The geochemical composition of calcite cement provides information about the type, source, temperature, and age of the fluids from which this mineral precipitated [1
]. Furthermore, the geochemical composition of adjacent host rocks also records the degree of fluid–rock interaction [6
]. Physical parameters such as temperature, ion saturation, and pressure during mineral precipitation have a strong influence on the morphology of calcite crystals [10
], which is also controlled by pH of fluids and the presence of organic compounds [15
]. The study of these parameters allows us to decipher processes such as boiling and effervescence during fluid migration, which also control the precipitation of calcites with different morphologies [12
]. However, fluid systems are complex, and the conditions at which these processes take place can change due to the input of exotic fluids [25
]. To identify these changes, petrographic observations, coupled with crystallographic and geochemical analyses, are needed.
The Southern Pyrenees is a fold and thrust belt where fluid migration has been well-studied by means of the geochemical and petrographic analysis of fracture-filling calcites [1
]. However, these previous works only focused on the type and origin of fluids from which calcites precipitated during different stages of compressional deformation. A more detailed study integrating the analysis of the morphology and geochemistry of fracture-filling calcite crystals will have strong implications on identifying processes such as boiling and/or effervescence, and changes in fluid salinity.
In the present work, we present an example of fracture-filling rose-like calcite crystal clusters of bladed calcites cropping out in the Southeastern Pyrenees. These uncommon calcite morphologies have been characterized by using optical, cathodoluminescence and electron microscope, X-ray diffraction (XRD), Raman spectroscopy, δ18O, δ13C, 87Sr/86Sr, and clumped isotopes, as well as major elements and rare earth element + yttrium analysis (REEs + Y). The results provide information about what parameters controlled the precipitation of these bladed calcite crystals in the Pyrenees area.
Field work has consisted of fracture and bedding data acquisition, description of their crosscutting relationships, and sampling of fracture-filling calcite cement and adjacent host rocks for petrographic studies and geochemical analyses.
Petrographic analysis was made by using optical, scanning electron, and cathodoluminescence microscopy. For scanning electron microscope, a FEI-Quanta 200 device (FEI Europe B.V., Eindhoven, The Netherlands) with and EDS Spectrometer (EDAX-Genesis) was used. A CL Technosyn cathodoluminescence device Model 8200 MkII (Technosyn Limited, Cambridge, UK) operating at 15–18 kV and 350 μA gun current was used.
XRD measurements were performed on as-collected large crystalline fragments of the calcite cement, in order to obtain information about the orientation of the crystals. The measurements were acquired with a Bruker D8-A25 powder diffractometer (Bruker AXS, Karlsruhe, Germany), equipped with a Cu X-ray source (Cu Kα radiation) and a LynxEye position sensitive detector (PSD). In order to obtain information about the orientation of calcite crystals, XRD measurements were performed on several as-collected crystalline fragments. The methodology employed here is similar to that used in Reference [63
], where platy calcite crystals were measured on a powder XRD mount with the short-length axis perpendicular to the plate. In that case, such measurements are allowed to confirm that the studied samples were oriented with the c-axis perpendicular to the XRD mount. In the present case, we have mounted the calcite crystals with the largest, best-developed face perpendicular to the XRD mount. The measurements were acquired with a Bruker D8-A25 powder diffractometer (Bruker AXS), equipped with a Cu X-ray source (Cu Kα radiation) and a LynxEye position sensitive detector (PSD). Phase identification and X-ray reflection assignment were carried out by using the DIFFRAC.EVA software and the Powder Diffraction File (PDF-2) database.
Room-temperature Raman microspectroscopy analyses were carried out on calcite cement and fluid inclusions, using a LabRam HR800 Jobin-Yvon™ spectrometer (HORIBA France SAS, Longjumeau, France). The measurements were performed with a 600 groove/mm grating, using a 532 nm (green) laser as excitation source. Acquisition timespan was of 30 or 60 s, with a total of 10 accumulations to improve the signal-to-noise ratios. The incident power density on the sample was carefully controlled, to avoid any damage on the studied areas. Fluid inclusions were analyzed to determine the presence of hydrocarbons and/or volatile species (CO2, CH4, N2, and H2S).
For carbon- and oxygen-isotope analysis, six microsamples were prepared by using a 500 μm thick dental drill to extract 60 ± 10 μg of powder from trims containing carbonate host rocks and calcite cement. The carbonate powder was reacted with 100% phosphoric acid for two minutes at 70 °C for calcite. The resultant CO2
was analyzed by using an automated Kiel Carbonate Device attached to a Thermal Ionization Mass Spectrometer Thermo Electron (Finnigan) MAT-252 (Thermo Fisher Scientific, Bremen, Germany), following the method of Reference [64
]. The obtained results were corrected by using the standard technique from References [65
], expressed in ‰, with respect to the VPDB (Vienna Pee Dee Belemnite) standard. Standard deviation is ±0.01‰ for δ13
C and ± 0.05‰ for δ18
Clumped isotopes thermometry was applied to the fracture-filling calcite cement in order to calculate the temperature and δ18
O of the former fluid from which it precipitated. To analyze this calcite cement, 2–3 mg aliquots were measured with the Imperial Batch EXtraction system (IBEX), an automated line developed at Imperial College of London. Each powdered sample was dropped in 105% phosphoric acid, at 90 °C, and reacted during 10 min. The reactant CO2
was separated by using a poropak-Q column and transferred into the bellows of a Thermo Scientific MAT 253 mass spectrometer (Thermo Fisher GmbH, Bremen, Germany). The mass spectrometric analysis of a single replicate consisted of 8 acquisitions in dual inlet mode, with 7 cycles per acquisition. Post-acquisition processing was completed with the Easotope software [67
values were corrected for isotope fractionation during phosphoric acid digestion, employing a phosphoric acid correction of 0.082‰, at 90 °C, for calcite [68
]. The data were corrected for non-linearity by applying the pressure baseline method (PBL) [69
] and projected into the absolute reference frame of [71
]. Carbonate δ18
O values were calculated with the acid fractionation factors of Reference [72
]. Samples were measured three times, and the average result was converted to temperatures, using the calibration method of Reference [73
]. The calculated δ18
O values of the fluid are expressed in ‰ with respect to the VSMOW standard (Vienna Standard Mean Ocean Water).
For 87Sr/86Sr analyses, two samples of 100% calcite cement and carbonate host rocks were analyzed. Samples were fully dissolved in 5 mL of 10% acetic acid and introduced in an ultrasonic bath for 15 min. After this time, samples were centrifuged during ten minutes, at 4000 rpm, and dried. The resultant product was reacted in 1 mL of 3 M HNO3 and dried. Finally, the sample was digested again in 3 mL of 3 M HNO3 and introduced in chromatographic columns. For the chromatographic separation of Sr, an extraction resin type SrResinTM was used (Trisken International) (crown-ether (4.4’(5’)-di-t-butylcyclohexano-18-crown-6). The Sr was recovered with HNO3 0.05 M as eluent. The fraction where Sr is concentrated was dried, charged on a Re single filament with 1 μL of H3PO4 1 M and 2 μL of Ta2O5, and analyzed on a TIMS-Phoenix mass spectrometer (Isotopx, Cheshire, UK). The acquisition method of data consisted of dynamic multicollection during ten blocks of 16 cycles, each one with a beam intensity in the 88Sr mass of 3 V. Analyses were corrected for possible interferences of 87Rb. 87Sr/86Sr ratios were normalized with respect to the measured mean value of the ratio 86Sr/88Sr = 0.1194 of the isotopic standard NBS-987, in order to correct possible mass fractionation during filament charge and instrumental analyses. The precision of the analytical standard error or internal precision is 0.000003.
Carbon-coated polished thin sections were used to analyze major, minor, and trace element concentrations on a CAMECA SX-50 electron microprobe (CAMECA SAS, Gennevilliers, France). The microprobe was operated, using 20 kV of excitation potential, 15 nA of current intensity, and a beam diameter of 10 µm. Analytical standards included natural silicates, carbonates, and oxides as follows: calcite (Ca), dolomite (Mg), Fe2O3 (Fe), rhodonite (Mn), and celestite (Sr). The detection limits were 135 ppm for Mn, 127 ppm for Fe, 102 ppm for Ca, 185 ppm for Mg, and 403 ppm for Sr. Precision on major element analyses averaged 0.64% standard error at 2σ confidence levels.
To determine the rare earth element and yttrium content (REEs + Y) of calcite cement and carbonate host rocks, four samples were analyzed by means of high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS), using a Thermo Scientific model Element XR (Thermo Fisher Scientific, Bremen, Germany). Up to 100 mg of powder was sampled from trims, using a 400/500 μm diameter dental drill. Powdered samples were dried at 40 °C during 24 h, and later, 100 mg of sample was acid digested in closed PTFE vessels, with a combination of HNO3
+ HF + HClO4
(2.5 mL: 5 mL: 2.5 mL v
). The samples were evaporated, and 1 mL of HNO3
was added, to make a double evaporation. Finally, the sample was re-digested and diluted with Milli-Q water (18.2 MΩcm-1) and 1 mL of HNO3
, in a 100 mL volume flask. In order to improve the sensitivity of the ICP-MS, a tuning solution containing 1 µg L−1
Li, B, Na, K, Sc, Fe, Co, Cu, Ga, Y, Rh, In, Ba, Tl, and U was used, and as internal standard, 20 mg L−1
of a monoelemental solution of 115
In. Reference materials were the BCS-CRM number 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 by 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° 393 for major oxides and JA-2 for trace elements). The analyzed elements and their detection limits expressed in ppm are as follows: La (0.21), Ce (0.32), Pr (0.04), Nd (0.15), Sm (0.03), Eu (0.01), Gd (0.02), Tb (0.003), Dy (0.02), Y (0.20), Ho (0.003), Er (0.01), Yb (0.01), and Lu (0.01). The detection limit was calculated as three times the standard deviation of the average of ten blanks. A Multielemental Solution IV-CCS-1 Rare Earths Standard in HNO3
, 125 mL (100 µg mL−1
) of Inorganic Ventures, was used in order to perform the calibration curves. REEs and Y data were normalized to the Post-Archean Australian Shale (PAAS) from Reference [74
Sulphur and oxygen isotopes were analyzed in two celestite samples to establish the origin of the precipitating fluid. The CO and SO2
gases produced from the samples were analyzed in a continuous flow elemental analyzer Thermo Delta Plus XP mass spectrometer (Thermo, Bremen, Germany), with a TC/EA pyrolizer for δ18
O and a Finnigan MAT CHNS 1108 analyzer (Finnigan, Bremen, Germany) for δ34
S. The results were calibrated with the international standards NBS-127, SO-5, and SO-6 [75
], and the internal standard YCEM (+12.78‰ CDT). The analytical error was ±0.4‰ CDT (Canyon Diablo Troilite) for δ34
S and ±0.5‰ VSMOW for δ18
The integration of petrographic, crystallographic, vibrational, and geochemical methods allowed us to decipher the origin of fracture-filling rose-like clusters of bladed calcite crystals in the northern sector of the Cadí thrust sheet (SE Pyrenees).
Optical and electron microscope observations, together with X-ray diffraction measurements, have allowed us to characterize the morphology and orientation of the studied calcite crystals. Petrographic observations of bladed calcites also indicate the presence of primary and secondary fluid inclusion assemblages (FIA1 and FIA2). Raman spectroscopy of FIA1 and FIA2 show several features that can be attributed to vibrational modes of the aromatic hydrocarbons and the benzene group. However, Raman peaks corresponding to the range of vibrations of the group the alkanes are also observed.
Clumped isotopes thermometry reflects that bladed calcite precipitated from meteoric fluids at 60–65 °C. The very low Mn and Fe content and the presence of negative Ce anomalies measured in this calcite indicate oxidizing conditions during its precipitation. High Sr content in bladed calcite suggests the interaction of fluids with celestite concretions developed within the carbonate member of the Corones Formation. The similar 87Sr/86Sr ratios of the Eocene marine carbonates and the studied bladed calcite (0.70789 and 0.70790, respectively), together with the Y/Ho ratios ranging from 20.94 to 41.88 and the Mg/Ca, Sr/Ca, and Mn/Ca ratios within the range of formation waters calculated for bladed calcites, also indicate interaction between meteoric fluids and the Corones Formation. The high Sr content in calcite, the presence of organic compounds within hydrocarbons trapped in fluid inclusions, and pH controlled the precipitation of bladed crystals with (104) rhombohedral faces. The presence of ‘nailhead’ calcites in non-cemented spaces by bladed crystals probably indicates later migration of shallow fresh groundwater with a lower salinity than that of bladed calcite.
The integration of the obtained results and their comparison with bladed calcites from other fluid systems worldwide reveal that the fracture-filling rose-like calcite clusters studied in the northern side of the Cadí thrust sheet precipitated due to a CO2 release by boiling meteoric fluids that mixed with benzene and aromatic hydrocarbons at ~60–65 °C. This mixture controlled the decrease of the boiling point at these temperatures.