Electron Microscopy of Magnetic Carriers in ~3.4 billion-year-old Cherts From the Barberton Greenstone Belt of Southern Africa

Julia Voronov

University of Rochester
Department of Earth and Environmental Sciences

OPT407: Practical Electron Microscopy
Spring 2008
Final Project

Introduction

Methods
Sample Preparation
Scanning Electron Microscopy
Transmission Electron Microscopy

Results
Scanning Electron Microscopy

Backscatter Detector with Elemental Analysis and Quantification
Backscatter Micrographs and Elemental Mapping of Thin Sections
Secondary Electron Micrographs and Elemental Mapping of Thin Sections
Electron Flight Simulation
Scanning Transmission Electron Micrographs and ImageJ Analysis
Transmission Electron Microscopy
TEM Micrographs
Scanning Transmission Electron Micrographs with Elemental Spectrum
TEM Micrograph of a Chain from a Modern Organism

Conclusions

Acknowledgements

References

 

Introduction

The goal of this project is to use microscopy to gain a better understanding of the magnetic particles of the ~3.4 billion year old Buck Reef Cherts of the Barberton Greenstone Belt, South Africa, since they magnetofossils, a product of magnetotactic bacteria, may be associated with these units. If so, this biogenic magnetite may be among some of the oldest microfossils. The existence of magnetofossils from magnetotactic bacteria is likely to indicate an existence of a magnetic field on Earth since it is unlikely an organism would expend extra energy to create magnetic particles without the needing to use the magnetic field. The magnetic field is currently known to exist at 3.2 billion years ago [5], so if magnetosomes are found within the 3.4 billion-year-old Buck Reef Chert, then they would indicate an earlier presence of a magnetic field.

The magnetic minerals of interest are from magnetotactic bacteria that synthesize magnetic particles with specific morphological shapes and narrow size ranges that fall within the single domain range [1;2]. See colorized TEM micrograph above for example of a chain formed by a magnetotactic organism.

 

Methods

Sample Preparation

Thin sections of the Buck Reef Chert were cut and sputter coated with about 40 Angstroms of gold to produce a conductive surface. In order to view the individual, small magnetic particles from the cherts, magnetic material had to be separated from the whole rock sample. The chert samples were crushed and placed in a vial with water. A test tube with a rare earth magnet was placed in the vial so any magnetic material would be attracted to the side of the test tube. That test tube was them removed and placed in a new vial of water. The samples were then placed on lacy carbon TEM grids.

Scanning Electron Microscopy

  • The thin sections of Buck Reef Cherts were analyzed in the Zeiss SUPRA 40VP Scanning Electron Microscope (SEM) using the backscatter detector with an accelerating voltage of 15kV.
  • Energy Dispersive Spectrometer (EDS) analysis using the EDAX detector and quantification software was used to analyze the particles in the thin section with the higher atomic number (brighter particles in the micrographs).
  • Elemental mapping was also performed on the thin sections to view the elemental composition across an area. This was useful to see the abundance and composition of elements across an area rather than one location.
  • The Everhart-Thornley secondary electron detector was used to view depressions or holes in the thin sections. An elemental spectrum was collected of an empty hole. An elemental mapping was performed on a depression which appeared to contain material within it.
  • Since the thin sections observed in the SEM are made of glass, the Electron Flight Simulation software was used to observe how the electron beam would interact with glass.
  • Magnetic separates were viewed with the Scanning Transmission Electron Microscope (STEM) detector in the SEM. Particles from STEM micrographs were measured using the ImageJ program, Version 1.41 [4].

Transmission Electron Microscopy

  • Magnetic separates were also viewed in the Transmission Electron Microscope (TEM). In order to view different faces of a crystal, the stage was tilted 16.15 degrees.
  • The STEM detector with an EDAX detector were used view the magnetic separates and collect elemental spectra.
  • Samples of modern magnetotactic organisms collected from the sediment-water interface from a local pond were viewed with the TEM.

 

Results

Backscatter Micrographs with Elemental Analysis and Quantification

In the thin sections, contrast is clearly visible between the dark areas indicating the presence of chert (silica) and the bright areas. The bright areas in the micrographs form laminations in the chert. The bright areas were analyzed using the EDS to view the elemental compositions.

The spectra collected from the bright area in this micrograph shows peaks of sulfur, oxygen, iron, and silica. Quantification is a useful program since it estimates weight and atomic percentages of elements seen in a x-ray spectrum. The program fits the spectrum peaks and background to calculate percentages. Since estimating the weight percentages by simply viewing the spectrum can lead to incorrect assessments, this method allows for more accurate weigth percentages. For example, in the figures below, the x-ray spectrum shows that the silica peak is higher than the oxygen peak, however the calculated weight percentage is higher for oxygen.

 

 

Backscatter Micrographs with Elemental Mapping of Thin Sections

Backscatter micrographs, with their corresponding elemental mappings are shown below. Particles could be distinguished since backscatter yield increases with atomic number, so particles composed of higher atomic number elements (ie. iron) appear brighter in the micrograph. The abundance of silicon and oxygen is due to the siliceous composition of the chert. The bright areas in the micrographs form laminations and correspond to the iron abundances in the mappings and the dark areas correspond to carbonaceous matter contained within the Buck Reef Chert.

 

 

Secondary Electron Micrographs and Elemental Mapping of Thin Sections

Secondary electrons are produced by inelastic collisions when the beam electrons collide with sample electrons and lose energy. Secondary electrons have more of an interaction volume closer to the surface so topographical structures can be seen. Within the thin sections, there appeared to be holes or depressions (see micrographs below).

While using the SE2 detector, some of the depressions appeared to be empty holes, (see figure below). The x-ray spectrum shows peaks of silicon and oxygen, indicating just the presence of chert.

 

While using the SE2 detector, other depressions appear to contain material (figure below). Elemental mapping shows that carbonaceous matter is contained in many of the depressions of the siliceous chert surrounding.

 

Electron Flight Simulation

Since the thin sections observed in the SEM are made of glass, the Electron Flight Simulation software was used to observe how the electron beam would interact with glass. The blue and green lines show the projected paths of the electrons within the sample and the red line represents the electron beam. With an acceleration voltage of 15kV, the interaction volume penetrates only to about 5 microns.

 

Scanning Transmission Electron Micrograph and ImageJ Analysis

The STEM detector allows for high resolution imaging of the magnetic separates with the SEM. The micrograph below shows magnetic particles from the Buck Reef Chert

ImageJ [4] was used to measure the lengths and widths of various particles. The length versus shape factor (length/width) can be plotted on the stability field diagram below [3]. Many of the grains imaged with the STEM detector fell within a stable single-domain range, typical of biogenic magnetite.

Transmission Electron Micrographs

TEM micrographs show Buck Reef Chert magnetic separates. Many crystals also fall within the size range typical of biogenic magnetite

 

Tilting the stage while viewing the sample in the TEM allows for observing the 3D structure of crystals in different planes. The red arrows in the micrographs (see figures below) show the the change in the crystal morphology with the rotation of 16.15 degrees. For example, the top arrow in the micrographs points to a slightly elongated crystal, but with a rotation, the crystal appears to have an equidimensional shape. Rotating the stage can be useful in order to view particles which may be blocked by others; the lower arrows point to a crystal where the shape is clearly more visible in the micrograph on the left.

 

 

STEM Micrograph with Elemental Spectrum

The STEM detector allows for high resolution imaging of the magnetic separates with the TEM. The micrograph below shows a bright area of the magnetic separates. X-ray elemental analysis shows peaks of iron, silica, and oxygen, possibly indicating the presence of iron oxides.

TEM Micrograph of a Chain in a Modern Organism

Samples of modern magnetotactic organisms collected from the sediment-water interface from a local pond were viewed with the TEM. A chain structures unique to magnetotactic organisms, of particles within the biogenic size range are shown in the micrograph below.

Conclusions

Thin sections of the Buck Reef Chert viewed with the backscatter, SE2, and EDAX detectors show laminations that contain iron, sulfur, and oxygen, indicating the presence of iron oxides and iron sulfides. Magnetic separates imaged in the SEM with the STEM detector, and those in the TEM contain particles that fall within a stable single-domain size range, typical of biogenic magnetite. Imaging of modern magnetotactic organisms will be useful for future research and understanding of the biogenic particles. The Buck Reef Chert appears to contain a mixture of secondary magnetic minerals, but some may also possibly be primary biogenic grains. More work is needed to trace the origin, structure, and composition of the magnetic particles.

Acknowledgements

Thanks to Brian McIntyre for his knowledge and for teaching us everything, and to Andreas Liapis for his patience, coffee, and help in the lab. Also thanks to John Tarduno and Rory Cottrell for help with paleomagnetism

References

1. Bazylinski, DA & Frankel, RB (2003) Biologically Controlled Mineralization in Prokaryotes.Reviews in Mineralogy and Geochemistry 54(1): 217-247.
2. Butler, RF (1992) Paleomagnetism: Magnetic Domains to Geologic Terranes. Blackwell Scientific Publications, Oxford.
3. Butler, RF & Banerjee, SK (1975) Theoretical single-domain grain size range in magnetite andtitanomagnetite. Journal of Geophysical Research 80: 4049-4058.
4. Rasband, WS (1997-2008) ImageJ. U.S. National Institute of Health, Bethesda, Maryland, USA.http://rsb .info.nih.gov/ij/.
5. Tarduno, JA, Cottrell, RD, Watkeys, MK, & Bauch, D (2007) Geomagnetic field strength 3.2 billion years ago recorded by single silicate crystals. Nature 446(7136): 657-660.

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