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1. The electron microprobe analysis

Thermic produced electrons are accelerated by a high voltage field and are bunched by electromagnetic lenses and apertures to a beam of less than 1 µm. The electrons interact with the solid sample, e.g. there form secondary-electrons, backscattered-electrons and x-ray. The characteristic x-ray is specific for every chemical element in the sample. Different detection systems (WDS and EDS) can disperse the x-ray spectral and the intensity of the single x-ray-signal can be detected. Comparison to standard samples and calculation of physical influencing variables provide the chemical composition of the analysed sample in a micrometer area. By measuring lots of adjacent points one acquires the extensive distribution of an element in the sample.

See also: Microprobe

We used a microprobe JEOL JXA 8900, which allowed us to choose various analytical methods:

• Backscattered-electron (BSE) images
• Secondary-electron (SEI) images
• Lines with quantitative wave-length dispersive (WDS) analysis
• Element mappings

We used the following parameters:

BSE- and SEI-images:

• Accumulation voltage: 15 kV
• Beam current: ~ 30 nA

WDS analysis:

• Accumulation voltage: 15 kV
• Beam current: ~ 30 nA

Map analysis:

• Accumulation voltage: 20 kV
• Beam current: ~ 60 nA

2. Laser-Ablation ICP-MS

Laser-Ablation ICP-MS combines the spatial resolution of an ultraviolet laser beam and the element sensitivity of an inductive coupled plasma mass spectrometer.
Samples, commonly thin sections, are located in an enclosed sample cell that is continuously purged with argon gas. The Laser light is focused on the sample surface and causes sample ablation. In this study the produced ablation craters had a diameter of 100 to 120 mm. The ablated material is transported in an argon carrier gas directly to the inductively coupled plasma.

Chemical analysis with inductively-coupled plasma (a state of matter
containing electrons and ionized atoms) is based on the principles of vaporization, dissociation, and ionization of chemical elements when introduced into
the hot plasma. These ions can then be separated according to their mass/charge ratio, using diverse separating mechanisms.

In this case the separating occurred with the “time of flight” (TOF) principle, patented by LECO with the “Renaissance ICP-TOFMS”. Segments of the ion beam are accelerated after the entrance into a free flight tube. After the accelerating all ions have the same kinetic energy, but according to their mass/charge ratio different velocities. Therefore the ions are separating into their individual groups as they travel the length of the flight path.
This way 20000 complete mass ranges can be detected every second.

3. Diffusion

In the presence of a concentration gradient the random motion of atoms results in a net transfer down the gradient, called diffusion.

The net flux Jx is defined in the Fick’s first law

D is the diffusion coefficient, which has the units of m2/s-1.
The Fick’s second law defines the change of the concentration distribution with time.

A simplified combination of the two laws results in the following equation, which defines the one-dimensional penetrating depth d of the diffusion front.

The diffusion coefficient D is a material specific variable, which can only be determined by experiments. The activation energy Ea required for one atom travelling to the material depends on several factors:

• Radius of the migrating ions
• Structure of the mineral, direction dependency, anisotropy
• Pressure
• Density of defects, especially vacancies

Like other temperature-activated processes, diffusion obeys an Arrhenius-type relation:

The strong temperature-dependency of D means that at lower temperatures the system is practical closed, that means the diffusion process is insignificant slow and the concentrations are not changing in the observed timescales.

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