Field notes by Kelsey Wagner

Seismic Anisotropy of the mid to lower crust is very difficult to measure in field by seismologists. Using a scanning electron microscope (SEM) to obtain crystal preferred orientations (CPO’s) of mineral grains from samples taken from this region allows us to find seismic anisotropy theoretically.

 Over the next 8 weeks, obtain velocity measurements and pole figures for 4 slides from the Pelona Schist that are cut parallel to foliation. The methods will be repeated on 3 corresponding slides that are cut perpendicular to foliation and compared with results of those parallel to foliation. Then 2 xenoliths will be analyzed using EBSD that have little to no biotite. Velocities are found using the

June 20, 2011 Biotite, plagioclase and quartz in cross polarized light.  Analyzed thin sections from Pelona Schist and Mojave region in Southern California to obtain modal volumes of minerals present in thin section samples. This reduces miscalculations due to indexing errors by the SEM.

Biotite tends to be an index mineral for determining anistropy. Amphiboles tend to be index minerals for determining anisotropy.  Metagraywackes: Mostly Biotite, plagioclase, and quartz. Some have small amount of hornblende. Some have high amounts of garnet. One is ≥ 60% plagioclase.  Xenoliths are composed of hornblende, actinolite, quartz, plagioclase and sphene.

 Index minerals determine general anisotropy of sample. Their CPO’s and symmetry determine velocity as well.  Biotite and amphiboles have unique hexagonal symmetry meaning they have a unique slow axis.  Having hexagonal symmetry is necessary for seismologists to analyze seismic anisotropy with modern methods. Sample containing large amounts of plagioclase as a unique fast axis.

SEM image. Green cross hairs indicate what section of the slide is being indexed.  Polished thin sections were analyzed with electron back scatter diffraction (EBSD) using an SEM. The slide was tilted 70 degrees incident to the electron sensor ray.  100x magnification was used.  Maps were created using various step sizes.  Standards generally included working distance of ~15.2, min band detection of 6, max -8. 4x4 binning, noise reduction of 3, timing of 80 ms/frame, chamber pressure of 70 Pa.  Slides were prepared by polishing with microdiamond polish and then colloidal silica polish to remove amorphous layer from previous polish.  Slides were not carbon coated and were viewed in low vacuum mode. This prevented electron interference with carbon coating however increased atmospheric interference due to presence of water.

Kikuchi patterns correspond to symmetry of minerals and their orientations. They are used to index the minerals and their CPO’s with the SEM by electron back scatter diffraction. Some background reading

 MAD = Mean Angular Deviation. When indexing minerals manually, the lower MAD the better the fit and more likely that it is actually that mineral and actually in that orientation.  EDS – gives elemental composition of mineral being analyzed. Very useful when multiple results are indexed or there is a poor fit of solution bands. Allows one to deduce exactly what mineral is.

Provide three parameters necessary to describe orientation of a mineral. Euler angles

 Pole figures are stereographic projections of mineral orientations in relation to planes and density of grains oriented in certain directions.  Velocity is found using a Mainprice computer program using the Voigt- Reuse-Hill averaging option that also calculates anisotropy using published mineral elastic constants and shear properties.

 Errors occur in SEM analysis due to chemical etching from colloidal silica polishing and differences in mineral hardness cause some minerals (quartz, plagioclase) to be under polished while biotite gets over polished.  Biotites tend to flow parallel to foliation during metaphorphism since they are sheet silicates. Therefore they were very difficult to index in samples that were cut orthogonal to foliation and had to be found manually to improve calculations.