Significance of Fracture-Filling Rose-Like Calcite Crystal Clusters in the SE Pyrenees

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.