Advances in submicron Raman spectroscopy mapping of serpentinite fault rocks

We present a series of recent advances in Raman spectroscopy analysis of serpentinite fault rocks. Serpentinite fault rocks often contain complex textural and mineralogical relationships between the various serpentine group minerals: antigorite, chrysotile, lizardite and polygonal/polyhedral/conical serpentine, with intergrowths and replacement textures at the submicron scale. Deciphering these textural relationships can be challenging because conventional techniques such as optical microscopy, XRD and SEM-EDS often fail to correctly identify the serpentine varieties, and lack the necessary spatial resolution. Transmission electron microscopy (TEM) is the only technique that can provide truly unambiguous identification of serpentine minerals, but complex sample preparation and very small sample sizes (c. 200-500 µm²) means that microstructural context is difficult to maintain.

Submicron Raman mapping distinguishes the main serpentine minerals within their microstructural context and can be performed directly on standard thin sections. The high spatial resolution ( 370 nm) over large areas (100's of µm to mm scale) allows the positive identification of serpentine mineral types and reveals delicate textural relationships. We have developed two new Raman-based techniques (Rooney et al. 2018, Tarling et al. 2018): 1) submicron Raman mapping performed directly on TEM grids, and 2) a crystallographic orientation mapping technique to measure c-axis orientations of lizardite, which can be performed if conventional EBSD is unsuccessful. We use these techniques to provide new information on the nature of crack-seal banding in serpentinite slickenfibres, and complex overprinting relationships between serpentine phases in long-lived continental shear zones.