3 Sampling and methods
3.3 Rock characterisation
The mineral compositions of the rock samples from the Schlattingen borehole were determined by X-ray diffractometry using a Philips PW3710 diffractometer. Rock material from the rim of the drillcores (typically 100 – 200 g) was ground in a ring mill to a grain size of less than 2 µm, mounted on a sample holder, disorientated with a stamp, and scanned with Cu Kα radiation from 2° to 70° 2θ angle. The contents of quartz, feldspars and carbonates (calcite, dolomite, siderite) were quantified by standardisation utilising the diffraction peak-intensity ratio of the mineral to that of an internal LiF-standard. The relative error of such determinations is about
± 5 %.
The standardisation used is valid for individual mineral contents up to approximately 50 wt.-%.
In limestones, the contents of calcite or dolomite often exceed 50 wt.-%. The relative contents of the carbonate minerals derived from the XRD peak intensities were calibrated using the total inorganic carbon content measured by infrared (IR) spectroscopy (see section 3.3.2). The pyrite content was similarly calculated from the total sulphur concentration measured by IR spectros-copy assuming that pyrite is the dominating sulphur-bearing mineral phase. Sheet silicates, which are mainly clay minerals, were estimated by difference with the percentages of all other phases.
3.3.2 Inorganic carbon, organic carbon, and total sulphur of whole rock
The concentrations of total carbon (Ctot), inorganic carbon (Cinorg) and total sulphur (Stot) were determined with a CS-Mat 5500 element analyzer (Ströhlein GmbH & Co, Germany).Approximately 0.02 – 0.1 g of rock powder is heated to 1350 °C in an O2 atmosphere to liberate all volatile components. Total carbon and sulphur are measured as CO2 and SO2 using separate NDIR (non-dispersive infrared absorbance) analysers. Inorganic carbon (i.e. essentially CO2 from carbonate minerals) is measured separately by IR spectroscopy after heating to 1550 °C in a N2 atmosphere and passing the produced CO2 through a Mg-perchlorate tube to remove water.
Organic carbon (Corg) is calculated by difference of Ctot and Cinorg. The method is calibrated with pure CaCO3 for carbon and Ag2SO4 for sulphur. The detection limits are around 0.1 wt.-% for Stot, Ctot and Cinorg.
3.3.3 Clay mineralogy
Total contents of sheet-silicate minerals were estimated by X-ray diffraction of whole-rock samples (section 3.3.1). For the identification of individual clay-mineral species, rock material from the rim of the drillcores was ground in a ring mill to a grain size of about 60 μm and the clay fraction (< 2 µm) was separated by sedimentation in a column of 0.01N NH4OH solution.
The clay fraction was removed from the supernatant suspension by centrifugation. The separa-ted clay material was mounsepara-ted by sedimentation on three sample holders in order to obtain oriented samples. Each sample holder was subjected to different treatments: (i) dried under air, (ii) saturated with ethylene glycol for the identification of expandable clay minerals, and (iii) heated at 550 °C over 1 hour for the distinction between kaolinite and chlorite. X-ray diffracto-metry scanning velocity was 2°/min from 2 to 40º 2θ angle using Cu Kα radiation.
The relative ratios of the individual clay-mineral contents were determined using the ARQUANT model (Blanc et al. 2006). This Excel-based tool compares the X-ray pattern of glycolated samples with patterns from a standard library, which contains a large number of patterns for clay minerals and other sheet silicates. These patterns were calculated using the NEWMOD code (Reynolds 1985). The work flow to quantify the clay mineralogy of samples was as follows:
Recalculation of the X-ray patterns (acquired using Cu-K radiation) to patterns referring to Co-K radiation, which was used for the standard library. This was performed in a spread-sheet ("Convertisseur") provided by P. Blanc (BRGM), the author of the ARQUANT code.
Inspection of the air-dried, glycolated and heated X-ray patterns, in order to qualitatively identify the main clay minerals present in the samples.
Setting parameters that reflect the conditions under which the diffraction pattern was obtained:
Calibration: The X-ray pattern of each sample was shifted in order to fit the known positions of minerals with well defined peaks (quartz, kaolinite).
As the ARQUANT model allows to include a maximum of 30 standard patterns into the calculation, only a subset of the patterns in the library can be considered in an individual model run. The selection procedure was performed on the basis of preliminary model calcu-lations. About 3 – 4 standard patterns were chosen for each clay-mineral group identified in the sample patterns (i.e. illite/smectite group, chlorite, kaolinite). To this initial set, further patterns from one of the mineral groups were added to reach the maximum number of standards of 30, and a model calculation was performed. Standard patterns for which the model yielded non-zero values were retained, while the others were eliminated. This process was repeated until all standards for the specific mineral group were considered in the calculation. The same procedure was then performed for the other mineral groups until a preferred set of standards was identified that is appropriate for the clay mineralogy of the samples. A particularly large number of standard patterns was included for illite/smectite mixed layers, in order to cover the whole range of contents of illite layers between 50 and 100 % in steps of 5 – 10 %.
After choosing the 30 standard patterns, they were fitted to the sample patterns for the angular range 6 – 32° 2θ (Co-K radiation). Lower angles were not considered due to the absence of relevant peaks in the sample patterns and due to the fact that the current version of NEWMOD used to calculate the standard patterns is known for not well reproducing the low-angle range (P. Blanc. pers. comm.). From these model fits, the relative abundances of kaolinite, chlorite, smectite and of the sum of illite and all illite/smectite mixed-layer phases were obtained.
In a second step, the fitted calculations were repeated for the angular range 6 – 16° 2θ (Co-K radiation). These calculations were used to quantify the relative contents of illite and all illite/smectite mixed-layer phases.
The reported relative amounts of clay-mineral species were obtained from the following simplifications of the model output:
− Illite: Sum of all species with illite content > 95 %.
− Illite/smectite mixed layers: Simplified to groups containing 90 – 95 %, 75 – 85 % and 50 – 70 % illite layers.
− Kaolinite: Sum of all kaolinites.
− Chlorite: Sum of all chlorites and chlorite/smectite mixed layers. Because there is no peak shift between the air-dried and the glycolated sample patterns, the presence of chlorite/smectite mixed layers appears unlikely. In the model calculations, the distinction between chlorite and mixed-layer phases is difficult due to the typically small and broad peaks, and so such a distinction would be an overinterpretation of the data.
3.3.4 Carbon and oxygen isotopes of carbonates
Carbon and oxygen isotope ratios of whole rock carbonate were measured on 1 g powder ali-quots of the same samples that were analysed for Sr. Vein calcite was micro-drilled with a 2 mm diamond drill. The stable C and O isotope composition was measured at Uni Bern on a Finnigan Delta V Advantage mass spectrometer equipped with an automated carbonate prepara-tion system (Gas Bench-II). The δ13C results are reported relative to the Vienna-Pee Dee Belem-nite (V-PDB) standard, whilst δ18O values refer to the Vienna Standard Mean Ocean Water (V-SMOW) standards; standardisation was accomplished using international standards NBS 19 and NBS 18 (Friedman et al. 1982). Analytical absolute errors (2) are ≤ 0.12 ‰ for δ13C and for δ18O.
3.3.5 Strontium isotopes of whole rock carbonate and vein calcite and celestite
Sr analyses of the whole rock carbonate fraction were performed on 0.02 to 0.05 g of dried rock powder, leached for at least one hour with 0.7 – 1 ml of acetic acid 0.5 M. After the leaching, the samples were centrifuged for 10 minutes and the solid fraction was removed. After drying of the solution, the sample was spiked with 84Sr, dried again and at least 100 μl of concentrated HNO3 was added and evaporated. The residue was dissolved with 300 μl of 1 M HNO3 and passed through a chromatographic column.
Sr isotopic analysis of vein samples was done with around 2 mg of powder (drilled from the vein sample) leached with around 1 ml HCl 6M. After leaching, the sample was dried and spiked with 84Sr. The following steps are the same as for the whole-rock carbonate procedure.
The Sr isotope measurements were made with an ICP mass spectrometer at Uni Bern. Every 5 – 6 samples, the international NBS987 standard (certified 87Sr/86Sr ratio of 0.710245) was measured to correct for analytical deviation. The strontium concentration (ppm) in the sample can be calculated from the sample and spike weights and the measured 84Sr/86Sr ratio. Absolute analytical error (2) on the 84Sr/86Sr ratio is generally around 0.000020.
3.3.6 Thin section investigations including carbonate staining
Thin sections were made from six Schattingen borehole core samples which contained silty-sandy layers. Rock chips were first cut with a diamond saw using little water. Thin polished sections were then prepared using paraffin oil instead of water to minimise disaggregation of the rock due to swelling. The sections were studied with a conventional optical polarising micro-scope under transmitted and reflected light.
3.3.7 Scanning electron microscopy and microprobe analysis
The thin sections were also studied by scanning electron microscopy. This included back scatter electron (BSE) and semi-quantitative EDX (energy dispersive x-ray) analyses of diagenetic and detrital mineral assemblages.
The chemical composition of biogenic and diagenetic carbonate minerals in six samples was measured with the microprobe (with a wave-dispersive energy system) and with EDX.