|Remains of amphibole in the ablation crater (center, right image) demonstrate relatively low birefringence color. The pictures were taken using different settings of camera and microscope, XPL.|
|Closeup of the crater in the same spot. Reflected light.|
Any kind of solid samples can be studied, or at least attempted to be studied. The ablated surface should be flat just for aiming purposes, polishing is not required. Ablation is a destructive technique, however diameter of a laser beam is usually around 30-50 µm. Besides the parameters of laser (such as ablation time), the depth of ablation heavily depends on the ability of material to absorb laser radiation. Normally, ablation depth exceeds 30 µm thickness of a regular section and in some cases could be up to 50 µm. In this regard, slightly thicker section was used.
On the picture above we see that the laser made a crater. Since the thickness decreased. the lower birefringence (grey 1st order) is observed in XPL. Assuming that the initial thickness was about 45 µm the birefringence value is equal 0.012 (the beginning of the second order). Using Michel-Lévy nomogramm (which I finally translated and made downloadable), I assume thickness went down to roughly 17 µm. Thus, the ablation depth is 45-17= 28 µm.
The resulted spectrum (count per seconds versus time) looks like this:
Note that the intensities measured for different elements increase and decrease in the same way. The patterns are identical. That means all the elements were ablated in the same way, without any fractionation.
Since I produced the thin section myself, it is not perfectly flat, so the edges of the section are slightly thinner.
|Before. Large crystals of alkali amphibole located on the edge of the section. The thickness is somewhat 30 um.|
The instrument (over)made its job. It produced an empty hole though the crystal. Apparently, the thickness on the edge of the section is around or even less than 30 µm.
|After. The laser beam made a hole through the crystal. Probably some of epoxy was ablated as well.|
The resulted spectrum: