Exploring the tectonic evolution of the Pamir mountains with microscopy

John Bershaw
University of Rochester
Dept. of Earth and Environmental Sciences

OPT407: Electron Microscopy
Spring 2010

1. Introduction
2. Methods
3. Results and Discussion
           3.1 Zircon Grains: Secondary Electrons
           3.2 Rock Thin-sections: Light Microscopy
           3.3 Rock Thin-sections: BSD and X-ray diffraction
4. Conclusion
5. References
6. Acknowledgements


1. Introduction

The Pamir mountains are the western expression of Himalayan deformation related to the collision of India into Eurasia ~55 million years (Ma) ago. One area of my research is focused on the tectonic evolution of the Pamir in relation to surrounding mountains such as the Tibetan plateau. In the fall of 2006, sedimentary rock samples ~150-20 Ma in age were collected near a town called Oytag (Wuyitake) in the eastern Pamir (located within rectangle "B" in Fig. 1). Detrital zircon grains were analyzed in an effort to constrain the age of mountain building. Thin-sections were also made to study changes in provenance, obstensibly related to the growth of mountains.

Figure 1: Area map. Samples collected from area labeled "B"

The aims of this project are two-fold:

1) The composition of sedimentary rocks can be compared to potential source terranes in the Pamir hinterland to determine provenance. I use backscatter electron microscopy (BSD), X-ray spectrometry, and light microscopy to constrain the mineralogical composition of sedimentary rock samples.

2) Zircon grains can be dated and correlated with source terranes of known age. However, age often does not enable one to distinguish between grains that are recycled from older sedimentary rocks versus those derived from primary igneous rocks. Zircon grains already prepared for U/Pb analysis are still encased in epoxy within a plastic cylinder. I propose to analyze these grains in the SEM to determine whether grains are euhedral and/or heavily abraded. The former would likely indicate an igneous origin while the latter might be derived from older sedimentary rocks.

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2. Methods

Both zircon and sedimentary thin section samples were mounted to aluminum stubs using carbon tape. Some samples were coated with gold to ensure conductivity which reduces the "charging" effect resulting in clearer micrographs. That said, copper tape was used on a large portion of a rock thin-section to preserve it for light microscopic work (Fig. 2). Copper tape was also used on the zircon samples embedded in epoxy to connect the gold coating with the aluminum stub beneath (Fig. 3). The instrument used for optical analyses was the Zeiss Supra 40 VP SEM. Stubs were placed in the SEM and sufficient vacuum was achieved. The electron beam was turned on and sample was centered under the beam using standard procedures. Thin-sections were analyzed using both secondary electron mode (SE2) and backscattered electron mode (QBSD) in both coated and uncoated instances under low (~100-200um) and medium (10-20um) magnifications. Thin-sections were also analyzed using a light microscope and digital photos were taken. Zircon crystals embedded in epoxy were scanned in only SE2 mode under low and medium magnifications to characterize crystal morphology.

Figure 2: Sedimentary rock thin-section 90% covered/grounded with copper tape and sputter-coated with gold

Figure 3: Zircon grains embedded in epoxy sputter-coated with gold and grounded with copper tape

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3. Results and Discussion

3.1 ZIRCON GRAINS: Secondary Electrons

The two zircon samples analyzed have very similar age-distributions as determined previously by U-Pb dating of grains. In fact, you can easily see the holes in grains where they were impacted with a laser to liberate material from grains for analysis. Sample PMR-25 comes from an older, Jurassic aged (~150-200Ma) sedimentary unit and PMR-46 is Miocene (~20Ma). Here, we are looking at how weathered grains are to determine whether PMR-46 is an eroded product of PMR-25. Micrographs at various magnifications were produced of PMR-25 (Figs. 4-7) and PMR-46 (Figs. 8-12) for comparison.

Figure 4: PMR-25 A handful of quartz grains sailing a sea of zircon
Figure 5: PMR-25 Zircon grains that have been shot with a laser pistol

Figure 6: PMR-25 A close-up of some of those grains

Figure 7: PMR-25 Do the grains have straight edges or are they weathered?

The first micrograph (Fig. 4) is at low magnification to give perspective. The larger grains running across the sample surface diagonally are quartz. These are used to calibrate zircon grain ages. In Figs. 5-7, it is easy to see the pocket-mark from laser ablation. All micrographs of this sample show fairly euhedral grains, though some wear can be seen along grain edges and at face boundaries.

Figure 8: PMR-46 This sample seems to have fewer zircons

Figure 9: PMR-46 The edges of this crystal are very straight (not so weathered)

Figure 10: PMR-46 This one may be moderately abraded

Figure 11: PMR-46 Here is a close-up of one of the laser pits

Figure 12: PMR-46 An uncoated sample... compare to the coated samples above

This second population of micrographs are of the younger rock sample PMR-46. The first micrograph (Fig. 8) is at a similar magnification as PMR-25 Fig. 4. The density of grains is much lower in PMR-46 and their relative size is much larger. This is likely a function of the type of sedimentary rock from which they were extracted reflecting different facies (parts of a basin). Striations are visible across all grain surfaces which are the result of polishing during sample preparation. One sample is highly magnified on the pocket-mark where laser ablation occurred to highlight the interesting morphology of these holes (Fig. 11). Also, Fig. 12 is a micrograph of an uncoated grain for comparison of methodology. Interestingly, the morphology around the grain boundary is much different in this micrograph when compared to coated grains. Some charging is visible and the grain is distorted as the laser hole is highly elliptical.

In this very preliminary and qualitative analysis, one can see that the euhedral nature of grains between samples PMR-25 and PMR-46 is not significantly different. In fact, the younger sample (PMR-46) shows the most euhedral grain of all (Fig. 9), suggesting the rock it was derived from may not have experienced more erosive/weathering processes than PMR-25.

3.2 THIN-SECTIONS: Light Microscopy

Three samples were analyzed that had previously been cut into thin-section. Samples are from the same sedimentary section mentioned above spanning a similar age range, from ~100Ma to ~20Ma. Considering that the goal is to get at compositional differences between samples, they were analyzed in the light microscope and in the SEM using both the backscattered electron detector (BSD) and X-Ray diffraction. In the light microscope, samples were viewed in cross-polarized light which facilitates mineral identification. Digital photos were taken of samples PMR-26 (Fig. 13), PMR-40 (Figs. 14-15), and PMR-46 (Figs. 16-17).

Figure 13: PMR-26 Cross-polarized light. Most of these grains are quartzite

Figure 14: PMR-40 Cross-polarized light. Also mostly quartzite.

Figure 15: PMR-40 Cross-polarized light and gold coating (on the right)

The first image (Fig. 13) is taken with the 10X objective and contains loads of quartzite grains (undulose extinction). There are also a couple intrusive igneous rock grains present. Sample PMR-40 is also dominated by quartzite grains with a few igneous rock grains as well. It is notable that the igneous grains in PMR-40 generally contain smaller crystals than PMR-26 suggesting the igneous source was shallow intrusive. Fig. 15 shows how gold coating obscures the optical properties of minerals under cross-polarized light. The right side of the photo is covered with a thin layer of gold.

Figure 16: PMR-46 Cross-polarized light

Figure 17: PMR-46 Plain polarized light (not as much contrast between grains)

Sample PMR-46 also consists of quartzite with some grains eroded from granite. This sample's composition is similar to the other two. Fig. 17 is included to show what these thin-sections look like under plain-polarized light. It becomes more difficult to differentiate between different mineral types.

3.3 THIN-SECTIONS: Backscattered Electrons and X-Ray Diffraction

Samples were also analyzed using backscattered electrons (BSD). In these analyses, one sample was coated with gold and the other was not. It is interesting to compare micrographs from samples subject to different methodologies. The gold coated thin-section (PMR-40) was scanned at low (Fig. 18a,b) and high (Fig. 19a,b) magnifications. Following are the BSD micrographs (a) and X-ray spectra for the same areas (b).

Figure 18a: PMR-40 Low magnification coated thin-section
Figure 18b: PMR-40 Low magnification X-ray spectra

Figure 19a: PMR-40 Higher magnification coated thin-section

Figure 19b: PMR-40 Higher magnification X-ray spectra

The low-magnification micrograph shows multiple grains and they all have a similar shade of grey which suggests they're the same thing! The X-ray spectra confirms this suggesting we're looking at grains of quartz (SiO2), which also corroborates observations from the light microscope above (quartzite). Higher magnification focuses on one individual grain so it's no surprise that one mineral is represented. The spectra indicates this mineral is, again, quartz. Note that the spectra do not include any gold (Au) even though this sample was coated. This is because small peaks labeled as gold were manually deleted from the spectra. The next sample (PMR-26 in Figs. 20-21) was not coated with gold. However, it still produces very usable compositional information.

Figure 20a: PMR-26 Very low magnification uncoated thin-section
Figure 20b: PMR-26 Very low magnification X-ray spectra

Figure 21a: PMR-26 Low magnification uncoated thin-section

Figure 21b: PMR-26 Low magnification X-ray spectra

Figure 22a: PMR-26 Higher magnification uncoated thin-section

Figure 22b: PMR-26 Higher magnification X-ray spectra

In this sample (PMR-26), the mineralogy is still dominated by quartz (SiO2). However, the spectra shows that some potassium (K), calcium (Ca), aluminum (Al), and even magnesium (Mg) exist in relatively small amounts. As such, feldspar (potassium and/or plagioclase) is likely present. The BSD micrograph does not indicate in any obvious way that the grains are different minerals. This might be because of the lack of conductivity resulting in "charging" and distortion, particularly on the edges of the micrographs.

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4. Conclusion

One goal of this project was to characterize the degree of weathering and erosion individual zircon grains had experienced from a stratigraphic section located in the far west of China. Using the SE2 detector in the SEM, we were able to produce quality micrographs of individual grains' surface morphologies for comparison. Though I expected the younger sample might have been derived from the older sample, consistently euhedral grains throughout both samples suggests this is not the case. It should be noted that this study was qualitative and limited in scope. A more systematic study of numerous (50+) grains in each sample should be done to confirm initial observations.

The second aspect of this project involved using compositional tools of a light microscope and the SEM including backscattered electrons and X-ray diffraction to detect compositional differences between samples from the same region of west China. As stated in the introduction, this information is useful in determining provenance, or where these sediments came from. The samples studied were all dominated by quartz (quartzite) grains, often with smaller fractions of feldspar. The light microscope enabled the distinction of unique igneous rocks in individual samples which may correlate to changes in provenance. More work should be done of those igneous grains including rare earth element analysis and analysis of radiogenic isotopes (strontium, neodymium) to conclusively say whether these sediments were sourced from unique source terranes.

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5. References

Faegri, K. and Iversen, J., 1989. Textbook of pollen analysis. New York.

Marshall, J., Whalley, W. and Krinsley, D., 1987. Clastic particles: scanning electron microscopy and shape analysis of sedimentary and volcanic clasts. Van Nostrand Reinhold Co., New York.

Reed, S., 2005. Electron microprobe analysis and scanning electron microscopy in geology. Cambridge Univ Pr.

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6. Acknowledgements

I would like to thank Lindsay Schoenbohm and Li Tao for assistance gathering samples in the field. I would also like to thank Brian McIntyre and Andreas Lapis for guidance in preparation of samples and use of the SEM.

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