Laboratory research

The laboratory work started with the preparation of 50 rock slices for macroscopic evaluation of the rock samples and re-evaluation of the macroscopic characteristics of the archaeological samples.

Moreover, the most representative rock samples were selected, and 42 thin and polished-thin sections were prepared for examination using the Optical and SEM-EDS microscopes. The slices and the sections were prepared in the Charles McBurney Laboratory for Geoarchaeology. The laboratory is based in the Department of Archaeology (West Building) at the University of Cambridge. The optical microscopic examination and the FTIR-ATR analyses were conducted in the same laboratory, while the SEM-EDS investigation (Brothwell and Higgs, 1969) took place in the Department of Earth Science, also at the University of Cambridge. A Quanta 650F scanning electron microscope (QEMSCAN 650F) equipment was used, which had two energy dispersive spectrometers (EDS) detectors (Bruker SSD Flash 6|30 detectors). The EDS analysis carried out using the Bruker software, ESPRIT. The polished-thin sections were carbon-coated and investigated in low vacuum conditions. In addition, the analyses were acquire

63 having a working distance of 13mm (±0.5mm), HV set at 15.00 kV and the spot size of the analysis at 4s.

Representative FTIR spectra (McBurney Laboratory protocol; Appendix II) obtained from all rock samples (n=62) by grinding a few tens of micrograms of the sample using an agate mortar and pestle (Parish et al. 2013; Smith, B. C. 2011; Hawkins et al. 2008). About 0.1mg or less of the sample was mixed with about 80mg of KBr (IR-grade). A 7mm pellet was then made using a hand press and the spectra were collected between 4000 and 400cm^-1 at 4cm^-1 resolution, using a Thermo Nicolet 380 spectrometer. The interpretation of the spectra was based on an internal library of infrared spectra of archaeological materials (Weiner, 2010). Moreover, the rock samples were examined with the ATR method to have a solid cross-reference database between the two techniques (fig.4.1). Similar (0.1mg) or less sample was used to collect ATR spectra and compared them with the ones of the FTIR equipment. The spectra were also collected between 4000 and 400cm^-1 at 4cm^-1 resolution and the same internal library was used to identify the minerals. The cross-examination between the two methods (i.e. FTIR and ATR) reduced the errors and overcame the lack of mineral reference and secured an accurate interpretation of the ATR spectra. The ATR equipment was less invasive than FTIR but was lacking in accuracy and the results deviated from acceptable values. This method obtained all the spectra from the artefact samples (n= 100) with the ATR technique and minimized the impact of this technique. Representative ATR spectra have been obtained from 100 artefact samples, under the same conditions as the rock samples. In addition, representative XRF spectra were obtained from most rock (n=60) and artefact samples (n=100), with a Bruker portable XRF, the Tracer III-V analyser (Bruker, 2010; Shackley, 1998). It was a non-destructive technique, and the sample was placed on the top of the analyser without any preparation. The collection of the spectrum was controlled from the S1PXRF software (KeyMaster Technologies, Inc. 2001), through which the properties of the measurements were arranged. The Baud Rate was set at the highest level (i.e. 115200) and the “Back scatter” and “PC Trigger” options were activated. Moreover, the High Voltage was set at the 40kV, the Anode Current at 20mA and the length of each measurement was placed at 60 seconds. Before the measurements started, the equipment was standardised with the Duplex 2205 stainless steel standard and the standardisation repeated every 40 minutes. The equipment for both these techniques belongs to the Department of Archaeology at the University of Cambridge, on whose premises the analyses were performed.

Moreover, elementary analyses were performed using Laser Ablation- Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) technique to determine the composition of the major, trace and rare earth elements (Speer, 2014; Neff, 2012). Through this method, 42 rock samples from Malta and Sicily and 129 archaeological samples from all the assemblages were examined. The equipment of this method is located in the Department of Earth Sciences, at the University of Cambridge. This high-resolution depth profiling technique initially employed an Analyte G2 excimer laser (Teledyne Photon

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Machines Inc) coupled with Thermo i-CapQ ICPMS The use of the Thermo i-CapQ ICPMS collision cell in kinetic energy discrimination (KED) minimized interferences on transitional mass elements (Tanner et al, 2002). The Laser Ablation system was optimized for high spatial resolution using an aperture slit of 60x20 μm to map the surface of the samples and 6Hz frequency with 1.8J/cm² laser fluence, while the laser speed scan along the tracks was set up at 2 μm/sec. In addition, approximately 1 μm of the top surface was removed using pre-ablation with 80x30μm laser spot to avoid any potential surface contamination. The ICP-MS sensitivity was optimized using NIST612 reference glass material for maximum sensitivity. Data reduction involved initial screening of spectra for outliers, subtraction of the mean background intensities (measured with the laser turned off) from the analysed isotope intensities, internal standardisation to 43Ca, and external standardisation using the NIST612 glass reference material. Finally, in-house NIST614 reference material was used to monitor long-term standards of reproducibility. However, this equipment was unable to perform elementary analyses on some of the Sicilian rock and artefact samples and therefore second type of equipment was employed.

These samples were more resilient and prevented the laser from producing the necessary plasma for an accurate analysis. This was an unexpected outcome which was observed while monitoring the LA-ICP-MS measurements. The high resilience of those samples had a negative impact on the produced results which were characterised by low accuracy and high possibility of error (i.e. Error> 20%, RSD>

20% and 80%<REC<120%). One suggested explanation might be the extremely high SiO2 content that these samples had (>95%) and was recorded with another LA-ICP-MS equipment (see below).

The research used the ESI NWR193 excimer Laser Ablation system interfaced to the Nexion 350D ICP-MS, which was much stronger and succeeded in analysing the remaining samples. A 100 µm diameter laser beam and a laser repetition rate of 10 Hz and laser power of 8 J cm^-1 was used for the entire study. The ICP-MS data acquisition settings in the Syngistix version 1.1 software were 1 sweep per reading, 60 readings, 1 replicate, and total data acquisition lasted 44 seconds in peak hopping mode. The data was acquired at a rate of one point for each element every 0.75 seconds. For all analyses, NIST614 was used for calibration of element sensitivity using the “Preferred Values” Ref 1 published on the GEOREM database. Calibration accuracy was checked by repeatedly analysing NIST610, NIST614, and BCR-2G as unknowns and comparing to GEOREM values. Standards were analysed at the beginning, end, and periodically within each laser session. For data processing and calculation of concentrations, Glitter Software (GEMOC, Australia) was used to process the raw data files containing the signal intensity vs time data (the output from the Elan software). This allows precise selection of blanks, signals, and rapid visualisation of the intensity data. The calculated concentrations,1 sigma error, and theoretical detection limits were exported to a statistical software and spreadsheet programs for further processing.

Special care was taken to secure consistency, precision and comparability between the results. In order to achieve this, samples were analysing with both equipment and the results of this

cross-65 examination is demonstrated below (fig.4.2). It is not expected to get the exact same values, especially when performing spot elementary analyses (LA-ICP-MS). Regardless of the measured values, prior geochemical analysis (e.g. Murray et al. 1992; Murray, 1994) have shown that the ratios between specific elements is consistent and it is based on these ratios that geochemical techniques are able to detect the features of the different rock samples.

The laboratory work was conducted in the Department of Earth Science at the University of Cambridge. The overall process of the results and the subsequent geochemical models were conducted with the use of the software GCDkit (ver. 3). It is the software that created the binary and ternary diagrams, and models used throughout the thesis.

Finally, the investigation of the typology and the craft techniques followed the work of Kowta (1980), Inizan et. al (1999), Andrefsky (2005), and Shea (2017). This method focused on the samples of the Brochtorff Xagħra Circle, Taċ-Ċawla, Ġgantija, Santa Verna and Kordin, and was conducted in the McBurney Laboratory. The important characteristics were identified, and the typology of each artefact was recorded. These features have contributed to identify the techniques employed on these artefacts that led them to their final form (i.e. current form).

Figure 4-1: Representative cross-examining FTRI-ATR spectra. The chert-rock samples (e.g. G2S1) from Malta have been examined with the FTIR (above) and ATR (below) to reduce the errors in interpretation and overcame the lack of mineral

reference of the ATR technique.

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Figure 4-2: Geochemical models used in this PhD research and compare the results between the two LA-ICP-MS equipment employed in this research. a) Ternary model using the concentrations of Fe, Al and Mn, b) Binary model using

the rations of Fe/Ti and Al/(Al+Fe) and c) The concentrations of the REE normalised with the World average shales standard (Piper, 1974).

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