University of Rochester, Department of Earth and Environmental Sciences
OPT307: SEM Practicum
An anorthositic gabbro from the Duluth Complex of northern Minnesota was analyzed using SEM to assess the textural relationships of zirconium (Zr)-rich phases such as zircon (ZrSiO4) and baddeleyite (ZrO2). These phases are often found as accessory minerals in igneous rock and can yield robust information about the relative sequencing of a rock's crystallization history. To assess these textural relationships, the cross-cutting relationships between the Zr-rich phases and their surroundings were analyzed and characterized with a combination of imaging and EDS techniques. Applying this relative dating method while anchored in an absolute time scale can bolster evidence for a particular crystallization history. For example, U-Pb geochronology uses phases such as zircon and baddeleyite to establish the absolute age of a sample. Trace elements such as uranium (U), hafnium (Hf), and other rare Earth elements (REEs) will substitute for zirconium in these minerals, making them excellent targets for high precision U-Pb dating, among other geochronological methods.
Previous BSE microscopy and EDS by Prof. Mauricio Ibanez-Mejia aided in identifying areas of interest in the sample. Zr-rich localities were established from a ZrLa elemental distribution map and correlated to the BSE image. The sample was also viewed via optical microscope to characterize the mineralogy and corroborate these findings.
II. Sample Description and Optical Microscopy
The optical (petrographic) microscope uses visible light in two ways: first as plane polarized light (PPL) by simply passing visible light through a polarizer, and second as cross polarized light (XPL) by passing the polarized light through a perpendicular polarizer.
Thin section FC-1-3 was viewed under both optical and electron microscopes. The sample is representative of an anorthositic gabbro, meaning that it is an intrusive (crystallized at depth and then later exposed) igneous rock. Observations in PPL and XPL that aid in mineral identification include cleavage, twinning, extinction, color/pleochroism, birefringence, relief, and zoning. The mineralogy of the section is dominated by coarse-grained plagioclase feldspar with minor pyroxene and olivine. Plagioclase feldspar grains have low relief, are colorless under reflected light, and show distinctive polysynthetic (albite-type) twinning under XPL.
III. Sample Preparation
An evaporative carbon coating had been previously applied to the thin section in order to make the sample conductive. Carbon tape was applied to connect the thin section to the sample stub in order to ground the sample and protect against charge accumulation.
IV. Scanning Electron Microscopy and Energy Dispersion X-Ray Spectroscopy (EDS)
Four areas of interest were imaged and analyzed using the Zeiss Leo DSM982 Scanning Electron Microscope of the Institute of Optics at the University of Rochester. The SEM allows for use of several electron detectors, such as secondary electron and backscatter detection. In this study, BSE imaging was primarily used due to its ability to show compositional features over topographic ones. This is especially useful when imaging flat geologic samples with multiple phases. The backscatter electron detector will show elements with a larger atomic number as brighter, as the element scatters more electrons from the incident beam. X-rays are produced when interaction causes electrons to be ejected from the inner shell, where the space is then filled by an outer shell electron that falls to a lower energy orbital to take its place. This high orbital energy is released in the form of characteristic X-rays. The EDAX detector was used to determine phase composition and to make elemental distribution maps.
The Zr-rich phase from this locality is high in silicon, characterizing it as zircon. As it is found as an inclusion within a plagioclase feldspar grain, it would likely have developed after the initial crystallization of the feldspar as a recrystallization event. Recrystallization is often an indicator of metamorphism (high pressures and/or temperatures).
Since baddeleyite is characterized by the absence of silicon in its chemical structure, the Zr-rich phase shown in Fig. 5 is mostly baddeleyite. However, as shown in Fig. 6 by the Si elemental map, there is a zone of silicon that stretches between the two Si-poor localities. This can be interpreted as recrystallization of zircon on top of baddeleyite.
The Zr-phase in this locality has very little silicon, so it is identified as a baddeleyite grain. The baddeleyite cross-cuts an iron-oxide and was thus likely formed by recrystallization of the mineral.
The tabular Si and Zr-rich phase here is zircon, cross-cutting an ilmenite grain. As before, the textural relationship between the zircon and its surrounding phases suggests that the zircon crystallized after formation of the ilmenite phase, i.e. recrystallization.
The Zr-rich phases from locality 2 (Fig. 5) were colorized to more clearly show the distinction between the baddeleyite phase and the zircon phase. The lighter pink areas are baddeleyite and the darker pink region in the center of the grain is zircon.
The study of thin section FC-1-3 with the SEM yielded robust results concerning the development of zircon and baddeleyite in the Duluth Complex. As both zircon, baddeleyite, and both phases together were identified in the sample, some assumptions can be made about the crystallization history of the rock. First, Fig. 5 shows zircon recrystallizing on top of the baddeleyite grains. From this qualitative data it is understood that baddeleyite recrystallization occurred first, followed by zircon recrystallization. Overall, both of the Zr-rich phases appear only as cross-cutting other phases, meaning that they are products of recrystallization. In terms of geologic history, a recrystallization event occurs under conditions of intense heat and/or pressure, creating new minerals and crystal structures. Therefore, for the Duluth Complex it can be assumed that following original crystallization, the region must have undergone a second heating or metamorphic event that caused recrystallization to occur. Further investigation of the sample would allow for clarifying the geologic history to a more precise resolution.
I would like to thank Prof. Mauricio Ibanez-Mejia for providing the samples and for guiding my research process. I would also like to thank Brian McIntyre for his endless patience and instruction, and Nursah for help on the labs.