Examination of Inclusions Within Archean Age Zircons in the Scanning Electron Microscope

Julia M. Nelson

University of Rochester

Department of Earth and Environmental Sciences

OPT 307/407: Practical Electron Microscopy

Spring 2009 ~ Final Project

Introduction and Background

Studies of Earth's earliest magnetic field can provide insight into the evolution of Earth's core. The Barberton area of South Africa contains some of the oldest rocks that could provide evidence for Earth's earliest magnetic field. These rocks contain zircons, an accessory mineral that might host inclusions capable of recording Earth's magnetic field at the time of their formation. Zircons have been used in the past for age-dating. The zircons collected from the Ancient Gneiss Complex (AGC) of the Barberton area have been dated as ~3.4 billion years old. It is around this age that scientists believe that the magnetic field may have "started up." Previous testing conducted in an Alternating Gradient Force Magnetometer (AGFM) has found that these zircons have an extremely weak magnetic moment which may be due to the relative rarity of inclusions and the size of the zircons themselves, ~200 m. While the magnetic moment may be weak, the zircons' inclusions could contain single domain carriers, such as magnetite that are ideal magnetic recorders.

Goal

The goal of this project was to examine the zircon crystals using the scanning electron microscope (SEM) to determine the elemental makeup of the inclusions within. By taking micrographs using the secondary electron detector (SE2) and the backscatter detector (BSD), the inclusions were able to be located and documented. In order to determine what elements made up the inclusions, x-ray analysis and elemental mapping were used.

Method and Materials

A group of AGC zircons were viewed under a light microscope and nine were chosen to be viewed in the SEM due to their classic zircon dipyramidal shape. Below are two of the zircons that were chosen to be viewed in the SEM.

zircon lm

In order to view the zircons in the SEM, they first had to be set in epoxy on a glass slide and polished to allow the side of each zircon to be flush with the surface of the epoxy. After this, a conductive circle was painted surrounding the group of zircons using a conductive graphite adhesive. The slide was then sputter coated in gold for 30-40 seconds. Below is a picture of the slide mounted on a sample stub before being placed in the SEM. The nine zircons are within the circle on the slide.

slide

Once in the SEM, a micrograph of each zircon was taken using the SE2 detector at 15kV. The SE2 images below are of zircon #3 and #4, respectively.

zircon 3 zircon 4

The zircons were then viewed using the backscatter detector (BSD) to determine if the inclusions seen using the SE2 detector were of a higher or lower atomic number than the surrounding zircon. This would mean that the inclusions were made not of ZrSiO2, but another material, possibly magnetite. The BSD images below are of zircons #3 and #4, respectively. The darker parts within the lighter zircon are the inclusions.

BSD zr 3 bsd zr 4

Here are colorized versions of the two micrographs above.

color bsd 3 color bsd 4

After viewing the AGC zircons using the SE2 and BSD detectors, x-ray analysis was used to help determine what the inclusions are made of. Spectra were taken using the ZAF32 program and elemental mapping was conducted using the IDX32 program. The EDAX detector was used in this part of the procedure. Elemental data collected can be found in the results section of this page.

Results

Four out of the nine AGC zircons examined in the SEM appear to have inclusions that contain the element iron (Fe). This could be representative of magnetite inclusions. The element Titanium (Ti) was also mapped because this could also be helpful in locating magnetite in the form of titano-magnetite.

Elemental maps of AGC zircon # 3: SE2 micrograph (far left), Zr map (left), Fe map (middle), Ti map (right)

zircon 3 whole map zr 3 map fe 3 map ti 3

Elemental mapping of AGC zircon # 4: SE2 micrograph (far left), Zr map (left), Fe map (middle), Ti map (right)

zircon 4 map zr 4 map fe 4 map ti 4

Elemental mapping of AGC zircon # 5: SE2 micrograph (far left), Zr map (left), Fe map (middle), Ti map (right)

zircon 5 whole map zr 5 map fe 5 map ti 5

Elemental mapping of AGC zircon # 7: SE2 micrograph (far left), Zr map (left), Fe map (middle), Ti map (right)

zircon 7 whole map zr 7 map fe 7 map ti 7

Here is an example of a spectrum taken of zircon # 4. More spectra will be collected on April 28th, 2009.

zr 4 spectra

Conclusions

It was found through elemental mapping that there were indeed inclusions of iron within the AGC zircons. While all of the AGC zircons examined contained inclusions, only half of those zircons held Fe-bearing inclusions. Mapping of the non Fe-bearing inclusions found them to contain calcium and aluminum. The presence of aluminum may be due to the polishing solution used to polish the slide before viewing. Further testing outside of the SEM will need to be conducted to further test the inclusions for possible magnetite inclusions and their Natural Remanent Magnetization (NRM).

Acknowledgments

Much thanks to Brian McIntyre for all his help, time, and humor and to Andreas Liapis for all his help during lab and SEM time. Also to John Tarduno and Rory Cottrell for help with paleomagnetism. Lastly, to Julia Voronov and Amanda Carey for all their support in and out of lab.

 

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