Submicron Raman spectroscopy mapping of serpentinite fault rocks

Abstract

Submicron Raman spectroscopy mapping is able to unambiguously distinguish the main serpentine minerals within their in situ microstructural context. The high spatial resolution (~370 nm), large‐area coverage (up to hundreds of micrometres in each dimension), and ability to map directly on polished thin sections allows novel interpretations to be made regarding the nature and evolution of serpentinite fault rock textures. The potential of this method is illustrated by examining submicron‐scale textures of scaly serpentinites (e.g., dissolution seams, mineral growth in pressure shadows, distribution, and intergrowth of serpentine minerals) from a lithospheric‐scale shear zone in New Zealand and a subduction‐related serpentinite body in California, USA.

Distinguishing the Raman spectrum of polygonal serpentine

“…Submicron Raman spectroscopy mapping was performed on an Alpha 300R+ confocal Raman microscope (WITec GmbH, Ulm, Germany) in the Chemistry Department at the University of Otago, New Zealand. A dry 100× objective (Zeiss), 1,200 g mm −1 grating and a 532‐nm laser (Coherent, Santa Clara, California) at 25 mW were used, resulting in a laser spot size of approximately 370 nm . A nano‐piezo stage was used to control the sample position during the Raman mapping process.…”

“…Raman spectroscopy has previously been used for rapid and cost‐effective identification of the most common serpentine minerals: lizardite, chrysotile, and antigorite . The high‐wavenumber region (3,600–3,710 cm −1 ) of the Raman spectra has been shown to be the most useful for distinguishing these serpentine mineral types . In this region, the well‐defined Raman spectra arise from inner and outer hydroxyl vibrations, due to the occurrence of Mg (OH) 2 brucite‐like octahedral sheets.…”

“…In this region, the well‐defined Raman spectra arise from inner and outer hydroxyl vibrations, due to the occurrence of Mg (OH) 2 brucite‐like octahedral sheets. Unsurprisingly, the symmetry and energies of these vibrations are sensitive to the different crystal structures of the various serpentine minerals . In particular, lizardite has planar TO layers, antigorite has modulated TO layers with periodic inversion of the tetrahedral sheet, and chrysotile has rolled TO layers, resulting in a cylindrical/fibrous habit .…”

“…To address whether polygonal serpentine has a distinct Raman spectrum, we performed high‐spatial resolution Raman mapping using methods recently introduced by Rooney et al, which allow for mapping at near diffraction‐limited resolution (~370 nm) . Raman mapping was performed directly on electron‐transparent regions of serpentinite samples that were characterized in detail using transmission electron microscopy (TEM).…”

Recent advances in linear and nonlinear Raman spectroscopy. Part XIII

“…Rooney et al reported submicron Raman spectroscopy mapping of serpentinite fault rocks. The potential of their method is illustrated by examining submicron‐scale textures of scaly serpentinites from a lithospheric‐scale shear zone in New Zealand and a subduction‐related serpentinite body in California, USA . Secchi et al described mineralogical investigations using X‐ray powder diffraction, X‐ray fluorescence, and Raman spectroscopy in a combined approach.…”