Abstract
Fracture-filling rose-like clusters of bladed calcite crystals are found in the northern sector of the Cadí thrust sheet (SE Pyrenees). This unusual calcite crystal morphology has been characterized by using optical and electron microscope, X-ray diffraction, Raman spectroscopy, δ18O, δ13C, 87Sr/86Sr, clumped isotopes, and major and rare earth elements + yttrium (REEs + Y) analysis. Petrographic observations and powder X-ray diffraction measurements indicate that these bladed crystals are mainly made of massive rhombic crystals with the conventional (104) faces, as well as of possibly younger, less abundant, and smaller laminar crystals displaying (108) and/or ( 1 ¯ 08) rhombic faces. Raman analysis of liquid fluid inclusions indicates the presence of aromatic hydrocarbons and occasionally alkanes. Clumped isotopes thermometry reflects that bladed calcite precipitated from meteoric fluids at ~60–65 °C. The 87Sr/86Sr ratios and major elements and REEs content of calcite indicate that these fluids interacted with Eocene marine carbonates. The presence of younger ‘nailhead’ calcite indicates later migration of shallow fresh groundwater. The results reveal that rose-like calcite clusters precipitated, at least in the studied area, due to a CO2 release by boiling of meteoric waters that mixed with benzene and aromatic hydrocarbons. This mixing decreased the boiling temperature at ~60–65 °C. The results also suggest that the high Sr content in calcite, and probably the presence of proteins within hydrocarbons trapped in fluid inclusions, controlled the precipitation of bladed crystals with (104) rhombohedral faces.
Highlights
The geochemical composition of calcite cement provides information about the type, source, temperature, and age of the fluids from which this mineral precipitated [1,2,3,4,5]
We present an example of fracture-filling rose-like calcite crystal clusters of bladed calcites cropping out in the Southeastern Pyrenees
The methodology employed here is similar to that used in Reference [63], where platy calcite crystals were measured on a powder X-ray diffraction (XRD) mount with the short-length axis perpendicular to the plate
Summary
The geochemical composition of calcite cement provides information about the type, source, temperature, and age of the fluids from which this mineral precipitated [1,2,3,4,5]. The geochemical composition of adjacent host rocks records the degree of fluid–rock interaction [6,7,8,9] Physical parameters such as temperature, ion saturation, and pressure during mineral precipitation have a strong influence on the morphology of calcite crystals [10,11,12,13,14], which is controlled by pH of fluids and the presence of organic compounds [15,16]. Fluid systems are complex, and the conditions at which these processes take place can change due to the input of exotic fluids [25,26,27] To identify these changes, petrographic observations, coupled with crystallographic and geochemical analyses, are needed
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