QUARTZ IN A NEW LIGHT: 3D SECOND HARMONIC GENERATION (SHG) MICROSCOPY OF MICROSTRUCTURES WITH SUB-MICROMETER RESOLUTION

Second harmonic generation (SHG) is a nonlinear optical process that occurs when non-centrosymmetric materials are probed with high-energy laser excitation. The process was discovered in quartz in 1962 and it occurs when two photons at the frequency of the laser excitation combine to emit a third photon with twice the frequency of the excitation. In the last 20 years, SHG microscopy has been widely used in biomedical research to image the distribution of non-centrosymmetric materials like collagen at the cellular level. During nonlinear optical microscopy experiments to characterize organic matter in sedimentary, metamorphic, and igneous rocks (Burruss et al., in press, Geology) we discovered that SHG images a wide range of microstructures in quartz in 3D with sub-micrometer resolution. These structures include fluid inclusions, Dauphiné and Brazil twins, low-angle grain boundaries, and veil-like features of unknown origin. SHG is a second-order nonlinear process that causes the intensity of the SHG signal to be a quadratic function of the power in the laser excitation. Therefore, in the tight focusing geometry of high numerical aperture microscope objectives (N.A. > 1.0), the signal at any point is generated only within the diffraction-limited focal volume with a diameter of about 300 nm and axial length of 1100 nm. Digital images are acquired as a Z stack of X-Y raster scans with typical image volumes of 350 x 350 x 100 μm. We have imaged fluid inclusions in quartz from a wide range of geologic environments: Dauphiné and Brazil twins in fracture-filling quartz from deep-basin shale gas systems and open-space filling quartz from an epithermal gold deposit, Dauphiné twins in grains of a quartz arenite petroleum reservoir, and microcrystalline quartz in mudstones. Images of low-angle grain boundaries associated with fluid inclusions in quartz grains that appear to be single crystals in polarized light provide new evidence on the trapping history of fluids during crystal growth. The well-known association of Dauphiné twinning with deformation of quartz suggests that SHG images of these twins that are not visible in polarized light may be useful for piezometry of deformed quartz-bearing rocks.