The Noble Gas Laboratory

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.



Our Research Profile



Contact Person:   Dr. Samuel Niedermann




Equipment

The noble gas laboratory  includes the following facilities:
 

• Two MM 5400 noble gas mass spectrometers, 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

• 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

MM5400 -Noble Gas Mass Spectrometer

The MM5400 is a sector field mass spectrometer optimized for noble gas analyses and produced by Micromass Co., Manchester, England. Its 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 ions 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.
  

Ultrahigh Vacuum Crusher

Ultrahigh vacuum crusher

A bellows-tightened spindle press is used to crush rocks or minerals under ultrahigh vacuum conditions. 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. Up to 6 samples are stored in a glass arm and can be run without breaking the vacuum. Crushed grains are pushed off the crushing space through a screw-driven bellows before dropping the next sample. 

Water Degassing Line

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.