Supplement of Influence of basement rocks on fluid evolution during multiphase deformation: the example of the Estamariu thrust in the Pyrenean Axial Zone

Abstract. Calcite veins precipitated in the Estamariu thrust during
two tectonic events are studied in order to (i) decipher the temporal and
spatial relationships between deformation and fluid migration in a
long-lived thrust and (ii) determine the influence of basement rocks on the
fluid chemistry during deformation. Structural and petrological observations
constrain the relative timing of fluid migration and vein formation, whilst
geochemical analyses (δ13C, δ18O,
87Sr∕86Sr, clumped isotope thermometry, and elemental composition)
applied to the related calcite cements and host rocks indicate the fluid
origin, pathways, and extent of fluid–rock interaction. The first tectonic
event, recorded by calcite cements Cc1a and Cc2, is attributed to the Alpine
reactivation of the Estamariu thrust. Analytical data indicate that these
cements precipitated from heated meteoric fluids (temperatures in the range
of 50 to 100 ∘C) that had interacted with basement rocks
(87Sr∕86Sr > 0.71) before upflowing through the thrust
zone. The second tectonic event, attributed to the Neogene extension, is
characterized by the reactivation of the Estamariu thrust and the formation
of normal faults and shear fractures sealed by calcite cements Cc3, Cc4, and
Cc5. Analytical data indicate that cements Cc3 and Cc4 precipitated from
hydrothermal fluids (temperatures between 130 and 210 ∘C
and between 100 and 170 ∘C, respectively) that had
interacted with basement rocks (87Sr∕86Sr > 0.71) and
been expelled through fault zones during deformation. In contrast, cement
Cc5 probably precipitated from meteoric waters that likely percolated from
the surface through small shear fractures. The comparison between our results and already published data in other
structures from the southern Pyrenees suggests that regardless of the origin
of the fluids and the tectonic context, basement rocks have a significant
influence on the fluid chemistry, particularly on the 87Sr∕86Sr
ratio. Accordingly, the cements precipitated from fluids that have
interacted with basement rocks have significantly higher 87Sr∕86Sr
ratios (> 0.710) with respect to those precipitated from fluids
that have interacted with the sedimentary cover (< 0.710), which
involves younger and less radiogenic rocks.



Methods
This study integrates a field compilation of structural data and samples as well as petrological and geochemical analyses of calcite cements and related host rocks. The structural data includes the orientation of bedding, foliations, and fractures in addition to crosscutting relationships and kinematics. Such data were plotted in equal-area lower-hemisphere projections and different fracture sets were stablished according to their type, strike, mineral infillings, and relative age deduced from crosscutting relationships. All these data were integrated in a schematic map and a cross-section of the Estamariu thrust and the Cerc basin ( Fig. 2A, B and 3). Samples considered representative of the involved host rocks and all calcite vein generations observed in the different fracture sets and fault-related structures were selected for petrological and geochemical analyses.
Thin sections of these samples were prepared and studied under a Zeiss Axiophot optical microscope and a Cold cathodoluminescence (CL) microscope model 8200 Mk5-1 operating between 16-19 kV and 350 µA gun current.
Thirty-five representative samples of the different generations of calcite cements and the carbonate portion of the Devonian rocks were sampled for stable isotope analysis. Sampling was carried out with a 500 µm-diameter microdrill. 50-100 µg of powdered samples were reacted with 100% phosphoric acid during two minutes at 70 o C. The resultant CO2 was analyzed with an automated Kiel Carbonate Device attached to a Thermal Ionization Mass Spectrometer Thermo Electron MAT-252 (Thermo Fisher Scientific) according to the method of (McCrea, 1950). For calibration, the International Standard NBS-18 and the internal standard RC-1, traceable to the International Standard NBS-19, were used. The standard deviation is ±0.03‰ for δ 13 C and ±0.05‰ for δ 18 O expressed with respect to the VPDB standard (Vienna Pee Dee Belemnite).
The elemental composition of twelve samples of calcite cements and related host rocks were analyzed using a high resolution inductively coupled plasma-mass spectrometry (HR-ICP-MS, model Element XR, Thermo Fisher Scientific). 100 mg of powdered samples were dried at 40 o C during 24 h and then they were acid digested in closed polytetrafluoroethylene (PTFE) vessels with a combination of HNO3 + HF + HClO4 (2.5 mL: 5 mL: 2.5 mL v/v). The samples were evaporated and 1 mL of HNO3 was added to make a double evaporation. Finally, the samples were re-dissolved and diluted with MilliQ water (18.2 MΩ cm -1 ) and 1 mL of HNO3 in a 100 mL volume flask. A tuning solution containing 1 g L -1 Li, B, Na, K, Sc, Fe, Co, Cu, Ga, Y, Rh, In, Ba, Tl, U was used in order to improve the sensitivity of the ICP-MS , and as internal standard, 20 mg L -1 of a monoelemental solution of 115 In. Reference materials are the BCS-CRM n o 393 (ECRM 752-1) limestone, JA-2 Andesite and JB-3 Basalt. The precision of the results was expressed in terms of two standard deviations of a set of eight reference materials measurements (reference material JA-2), whereas accuracy (%) was calculated using the absolute value of the difference between the measured values obtained during the analysis and the certified values of a set of eight reference material analysis (reference material BCS-CRM n o 393 for major oxides and JA-2 for trace elements). The detection limit (DL) was calculated as three times the standard deviation of the average of ten blanks.
The 87 Sr/ 86 Sr ratio was analyzed for eight representative samples of calcite cements and host rocks. Powdered samples were dissolved in 5 mL of 10% acetic acid. After centrifugation, the supernatant was dried and dissolved in 1 mL of 1M HNO3. The solid residue generated after evaporation was diluted in 3 mL of 3M HNO3 and loaded into chromatographic columns to separate the Rb-free Sr fraction, using SrResinTM (crown-ether (4,4'(5')-di-t-butylcyclohexano-18-crown-6)) and 0.05M HNO3 as eluent. After evaporation, samples were loaded onto a Re filament along with 1 µL of 1 M phosphoric acid and 2 µL of Ta2O5. Isotopic analyses were carried out in a TIMS-Phoenix mass spectrometer performing a dynamic multicollection method, during 10 blocks of 16 cycles each one keeping a 88 Sr beam intensity of 3-V. Possible 87 Rb interferences and possible mass fractionation during sample loading and analysis were corrected and normalized with the reference value of 88 Sr/ 86 Sr = 0.1194. The isotopic standard NBS-987 was analyzed six times during sample analysis, yielding an average value of 0.710243 ±0.000009 (standard deviation, 2σ). NBS 987 data have been used to correct the sample ratios for standard drift from the certified value. The analytical error in the 87 Sr/ 86 Sr ratio, referred to two standard deviations, was 0.01%, whereas the internal precision is 0.000003. Sr procedural blanks were always below 0.5 ng.
The 143 Nd/ 144 Nd ratios were analyzed in seven samples of calcite cements and host rocks. Samples were weighted in Teflon® vessels, with enriched spike solution ( 149 Sm-150 Nd -Oak Ridge) and dissolved in 5 mL of ultrapure HF and 3 mL of ultrapure HNO3 (Merck-Suprapur TM ). The PFA-vessels were placed 65 hours at 120 o C into an oven. After that, cold vials were evaporated at 120 o C on a heat plate. 4 mL of 6N distilled HCL were added to the dried samples and placed at 120 ºC in an oven overnight. The solid residue generated after evaporation was dissolved in 3 mL of distilled and titrated 2.5 N HCl. Samples were centrifuged at 4000 rpm for 10 minutes to separate the possible dissolved fraction from the residue. Chromatographic separation of the whole group of REE was performed with a previously calibrated cation exchange resin DOWEX 50W-X8 200-400 mesh. After that, recovered REE fractions were dried and again dissolved in 200 µL HCl 0.18N. Such solutions were passed in a new chromatographic step (Ln-resin). The result is a complete separation between the Nd and the Sm fractions, using 0.3N HCl and 0.4N HCl as eluent, respectively. Dried Sm and Nd samples dissolved with 2 µL of 0.05M phosphoric acid were loaded onto a side Rhenium (Re) filament of a triple Re filament arrangement. Nd ratios were analysed in a mass spectrometer TIMS-Phoenix  , using a dynamic multicollection method, through 160 cycles at a stable intensity of 1V for the 144 Nd mass. In turn, Sm ratios were analysed in the same spectrometer, using a single static method through 112 cycles keeping 1V intensity for the 149 Sm mass. Nd analyses were corrected for 142 Ce and 144 Sm interferences, if any, and normalized to a ratio of 146 Nd/ 144 Nd = 0.7219 to correct the possible mass fractionation during the processes of loading and analysing at the TIMS. Nd isotopic standard JNdi-1 was checked to correct the sample ratios for standard drift from the certified value. The analytical error (2STD) was 0.1% in the 147 Sm/ 144 Nd ratio and 0.006% in the 143 Nd/ 144 Nd ratio. Procedural blanks were always below 0.1 ng.
Clumped isotope thermometry of the calcite cements was carried out in order to determine the temperature and composition (δ 18 Ofluid) of the vein-forming fluids. 2-3 mg aliquots from cements were measured with an automated line, the Imperial Batch Extraction system (IBEX), developed at Imperial College. Samples were dropped in 105% phosphoric acid at 90 o C and reacted during 30 min. The reactant CO2 was separated with a poropak-Q column and transferred into the bellows of a Thermo Scientific MAT 253 mass spectrometer. The characterization of a replicate consisted of 8 acquisitions in dual inlet mode with 7 cycles per acquisition. The post-acquisition processing was completed with Easotope, a software for clumped isotope analyses (John and Bowen, 2016). During phosphoric acid digestion, Δ47 values were corrected for isotope fractionation using a phosphoric acid correction of 0.069‰ at 90 o C for calcite . The data were also corrected for non-linearity applying the heated gas method (Huntington et al., 2009) and projected into the reference frame of (Dennis et al., 2011). Carbonate δ 18 O values were calculated with the acid fractionation factors of (Kim and O'Neil, 1997). Results were converted to temperatures applying the calibration method of (Kluge et al., 2015).