g. U/Th-4He, 40K-40Ar, surface exposure dating).
Over the history of the Earth, such processes have modified the noble gas isotopic compositions in distinct terrestrial reservoirs (mantle, crust, atmosphere). The isotopic signature of noble gases yields therefore important information about the origin and history of a rock or fluid sample.
Equipment & location: B264/B266
The noble gas laboratory includes the following facilities:
- Two noble gas mass spectrometers (MM 5400 and Helix SFT), both fitted with
- an ultrahigh vacuum furnace for heating and melting of rock samples
- a gas preparation line for removal of active gases
- a cryogenic adsorber for the separation of noble gases from each other
- pipette systems for calibration using noble gas standards
And in addition:
- a water degassing line including an attachment connection for gas samples
- an ultrahigh vacuum crushing device for the mechanical extraction of gases from rocks and minerals
The Noble Gas Mass Spectrometers
MM5400 and Helix SFT are sector field mass spectrometers, which have been optimized for noble gas analysis by ThermoFisher Scientific company and its predecessors. The main components include:
- Ion source: A modified Nier-type ion source ("bright source") is used for ionizing the gas atoms by electron bombardment.
- Ion optics: Ions are accelerated in a 4.5 kV high voltage and focused through a system of electric lenses.
- Magnet: In the 90° magnetic sector field, ions are deflected according to their mass to charge ratio. By setting the magnetic field to an appropriate value, ions of one specific mass to charge ratio are enabled to reach the detectors while all others will hit the mass spectrometer walls. At distinct magnetic field settings distinct noble gas isotopes can therefore be detected and their abundance be determined.
- Detectors: Relatively large ion beams (~10-13 to 10-10 A) are detected in a Faraday cup, smaller beams in a secondary electron multiplier fitted for single ion counting.
Both the older MM5400 and the new Helix SFT (acquired 2012) reach a mass resolution m/delta-m > 600 in the electron multipliers, which is essential for the separation of 3He+ (3.016 amu) from HD+ (3.022 amu) and thus for a precise determination of the 3He/4He ratio. In addition, owing to its “Split Flight Tube“ the Helix SFT allows for the simultaneous detection of 3He und 4He. An additional electrostatic filter suppresses scattered 4He ions in the 3He beam, enabling the precise measurement of very small 3He/4He ratios. The heavier noble gases Ne, Ar, Kr and Xe are analyzed in “peak jump” mode in both mass spectrometers, i.e., the isotopes are detected one after another by setting the magnetic field accordingly.
Ultrahigh Vacuum Furnace and Sample Crusher
For gas extraction from rock samples, two double-walled ultrahigh vacuum furnaces with resistance heating are employed. In an external vacuum (~10–6 mbar), a cylindrical heating element made of graphite surrounds the tantalum crucible. In the ultrahigh vacuum (~10–8 mbar) on the inside, a molybdenum liner protects the crucible from melt accretion and corrosion. Samples are dropped into the crucible from a carrousel with 14 or 21 positions and can be heated up to a maximum of 2000°C. A W/Re thermocouple is used for temperature control.
A bellows-tightened spindle press is used to crush rocks or minerals under ultrahigh vacuum conditions (see Figure). The sample is repeatedly squeezed between two hard-metal surfaces and released again until no cracking can be heard or felt any more. By this procedure gases from fluid inclusions or vesicles are expelled. Samples are loaded individually or can be dropped from a glass branch. Adsorbed atmospheric gases are pumped away before gas extraction during a 24 hour bake at 100°C.
Water Degassing Line
Water is sampled in copper tubes of 10mm diameter which are closed in a gas-tight way by stainless steel clamps. After attaching the tubes to our degassing line and pumping down to ~10-3 mbar, the lower clamp is opened and the water released to a spherical glass bulb immersed in an ultrasonic bath. The dissolved gas is extracted from the water by 15 minutes of ultrasonic agitation. During that time, a cold trap on the other side of a capillary is cooled by liquid nitrogen (-196°C), resulting in a pressure gradient between the extraction bulb and the cold trap which produces a laminar H2O gas flow. All the noble gases are entrained within the flow and quantitatively transferred to the cold trap, along with <5% of the water. The gas is then expanded into a larger volume and an appropriate split is used for noble gas analysis.
Gas containers can be attached directly to a pipette system on the vacuum line. A pipette of gas is expanded into the line and again an appropriate split is used for analysis.
Gas Preparation Line and Cryogenic Adsorbers
In the gas preparation line, condensable gases such as H2O are first frozen to a dry-ice-cooled trap. Other chemically active gases (e.g. O2, N2, CO2, H2, hydrocarbons etc.) are consecutively absorbed to two titanium sponge and two ZrAl („SAES“) getters. The remaining background of active gases is kept at low levels during analyses by additional ZrAl getters inside the mass spectrometer.
By means of expansion cooling in a closed circuit of compressed helium gas, the cryogenic adsorbers reach temperatures down to 12 K (-261°C). At this temperature all noble gases, including helium, are physically adsorbed to activated charcoal or a stainless steel frit. The noble gases are then individually released by incremental heating and can be admitted to the mass spectrometer and analyzed separately. Like this, mutual influences which could affect the accuracy of the measurements are avoided.