Spring 2017 Project


Contents

Introduction
Prep Work
Methods
Samples
X-Ray Analysis
Distinguishing Possible Remanence Carriers
Possible Remanence Carrier
Flight Simulator
Conclusion
Future Work
Acknowledgments
References

Introduction

It has been shown that crystals of silicate minerals can house single domain (SD, .02 - 0.1) and pseudo-single domain (PSD, 0.1 - 2.0 µm)1 grains of magnetite and other magnetic remanence carrying minerals1 . By isolating these silicate hosts, which shield remanence carriers from alteration during thermal experiments, paleointensity studies can be performed and the recorded magnetic field can be determined from these carriers, which can preserve remanences on billion-year time scales.

Typically, optically clear samples are chosen under a light microscope, as these are more likely to contain the SD and PSD grains and not the larger MD grains that may have undergone alteration, making them less suited for paleointensity work. Hysteresis tests are then performed to determine if any carriers exist and whether testing should continue. Six feldspar crystals were selected from a mafic intrusion and through the utilization of a Scanning Electron Microscope (SEM), along with a combination of the following methods, I hope to determine if this testing method can be improved upon.


Prep Work


Methods


Samples

The top portion of each segment is the image collected by using the secondary electron detector on the SEM. The center portion is the image collected by using the backscatter detector on the SEM. The bottom portion is the spectral analysis of the entire grain. Analysis indicates each is a feldspar. Specifically, Sample 1 is most likely an albite, Samples 2 through 4 are alkali feldspars and Sample 5 & 6 are plagioclase feldspars.

       

       

Figure 2: (Top Left) Sample 1. (Top Center) Sample 2. (Top Right) Sample 3. (Bottom Left) Sample 4. (Bottom Center) Sample 5. (Bottom Right) Sample 6.


X-Ray Analysis

Elemental mapping of the grains allows for recognition of the major elemental contributors to each sample. Results indicate x-ray analysis presents a good starting point of where to look within the grain for potential magnetic remnance carriers. Clusters of Fe (iron) pixels appear to correlate with PSD size grains and larger. The high pixel density correlation of Na (sodium) and Cl (chlorine) in Sample 4 is possibly a hydrosaline melt. 3 Other Na and Cl correlations are due to a combination of feldspar composition and the presence of known chlorite in bulk sample

Figure 3: Sample 1

Figure 4: Sample 2

Figure 5: Sample 3

Figure 6: Sample 4

Figure 7: Sample 5.

Figure 8: Sample 6.


Distinguishing Possible Remanence Carriers

Spectral analysis permits discernment of inclusions within the larger feldspar grain by confirming their composition. The brightness of the grains is a function of the average atomic weight of the sample. The brighter grains appear to be zinc sulfide while the dimmer grains appear to be an iron oxide.

Figure 9: Colorized section of Sample 4 chosen for magnetic remanence identification due to abundance of bright grains, indicative of a higher atomic number element present. Left spectra indicate presence of Fe in selected grains. Right spectra indicate presence of Zn in selected grains.


Possible Remanence Carrier

Natural magnetites are known to have irregular shapes4 and even form parallelepiped crystals.3

Figure 10: Section of Sample 4 chosen for magnetic remanence identification. Red box showcases two potential remnance carriers.

Figure 11: Colorized InLens image of highlighted grains in Figure 10. Lengths and widths place grains in PSD size range.


Flight Simulator

Figure 12: Simulations demonstrating the generation and emission of Oxygen (left) and Iron (right) electrons from the interaction volume produced from a 20 kV electron beam striking a particle of hematite (Fe2O3). While electrons of both elements are generated throughout the entire interaction volume, a portion of Oxygen electrons do not escape due to their lower atomic number.


Conclusion

X-Ray analysis/elemental mapping is a good starting point for magnetic remanence carrier identification prior to susceptibility testing. The best results are returned when coupled with BSD images and spectral analysis.


Future Work

Going forward, colloidal silica polishing would be performed to remove surface debris and re-examined to better distinguish between imbedded grains and surface particles. Afterwards, paleointensity measurements would be carried out to confirm particle identities and justify implentation of this procedure.


Acknowledgements

I would like to thank Brian McIntyre for all his help and guidance during this project, Rakan Ashour for the training on the SEM, Tim O’Brien for his aid in sample preparation and Prof. John Tarduno for the opportunity to work with these samples.


References

1. Dunlop, D. J., & Özdemir, Ö. (2001). Rock magnetism: fundamentals and frontiers (Vol. 3). Cambridge university press.
2. Lowenstern, J.B., 2000, A review of the contrasting behavior of two magmatic volatiles: chlorine and carbon dioxide: Journal of Geochemical Exploration v. 69-70, p. 287-290
3. Tarduno, J. A., Cottrell, R. D., Davis, W. J., Nimmo, F., & Bono, R. K. (2015). A Hadean to Paleoarchean geodynamo recorded by single zircon crystals. Science, 349(6247), 521-524. doi:10.1126/science.aaa9114
4. Thomas-Keprta, K. L., Bazylinski, D. A., Kirschvink, J. L., Clemett, S. J., Mckay, D. S., Wentworth, S. J., . . . Romanek, C. S. (2000). Elongated prismatic magnetite crystals in ALH84001 carbonate globules: Potential Martian magnetofossils. Geochimica et Cosmochimica Acta, 64(23), 4049-4081. doi:10.1016/s0016-7037(00)00481-6

Please enter any comments, criticisms, questions, etc. below.

Your name:

Email address: