Analytical methods

4.3.1. Whole rock

Whole-rock major elements were measured by wide-angle X-ray fluorescence (WDXRF) with a sequential spectrometer Bruker S4 Pioneer at the analytical services of the IACT (CSIC, Granada, Spain), using an Rh X-ray tube (160 kV, 159 mA). Veins and xenoliths that were apparent in hand specimens were removed. The samples were then crushed and finely powdered (< 100 µm) in an agate mill. Glass bead were prepared by weighing 1 g of powder with a Li2B4O7

flux (8:1 flux:rock). The mixture is then fused at 1000 °C during 15 minutes in Pt crucibles under

Chapitre 3 – Les inclusions fluides et les minéraux riches en volatils comme traceurs des fluides

a CH4-O flame using a Fluxana Vulcan 4M fusion machine. The produced melt is automatically poured onto a 40 mm diameter, flat-bottom Pt holder and immediately cooled with compressed air, thus producing homogeneous glass beads with a smooth flat surface. Measurements were carried out under constant voltage for all elements to achieve maximum long term stability and precision. The concentrations of the major elements were measured by comparing the X-ray intensity for each element with the intensity for two beads, each of nine reference geological standard samples (PCC-1, BCR-1, BIR-1, DNC-1, W-2, AGV-1, GSP-1, G-2, and STM -1, using the values recommended by Govindaraju, 1994). Two beads of pure quartz were used as blanks for all elements except Si.

Loss on ignition (LOI) for each sample were determined by weighing 1 g of powder in a ceramic crucible, putting it under a Muffle oven at 900 °C during 1 hour, and reporting the weight loss between the original and the devolatilized sample. The LOI content can be semi- quantitatively used as an estimation of the volatile content (CO2, OH, S, Cl, and F) of a sample.

Whole-rock Rare Earth Elements (REE) were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) on an Agilent 7700x mass spectrometer at the University of Montpellier. 100 mg of previously prepared powder were weighed using a precision balance (± 1 mg), then digested 3 times in an HF+HNO3+HClO4 solution. Each digestion is followed by a 24 h evaporation at 150 °C. The solid residues are then dissolved in H2O+HNO3 and diluted 8000 times to avoid saturation of the ICP-MS. Internal standards were ¹¹⁵In and ²⁰⁹Bi, external standards were international reference materials AVG-1 and BEN. Interferences between elements were corrected using external standards and reference values for Zr and Hf.

Bulk rock sulfur and carbon concentrations (total carbon and total organic carbon) were measured for each sample by element analyzer (IACT, Granada, Alt et al., 2012). Total inorganic carbon (TIC, or carbonate carbon) was removed by reaction with dilute (3 N) HCl, followed by washing in distilled H2O. To minimize adsorption of atmospheric CO2, powders were degassed at 100 °C and stored under vacuum in a desiccator. Standard deviations were between 10–40 ppm for sulfur and 10 ppm for carbon.

4.3.2. Minerals

Mineral major element compositions have been acquired on a CAMECA SX100 electron microprobe at the Microsonde Sud service of the University of Montpellier. The operating

conditions were a beam current of 10 nA, an accelerating voltage of 20 keV, 20 s counting time per element, and a focused 1 µm beam. For apatite, to avoid loss of volatiles (F, Cl, S), the conditions were adjusted to 10 nA, 15 keV, 40 s per element and the beam was defocused to 10 µm. Accuracy is below 0.5 wt.% for major elements.

Trace element analyses for sulfides and clinopyroxenes were acquired by Laser ablation, inductively coupled plasma mass spectrometry (LA-ICP-MS) using a GeoLas Q+ with an Excimer CompEx102 laser and a ThermoFinnigan Element XR plasma source mass spectrometer at Geosciences Montpellier Laboratory, Montpellier, France. Data acquisitions were conducted in-situ on 150 µm thick polished sections. Analytical conditions for clinopyroxene were 6 Hz, 10 J/cm² and a beam diameter of 51 µm, and they were adjusted for sulfides with 10 Hz, 12 J/cm² and a beam diameter between 5 and 26 µm depending on the sulfide’s size. Internal standards used were CaO wt.% for clinopyroxenes and S wt.% for sulfides, both determined with electron microprobe. External standards were Nist612 glass for clinopyroxene and PGE-A, JK37 steel and a Re-Os bead for sulfides. All data were processed using the GLITTER software (Van Achterbergh et al., 2001).

4.3.3. Fluid inclusions

Fluid inclusions were investigated in 150 µm doubly-polished single olivine crystals.

Microthermometric measurements were done with a Linkam^® THM 600 heating-cooling stage at Geosciences Montpellier Laboratory (France) and calibrated with SYNFLINC^® synthetic pure H2O and H2O-CO2 fluid inclusion standards. Accuracy of CO2 triple-point measurements is better than ±0.2 °C, and reproducibility of melting and homogenization temperatures is better than ±0.2

°C. Fluid inclusion density was derived from the homogenization temperatures using the phase diagram of Thiery et al. (1994) and FLUIDS Computer package (Bakker, 2008), and assuming a composition of pure CO2. Isochores were calculated using Span and Wagner (1996) equations of state for pure CO2 with the ISOC computer program (Bakker, 2003).

Fluid inclusions were analyzed by Raman spectroscopy using a LabRAM HR Spectrometer (Horiba Jobin Yvon) equipped with a 600 g/mm grating and an edge filter at GeoRessources Laboratory, Nancy, France. The excitation beam is provided by a Stabilite 2017 Ar+ laser (Spectra Physics, Newport Corporation) at 514.53 nm and a power of 200 mW (resulting in ~20 mW at sample), focused in the sample using a x50 objective (Olympus).

Chapitre 3 – Les inclusions fluides et les minéraux riches en volatils comme traceurs des fluides

Acquisition time is optimized to have the spectrum maximum intensity between 1/3 and 2/3 of the CCD saturation level (i.e. 20,000–40,000 counts). A total of 10 acquisitions were accumulated to provide a satisfactory spectrum: the resulting noise-to-signal ratio (N/S) is by far lower than 1%. To characterize the bands in the Raman spectra and identify the phases present, the online database of the French Society of Mineralogy and Crystallography (http://wwwobs.univ-bpclermont.fr/sfmc/ramandb2/index.html) and the software CrystalSleuth with the RRUFF Project database (Downs, 2006) were referred to. Raw data were processed using LabSpec software designed for Jobin-Yvon Horiba LabRAM instruments.