Large area EBSD mapping of quartz layers sheared around rigid porphyroclasts

Introduction

Microstructural information in geological samples can be gathered and imaged using an SEM equipped with an EBSD detector and combined with specialist software. EBSD enables the identification of mineral phases based on their particular crystallographic characteristics and allows the orientation of mineral grains to be determined. Large-area EBSD mapping, with Oxford Instruments' AZtec® software, allows large quantities of information relating to microstructural deformation to be obtained, enabling the history and evolution of a geological sample to be revealed. This study investigates the microstructural response of quartz layers to flow around rigid secondary garnet porphyroclasts during ductile deformation.

Large Area Mapping

The quartz-rich mylonites analysed here are from the Alpine Fault Zone of New Zealand (Fig. 1). Due to their location in an active fault zone, they have been subject to high levels of shear strain. The mylonites, which contain large garnet porphyroclasts of up to ~5 mm, were analysed in thin section over a broad area of approximately 30 x 5 mm using the large area EBSD mapping tool in AZtec.

Two samples were analysed using an Oxford Instruments' NordlysF EBSD camera fitted to a Zeiss Sigma VP FEG-SEM and operated at a 30 kV accelerating voltage. One sample (STO-2-03, shown below) was mapped using an Oxford Instruments' NordlysNano detector fitted to a FEG-SEM and operated with a 20 kV accelerating voltage. The automated stage movement, controlled through AZtec, allowed the automatic collection of data from up to 150 frames and, in this particular example, at magnifications of up to 300x.

For each frame, EBSD patterns were acquired and automatically indexed in real-time, using a fixed step size of between 2 and 5 µm. During analysis, the frames were automatically montaged together to create a large area map. The large area EBSD map generated by AZtec was processed to investigate the orientation of the mineral grains within the sample and the distribution of grains with different sizes. Of particular interest was the microstructural response of the quartz layers to flow around the rigid garnet porphyroclasts.

Fig. 1. Map of New Zealand showing location of the Alpine Fault.

Fig. 2. (Top) A band contrast image of sample STO-2-03 which has been divided into sub-regions with contoured pole figures (equal area, lower hemisphere) shown for each. (Bottom) A grain shape orientation plot of the quartz grains in the mapped layer. The different colours represent the angle between each grains long axis and the foliation plane (horizontal).

Results and Interpretation

The results of EBSD analysis over a large area reveal marked asymmetry in the grain shapes, sizes, and crystallographic preferred orientation of quartz grains around the garnet porphyroclasts, indicating a dominant non-coaxial shear component. The plotting of C-axis pole figures reveals a dominance of top-to-the-west simple shear with a smaller pure shear component. An increase in fabric strength and the decrease in grain size towards the point of maximum quartz layer deflection around the garnet porphyroclast were observed from the large area map, and indicate that dislocation creep is a dominant process. It is apparent that the concentration of stress around these rigid porphyroclasts has reduced the size of the quartz grains and increased the fabric strength. The quartz grains with the largest grain sizes and weakest grain shape alignment are found directly 'upstream' of a garnet porphyroclast.

Conclusion

This study has demonstrated that Large Area EBSD Mapping in AZtec is a powerful tool in the collection of microstructural information from a large area of a sample. This allows the microstructural response of minerals to strain to be imaged and analysed over a relatively large area. In this particular study AZtec has been used to gain a greater understanding into shear zone kinematics in geology.

Oxford Instruments thanks Andrew Cross and Professor David Prior of the University of Otago for their contribution to this note.