^1,2Craig Hardgrove,¹A. Deanne Rogers,¹Timothy, D. Glotch,^1,3Jessica, Anne Arnold
¹Stony Brook University, Department of Geosciences, Stony Brook, NY
²Arizona State University, United States
³University of Oxford, United Kingdom

Distinguishing between micro and macrocrystalline mineral phases can help constrain the conditions under which those minerals formed or the degree of post-depositional alteration. This study demonstrates the effects of crystal size and surface roughness on thermal infrared emission spectra of micro and macrocrystalline phases of the two most common minerals on Earth, quartz and calcite. Given the characteristic depositional and environmental conditions under which microcrystalline minerals form, and the recent observations of high-silica deposits on Mars, it is important to understand how these unique materials can be identified using remote infrared spectroscopy techniques. We find that (a) microcrystalline minerals exhibit naturally rough surfaces compared to their macrocrystalline counterparts at the 10 µm scale; and that (b) this roughness causes distinct spectral differences within the Reststrahlen bands of each mineral. These spectral differences occur for surfaces that are rough on the wavelength scale, where the absorption coefficient (k) is large. Specifically, the wavelength positions of the Reststrahlen features for microcrystalline phases are narrowed and shifted compared to macrocrystalline counterparts. The spectral shape differences are small enough that the composition of the material is still recognizable, but large enough such that a roughness effect could be detected. Petrographic and topographic analyses of microcrystalline samples suggest a relationship between crystal size and surface roughness. Together, these observations suggest it may be possible to make general inferences about microcrystallinity from the thermal infrared spectral character of samples, which could aid in reconstructions of sedimentary rock diagenesis where corresponding petrographic or micro-imaging is not available.

Reference
Hardgrove C, Rogers AD, Glotch TD, Arnold JA (2016) Thermal Emission Spectroscopy of Microcrystalline Sedimentary Phases: Effects of Natural Surface Roughness on Spectral Feature Shape. Journal of Geophysical Research, Planets (in Press)
Link to Article [DOI: 10.1002/2015JE004919]
Published by arrangement with John Wiley & Sons

¹James R. Darling, ²Desmond E. Moser, ²Ivan R. Barker, ³Kim T. Tait, ⁴Kevin R. Chamberlain, ^5,6Axel K. Schmitt, ³Brendt C. Hyde
¹School of Earth and Environmental Sciences, University of Portsmouth, Portsmouth PO1 3QL, UK
²Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada
³Department of Natural History, Mineralogy, Royal Ontario Museum, Toronto, Ontario M5S 2C6, Canada
⁴Department of Geology and Geophysics, University of Wyoming, 3006, Laramie, WY 82071, USA
⁵Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095, USA
⁶Institut für Geowissenschaften, Universität Heidelberg, 69120 Heidelberg, Germany

The accurate dating of igneous and impact events is vital for the understanding of Solar System evolution, but has been hampered by limited knowledge of how shock metamorphism affects mineral and whole-rock isotopic systems used for geochronology. Baddeleyite (monoclinic ZrO2) is a refractory mineral chronometer of great potential to date these processes due to its widespread occurrence in achondrites and robust U–Pb isotopic systematics, but there is little understanding of shock-effects on this phase. Here we present new nano-structural measurements of baddeleyite grains in a thin-section of the highly-shocked basaltic shergottite Northwest Africa (NWA) 5298, using high-resolution electron backscattered diffraction (EBSD) and scanning transmission electron microscopy (STEM) techniques, to investigate shock-effects and their linkage with U–Pb isotopic disturbance that has previously been documented by in-situ U–Pb isotopic analyses.

The shock-altered state of originally igneous baddeleyite grains is highly variable across the thin-section and often within single grains. Analyzed grains range from those that preserve primary (magmatic) twinning and trace-element zonation (baddeleyite shock Group 1), to quasi-amorphous ZrO2 (Group 2) and to recrystallized micro-granular domains of baddeleyite (Group 3). These groups correlate closely with measured U–Pb isotope compositions. Primary igneous features in Group 1 baddeleyites (n=5)(n=5) are retained in high shock impedance grain environments, and an average of these grains yields a revised late-Amazonian magmatic crystallization age of 175±30 Ma175±30 Ma for this shergottite. The youngest U–Pb dates occur from Group 3 recrystallized nano- to micro-granular baddeleyite grains, indicating that it is post-shock heating and new mineral growth that drives much of the isotopic disturbance, rather than just shock deformation and phase transitions.

Our data demonstrate that a systematic multi-stage microstructural evolution in baddeleyite results from a single cycle of shock-loading, heating and cooling during transit to space, and that this leads to variable disturbance of the U–Pb isotope system. Furthermore, by linking in-situ U–Pb isotopic measurements with detailed micro- to nano-structural analyses, it is possible to resolve the timing of both endogenic crustal processes and impact events in highly-shocked planetary materials using baddeleyite. This opens up new opportunities to refine the timing of major events across the Solar System.

Reference
Darling JR, Moser DE, Barker IR, Tait KT, Chamberlain KR, Schmitt AK, Hyde BC (2016)
Variable microstructural response of baddeleyite to shock metamorphism in young basaltic shergottite NWA 5298 and improved U–Pb dating of Solar System events. Earth and Planetary Science Letters 444, 1–12
Link to Article [doi:10.1016/j.epsl.2016.03.032]
Copyright Elsevier