Origin and timing of Dauphine twins in quartz cement in fractured sandstones from diagenetic environments: Insight from fluid inclusions

Electron backscattered diffraction techniques (EBSD) show that Dauphine twins in quartz are widespread in many tectonometamorphic environments. Our study documents that under diagenetic temperatures (<200 degrees C) and burial depths <5 km Dauphine twins are common in isolated fracture quartz deposits spanning between fracture walls (i.e., quartz bridges) in low-porosity quartz-cemented sandstones. Using examples from East Texas and Colorado cores, we show that twins are associated with crack-seal microstructure and fluid inclusions. Fracture wall-parallel and wall-normal inclusion trails contain coexisting aqueous and hydrocarbon gas inclusions, so homogenization temperatures of aqueous inclusions record true trapping temperatures. Inclusions in alignments normal to fracture walls are large and irregularly shaped compared to those aligned parallel to walls, but both show similar liquid-to-vapor ratios. Stacking transmitted light images with scanning electron microscope cathodoluminescence (SEM-CL) and EBSD images demonstrates that Dauphine twin boundaries are localized along wall-normal inclusion trails. Trapping temperatures for wall-normal inclusion trails are usually higher than those aligned parallel to the fracture wall. Wall-normal fluid inclusion assemblage temperatures typically match the highest temperatures of wall-parallel assemblages trapped during sequential widening, but not necessarily the most recent. In context of burial histories for these samples, this temperature pattern implies that wall-normal assemblages form at discrete times during or after crack-seal fracture widening. Localization in isolated, potentially high-stress quartz deposits in fractures is compatible with a mechanical origin for these Dauphine twins. Punctuated temperature values and discrepant sizes and shapes of inclusions in wall-normal trails implies that twinning is a by-product of the formation of the wall-normal inclusion assemblages. The association of Dauphine twins and fluid inclusion assemblages from which temperature and possibly timing can be inferred provides a way to research timing as well as magnitude of paleostress in some diagenetic settings. (C) 2016 Elsevier B.V. All rights reserved.