The copper tube, equipped with the desired sample, is installed at position B as shown in figure 3.1. Hereby an airtight connection between line and copper tube is enabled using a brass ferrule nut set (Swagelok), which is used for each sample once. The hydraulic crusher, as shown in figure 3.2 exerts a pressure of 200 - 300 bar onto the copper tube. A small barrier (2 mm height) prevents a complete compression of the tube, which would lead to an interruption of the water vapour flow. The piston in the stainless steel jack is moved by a hydraulic hand pump. Since the piston has a diameter of 4 cm, very large samples require crushing in two steps. To crush the
Figure 3.2: Schematic illustration of the hydraulic crusher, which consist of a piston fixed in a stainless steel jack and a hydraulic hand pump. It exerts a pressure of 200 -300 bar onto the sample, whereby a small barrier prevents a complete compression of the tube. All used components and corresponding companies are listed in table A.2.
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sample the oven door must be opened, which results in a decrease in temperature and disturbs the water vapour background signal (see figure 3.4). An experienced user can complete this process in less than 30 seconds. Then the disturbance has no effect on the measurement signal as it reaches the analyser with a 90 second delay and is taken into account in the evaluation (see section 3.4). For the analysis of the sample signal, this intermediate interval is set to two minutes, which ensures that the undisturbed water vapour background is used to calculate the background signal.
Particle size analysis
I carried out a grain size analysis at the Institute of Earth Sciences Heidelberg to test the efficiency of the hydraulic crusher. The samples were analysed with the Analysette 22 Micro Tec (Fritsch - Achern, Germany) based on the principle of laser diffraction analysis. It follows the fundamental that particles of different sizes produce different diffraction patterns. Thus a simple and fast measurement of the geometrical dimen-sions of the particle is possible [Beuselinck et al., 1998]. For particles with a diame-ter above 10 µm the laser diffraction analysis is based on the Fraunhofer diffraction, whereas Mie diffraction plays a role for smaller particles, which will not be discussed here [de Boer et al., 1987]. The Fraunhofer diffraction theory states that the intensity of light, scattered by a particle, is directly proportional to the size of the particle.
Thus, the grain size can be determined by measuring the diffraction angle of the laser beam. The relation is inversely proportional: the smaller the particle, the larger the diffraction angle. The detailed measurement process is described in the appendix A.2.
I have selected five already crushed and measured samples for fluid inclusion to deter-mine the particle size. The results of the particle size analysis are shown in figure 3.3 and listed in table 3.1, with each sample measured ten times. The left side of figure 3.3 shows the development of the median over the number of measurements. Here the median or d50 describes the average grain size. There is a decrease in the median with progressive measurements, indicating a change in the particle size distribution.
This observation implies that some particles stick together and only gradually separate
sample name weight median
Table 3.1: Five samples already crushed and measured for fluid inclusion were anal-ysed for particle size analysis. The mean value or d50 is the value of the particle diameter at 50 % in the cumulative distribution.
with running an ultrasonic bath. During crushing a high pressure (up to 300 bar) is performed on the sample. This pressure leads to crushing of the stalagmite as well as to a compression and compaction of the fine material.
Due to the non constant median, I used the last five measurements of each sample to determine the mean particle size distribution (figure 3.3 right panel). Here the mass fraction in % is plotted against the particle diameter in µm. Two peaks have to be distinguished. Peak I at a particle diameter of about 20 - 30 µm represents the mean diameter of single particles, whereas peak II rather correspondence to several particles that stick together due to the high pressure exerted during crushing. As the grain size measurement progresses a decrease of the peak II is visible, corresponding to a dissolution of the clumping particles, at the same time peak I increases. All samples are showing a similar particle size distribution, which speaks for a homogenous and reproducible crushing routine. With a mean grain size around 30 µm, it is feasible to open almost all fluid inclusions as they have typically a size from 1 to 50 µm [Schei-degger et al., 2010] or 10 to 50 µm [Schwarcz et al., 1976].
Compared to other techniques, the hydraulic crusher of the fluid inclusion line achieves a very good crushing efficiency with an average d50 value of 37 µm. A similar crushing procedure developed by Affolter et al. [2014] results in a d50 value of 495 µm with the
0 2 4 6 8 1 0
Figure 3.3: Left: Median or d50 with progressive measurements for five different samples illustrated in different colours. The median measurement was repeated ten times. A decrease of the median indicates a change in the particle size distribution.
Right: Grain size distribution for all five samples, whereby the last five measurements were taken into account. Two particle diameters are dominant with peaks at 20 -30 µm (peak I ) and 400 µm (peak II ), while peak II does not correspond to a single particle size, but to the size of several particles clumping together due to the high pressure exerted during crushing. The mean diameter of single particles is dominant at a particle diameter of about 20 - 30 µm indicated as peak I. Detailed data for all samples and the 10 individual measurements are listed in the appendix (A.4, A.5, A.6, A.7, A.8 and A.9).
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difference that the barrier is adjusted to a higher level and therefore the copper tube is not compressed as much as for this setup. Kluge et al. [2008] has developed a crushing method for noble gas analysis, where they achieved an average grain size of 630 µm.
With the Amsterdam Device of Vonhof et al. [2006] a grain size distribution from 100 up to 1000 µm was achieved.