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To establish how prone to acquiring a magnetisation the rocks of the BGB are, room tem- perature susceptibility measurements (χ) were carried out on a Bartington Susceptibility meter. Samples were weighed and their susceptibility was measured at both low (465Hz) and high (4.7 KHz) frequencies. During the later stages of this research project an Agico Kappa Bridge Susceptibility meter (MFK1) was acquired by the Geomagnetism Laboratory at Liverpool allowing both room temperature and temperature dependant susceptibility measurements to be carried out. Using the Kappa Bridge magnetic bulk susceptibility as function of low temperatures (-196°C to 0°C) and of high temperatures (20°C to 700°C) was measured. Here samples of a weight of approximately ~330mg were crushed to a fine powder and inserted into a tube sample holder. The thermocouple is also inserted into the sample holder, ensuring there is sufficient sample to wrap around the base of the thermocouple. The high temperature experiment was carried out in argon, in order to reduce the potential for oxidation of magnetic minerals, thus minimising the effects of thermochemical alteration. The sample is then water cooled back down to room tem- perature. During the cooling experiment, the sample is cooled down to -196°C by adding liquid nitrogen and then continuously heated by the furnace in the Kappa Bridge up to 0° C.

Measuring bulk susceptibility as a function of temperature allows further characterisation of the magnetic carriers and was conducted in order to compliment the results obtained from the thermomagnetic experiments on the VFTB. From the low temperature experi- ment, the Verwey Transition (Tv) can be obtained (Verwey, 1939), which can indicate the

presence of magnetite in a sample. At below 120K the lattice structure of magnetite is disrupted and changes from being cubic to slightly monoclinic as a result of reordering of Fe2+ and Fe3+ ions (Dunlop and Özdemir, 1997). The reordering in lattice structure leads to an abrupt change at Tv when measured on the Kappa Bridge (an example of which is

Figure 3.6. The Agico Kappa Bridge at the Geomag- netism Laboratory , University of Liverpool.

shown in Fig. 3.6), given that susceptibly measurements are dependent on crystalline ani- sotropy. However, the sharp peak is only observed is a sample contains pure magnetite, which is rare in nature. A broad peak, or a peak at lower Tv is not uncommon and indi-

cates the remanance carrier contains impurities of titanium or may have been oxidised during the experiment. High temperature experiments are comparable to the VFTB ther- momagnetic curves experiments but as the experiment is measured in much weaker fields than those applied during the VFTB experiments, it is less likely that paramagnetic contri- butions will swamp the ferromagnetic signal. From the high temperature experiment it is possible to observe Tc as a sharp drop in susceptibility (generally preceded by a peak).

Measuring frequency dependant susceptibility ( χfd) can be used to establish grain size determination of the presence of superparamagnetic (SP) grains within a sample. Very narrow hysteresis loops can indicate the presence of SP grain sized particles (Dunlop and Özdemir, 1997; Tauxe, 1996). Very fine grained ferromagnetic particles show strong fre- quency dependence. Changing the frequency to which the sample is exposed to is equiva- lent to changing the amount of time available to the magnetic grains within the sample to react to a change in the applied field strength. Magnetic susceptibility as a function of fre- quency, from 976Hz to 3904Hz and onto 15616Hz, was measured at room temperature from individual, whole, specimens. The superparamagnetic constituent in a sample is given by:

Where χlf is the measured low frequency susceptibly and χhf is high frequency susceptibil-

ity.

3.2.2.3. Microscopy

Rock magnetic characterisation experiments were supplemented with microscopy studies to: a) better constrain the magnetic carriers, b) better understand the metamorphic al- teration in the rocks of the BGB and its effect on the potential magnetic carriers. A selec- tion of samples from each of the four BGB formations were studied using transmitted light microscopy and on a Philips XL30 tungsten filament Scanning Electron Microscope (SEM) fitted with for Electron Back Scatter Diffraction (EBSD) and Energy Dispersive Spectros- copy (EDS/EDX) technology. Thin sections were prepared by Paul Hands, at Birmingham

University. Details of which samples were studied and the selection rational are given in the individual Formation results chapters. Samples were studied using transmitted light microscopy prior to any SEM investigations, as thin sections require a carbon coat during SEM analysis to minimise charging effects (Prior et al., 1996), which obscures mineralogy observations in transmitted light.

Transmitted light microscopy was carried out on Meiji Technology Microscopes fitted with a Nikon camera, to establish the extent to which the samples had been altered during ser- pentinization and reheating. Identification of iron oxides was crucial and understanding their relationship to the surrounding mineralogy important to ascertain whether they are associated with the primary mineralogy of the sample. Given the complex history of the rocks of the BGB, the mineralogy of the samples is often very complex and it was difficult to establish a clear cooling history. Nonetheless amongst others, observation of freshness of minerals, as well as alteration of crystal rims and cores and textural relationships ( e.g. phenocrysts in a fine grained matrix indicates a two stage cooling history; embayment of minerals indicates addition of mew magmatic material during emplacement of the rocks) were studied in order to better understand the mineralogy. All thin sections were studied under plain polarised light (PPL), cross polars (XPL) and reflected light (which is ideal for the study of opaque minerals, such as iron oxides).

Detailed discussion of SEM and BSE image acquisition is given in Prior et al., (1999 and 2002) and references therein. In BSE imaging the variation seen in the intensity of grey- scale colours indicate rough differences in mineral chemistry. Heavy elements (high atomic numbers) such as iron (Fe) appear brighter than atoms which are lighter, such as silicon (Si). This results from a shallower interaction of the electron beam with the tar- geted mineral. Therefore, this means iron oxides tend to be easily identifiable during the SEM investigations. EDX spectra can then be used to identify the main elements within the targeted mineral and so ascertain its exact mineral composition. In addition element maps can be created. The beam scans a given area of the thin section highlighting the presence of preselected atoms. This is useful to quickly identify areas in which there are large concentrations of Fe, which may indicate the presence of iron oxides (keeping in mind that minerals such as pyroxene, commonly found in volcanic rocks, can also be Fe rich).