Host: The Japanese Association of Mineralogists, Petrologists and Economic Geologists
Cathodoluminescence (CL) is luminescence induced by electron bombardment. Detectable variations of CL wavelength and intensity, due to non-stoichiometry, structural imperfection or impurities in quartz and/or other crystals give insights into many geological processes. The aim of this study is to infer deformation and fracture characteristics in igneous quartz, by trace element and microdefect analysis, using a combination of SEM-CL, SIMS, EPMA, TEM and fluid inclusion microthermometry.Double polished (100mm thick) quartz sections were prepared for samples from the Takidani Granodiorite, for SEM-CL, SIMS, EDX and fluid inclusion analysis.Most quartz samples contain abundant liquid- and vapor-rich (secondary) fluid inclusions, with homogenization (Th) temperatures that range from 200 to 400°C. The salinity of most of the liquid-rich inclusions varies from 0 to 20wt% NaCl eq., and up to 60wt% NaCleq. for boiled fluids. High temperature (>600°C), hypersaline, halite-bearing fluid inclusions (∼40wt% NaCl) of magmatic origin were also trapped.CL observation of igneous quartz crystals from the Takidani Granodiorite reveals evidence of crystal zonation, dissolution, re-crystallization, fracturing and crack healing. The quartz characteristically shows euhedral crystal zonation, but also crystal rounding indicative of resorption. Most quartz crystals contain CL-dark luminescent lines, corresponding to healed cracks, which could not be readily detected using other observational tools (e.g. optical microscopy), with a well-defined alteration halo around the healed cracks.SIMS trace element analysis indicates that the alteration halo is enriched in sodium, potassium and magnesium. In addition, trace element analysis using EDX indicate that some of the healed cracks were sealed by aluminum-rich quartz. The deformation and fracturing history of the Takidani Granodiorite indicate quartz crystallization was a late-stage process, with crystal growth filling voids between plagioclase, orthoclase and accessory minerals. Fluid inclusions were trapped due to fracturing of the granodiorite at ∼400°C, based on the fluid inclusion (Th) data, which also marks the low temperature limit for brittle-ductile transition conditions in the Takidani Granodiorite. Chemical interactions occurred as a consequence of hydrothermal fluids moving through the cracks, leading to a sodium, potassium and magnesium-rich halo in the host rock, and crack healing. The occurrence of aluminium-rich quartz-healed microcracks, crosscutting alteration halos associated with other healed microcracks highlights the relative timing of microcrack generation in the igneous quartz.The application of CL observational techniques, complemented by SIMS/EDX analysis is useful for understanding hydrothermal quartz vein formation, and our experimental study shows the approach is also effective for deducing crystallization processes of igneous quartz and deformational history of granitoid intrusions.
