Response of cathodoluminescence to crystal-plastic deformation in zircon

Geochemical and geochronological studies of zircon are commonly supplemented by cathodoluminescence (CL) imaging because it provides a means of recognizing different generations of zircon growth at high-spatial resolution. Crystal-plastic deformation of zircon can have significant effects on zircon geochemistry. Detailed analyses from electron backscatter diffraction mapping combined with panchromatic CL imaging and hyperspectral CL mapping of several crystal-plastically deformed grains from different geological settings are used to establish the relationships between crystal-plastic deformation and CL in zircon. Results show a strong spatial association between deformation microstructures and CL response that lead to modification of CL that commonly cross-cuts primary zoning. Variable contributions from two fundamental deformation-related processes result in a variety of CL characteristics: A defect control on panchromatic CL intensity, particularly at low-angle (subgrain) boundaries; and changes in spectral CL response due to deformation-related modification of CL-active REE geochemistry. A framework is provided for the recognition of deformation-related microstructures using CL and the usefulness of CL imaging in the discrimination of these microstructures is critically evaluated

Nanoscale records of ancient shock deformation: Reidite (ZrSiO4) in sandstone at the Ordovician Rock Elm impact crater

The terrestrial record of meteorite impacts is difficult to decipher because unequivocal evidence of impact is increasingly destroyed with time by erosion, burial, and tectonics. Zircon survives these processes as a shocked mineral, and above 20 GPa transforms to reidite, a high-pressure ZrSiO4 polymorph diagnostic of impact. However, the utility of reidite has been limited by its occurrence; it has only been reported from three relatively young (<36 Ma) impact craters globally. Here we report a new occurrence of reidite in brecciated sandstone from the Ordovician Rock Elm impact crater in Wisconsin, United States. Electron backscatter diffraction mapping was used to identify reidite and microtwins within shocked zircons smaller than 50 µm in diameter. Reidite occurs both as 200–500-nm-wide lamellar intergrowths and as nanoparticulate grains, and not only provides the first diagnostic evidence for ultrahigh-pressure shock metamorphism at Rock Elm, but is also the oldest reported occurrence of reidite. Considering its small size, and the ubiquitous presence of detrital zircon in siliciclastic rocks, reidite may be more common in the rock record than has been reported but has potentially gone undetected. The recognition that nanoscale reidite can be preserved over deep time within zircon in shock-metamorphosed sandstone presents new opportunities for investigating Earth’s impact record, as it could potentially preserve nanoscopic evidence of impact events much older than the one that formed Rock Elm. Given that shocked zircons have been shown to survive sedimentary cycling, the identification of reidite within zircons in siliciclastic rocks could facilitate investigating the impact chronology over much of the geological time scale, as the oldest terrestrial minerals known are detrital zircons

The variation and visualisation of elastic anisotropy in rock forming minerals

Acknowledgements David Healy thanks John Wheeler (Liverpool) for discussions and Ross Angel (Padua) for discussions and a reprint. This paper is dedicated to the memory of John Frederick Nye (1923–2019), whose seminal text book, first published in 1957 (Physical Properties of Crystals: Their Representation by Tensors and Matrices; reprinted as Nye, 1985), has had a huge influence on the lead author. Financial support This research has been supported by the NERC (grant no. NE/N003063/1).Peer reviewedPublisher PD

Grenville Skarn Titanite: Potential Reference Material for Sims U–Th–Pb Analysis

We have investigated the homogeneity, chemical composition, structure, degree of radiation damage, and post-formation evolution of titanite crystals from skarns of the Grenville Province of the Canadian Shield using SHRIMP, TIMS, Raman and PL spectroscopy, EBSD, and EPMA–WDS. These results are used to assess the potential of the titanite as Reference Material (RM) for micro-analytical U–Th–Pb age dating. The SHRIMP data show that these megacrysts (5–31g) have concordant U–Pb isotope systematics, 60 to 500 ppm U, 120 to 1200 ppm Th , 206Pb/204Pb between 500 and 2500, ages of ~1 Ga, and excellent homogeneity at the scale of the analytical volume of the ion probe. The ID–TIMS titanite data for OLT1, OLT2 and TCB show that these crystals are essentially concordant. Data for OLT1 and OLT2 show slight scatter (i.e., in excess of that expected from the uncertainty in an individual analysis). For OLT1, one of seven analyses shows Pb loss or, possibly, a younger period of growth. Crystals OLT1 and OLT2 have respective TIMS concordia ages of 1014.8 ± 2.0 Ma (2s, n = 6, MSWD = 1.8) and 998.0 ± 4.5 Ma (2s, n = 3, MSWD = 3.3) for domains that have not lost Pb.The TIMS analyses of TCB are tightly clustered and give a concordia age of 1018.1 ± 1.7 Ma (2s, n = 4, MSWD = 0.92). Raman and PL spectra show a low to moderate degree of accumulated radiation-induced damage in the Grenville Skarn Titanite crystals and uniform internal distributions of this damage. The EDSB contrast images indicate little or no crystallographic misorientation. The EMPA–WDS data show that the outer 50–100 mm of the OLT1 and TCB crystals are enriched in Al and F, and depleted in Fe and Nb, when compared with the interior. In spite of the variation in composition and degree of radiation damage amongst samples, there are no identifiable matrix effects in our SHRIMP data. Some Grenville skarn titanite (GST) crystals have potential as RM for micro-analytical U–Th–Pb age dating. Crystal TCB has excellent homogeneity of U–Th–Pb isotopic composition. Crystals OLT1 and OLT2 have minor TIMS age heterogeneity. However, this heterogeneity is smaller than that of the Khan titanite, our current in-house titanite standard. Careful selection of analysis areas during SIMS, and of chips for TIMS analysis, allows high-quality isotopic data to be obtained from these large crystals of titanite

Enhanced diffusion of Uranium and Thorium linked to crystal plasticity in zircon

The effects of crystal-plasticity on the U-Th-Pb system in zircon is studied by quantitative microstructural and microchemical analysis of a large zircon grain collected from pyroxenite of the Lewisian Complex, Scotland. Electron backscatter diffraction (EBSD) mapping reveals a c.18° variation in crystallographic orientation that comprises both a gradual change in orientation and a series of discrete low-angle (<4°) boundaries. These microstructural data are consistent with crystal-plastic deformation of zircon associated with the formation and migration of dislocations. A heterogeneous pattern of dark cathodoluminescence, with the darkest domains coinciding with low-angle boundaries, mimics the deformation microstructure identified by EBSD. Geochemical data collected using the Sensitive High Resolution Ion MicroProbe (SHRIMP) shows a positive correlation between concentrations of the elements U, Th and Pb (ranging from 20–60 ppm, 30–110 ppm, and 14–36 ppm, respectively) and Th/U ratio (1.13 – 1.8) with the deformation microstructure. The highest measured concentrations and Th/U coincide with low-angle boundaries. This enrichment is interpreted to reflect enhanced bulk diffusion of U and Th due to the formation and migration of high-diffusivity dislocations. (207)Pb/(206)Pb ages for individual analyses show no significant variation across the grain, and define a concordant, combined mean age of 2451 ± 14 Ma. This indicates that the grain was deformed shortly after initial crystallization, most probably during retrograde Inverian metamorphism at amphibolite facies conditions. The elevated Th over U and consistent (207)Pb/(206)Pb ages indicates that deformation most likely occurred in the presence of a late-stage magmatic fluid that drove an increase in the Th/U during deformation. The relative enrichment of Th over U implies that Th/U ratio may not always be a robust indicator of crystallization environment. This study provides the first evidence of deformation-related modification of the U-Th system in zircon and has fundamental implications for the application and interpretation of zircon trace element data

Quantitative characterization of plastic deformation of zircon and geological implications

The deformation-related microstructure of an Indian Ocean zircon hosted in a gabbro deformed at amphibolite grade has been quantified by electron backscatter diffraction. Orientation mapping reveals progressive variations in intragrain crystallographic orientations that accommodate 20° of misorientation in the zircon crystal. These variations are manifested by discrete low-angle (<4°) boundaries that separate domains recording no resolvable orientation variation. The progressive nature of orientation change is documented by crystallographic pole figures which show systematic small circle distributions, and disorientation axes associated with 0.5–4° disorientation angles, which lie parallel to rational low index crystallographic axes. In the most distorted part of the grain (area A), this is the [100] crystal direction. A quaternion analysis of orientation correlations confirms the [100] rotation axis inferred by stereographic inspection, and reveals subtle orientation variations related to the local boundary structure. Microstructural characteristics and orientation data are consistent with the low-angle boundaries having a tilt boundary geometry with dislocation line [100]. This tilt boundary is most likely to have formed by accumulation of edge dislocations associated with a 〈001〉{100} slip system. Analysis of the energy associated with these dislocations suggest they are energetically more favorable than TEM verified 〈010〉{100} slip. Analysis of minor boundaries in area A indicates deformation by either [01¯0] (001) edge, or [100](100) and [001](100) screw dislocations. In other parts of the grain, [11¯0] cross slip on (111), (111¯) and (112) planes seems likely. These data provide the first detailed microstructural analysis of naturally deformed zircon and indicate ductile crystal-plastic deformation of zircon by the formation and migration of dislocations into low-angle boundaries. Minimum estimates of dislocation density in the low-angle boundaries are of the order of ∼3.1010 cm−2. This value is sufficiently high to have a marked effect on the geochemical behavior of zircon, via enhanced bulk diffusion and increased dissolution rates. Therefore, crystal plasticity in zircon may have significant implications for the interpretation of radiometric ages, isotopic discordance and trace element mobility during high-grade metamorphism and melting of the crust

Grain boundary networks and shape preferred orientation : A fresh angle on pattern quantification with GBPaQ

A quantitative understanding of grain shape preferred orientation (SPO) and grain boundary networks as fundamental characteristics of rocks and other crystalline solids is of major interest in geology and material science. Grain boundary networks contain useful information on the deformation history of polycrystalline aggregates, and their diagenetic and metamorphic histories. SPO can have a major impact on material characteristics such as permeability, acoustic velocity and mechanical strength, and on reaction surfaces. The objective of this study is to present a semi-automated toolbox of MATLAB™ scripts, named Grain Boundary Pattern Quantification (GBPaQ), that incorporate different methods for grain boundary pattern quantification for their application to, for example, seismic wave attenuation estimation. GBPaQ uses grain boundary statistics and calculates radial scan line intercepts. In this paper, GBPaQ is tested on two example grain boundary patterns, a granular texture and a foam texture with equant grains, which have been digitally stretched (deformed) to analyse their SPO evolution. The results show that a combination of grain ellipse, grain boundary segment orientation, and grain boundary segment intercept density rose diagrams provide a complete, detailed quantification of grain boundary pattern anisotropy. Grain boundary segment intercept (GBSI) analysis using GBPaQ yields a new grain boundary network parameter – the minimum intensity of grain boundary intercepts (Imin) – which follows a power law relationship with the average axial ratio of grain-fitted ellipses (r) during SPO development. We propose that Imin can be used for the quantitative analysis of SPO strength as a useful tool to assess the deformation history of polycrystalline aggregates. Further studies involving a broader range of different patterns and strain histories are necessary to fully investigate the potential of Imin versus r diagrams

Exploring the relative contribution of mineralogy and CPO to the seismic velocity anisotropy of evaporites

We present the influence of mineralogy and microstructure on the seismic velocity anisotropy ofevaporites. Bulk elastic properties and seismic velocities are calculated for a suite of 20 natural evaporate samples, which consist mainly of halite, anhydrite, and gypsum. They exhibit strong fabrics as a result of tectonic and diagenetic processes. Sample mineralogy and crystallographic preferred orientation (CPO) were obtained with the electron backscatter diffraction (EBSD) technique and the data used for seismic velocity calculations. Bulk seismic properties for polymineralic evaporites were evaluated with a rock recipe approach. Ultrasonic velocity measurements were also taken on cube shaped samples to assess the contribution of grain-scale shape preferred orientation (SPO) to the total seismic anisotropy. The sample results suggest that CPO is responsible for a significant fraction of the bulk seismic properties, in agreement with observations from previous studies. Results from the rock recipe indicate that increasing modal proportion of anhydrite grains can lead to a greater seismic anisotropy of a halite-dominated rock.Conversely, it can lead to a smaller seismic anisotropy degree of a gypsum-dominated rock until anestimated threshold proportion after which anisotropy increases again. The difference between thepredicted anisotropy due to CPO and the anisotropy measured with ultrasonic velocities is attributed to the SPO and grain boundary effects in these evaporites

Rapid hydration and weakening of anhydrite under stress : Implications for natural hydration in the Earth’s crust and mantle

Acknowledgements JThis research has been supported by an Aberdeen–Curtin Alliance international postgraduate scholarship, by a Curtin publication grant, and by the Natural Environment Research Council (grant no. NE/T007826/1). Enrique Gomez-Rivas acknowledges the “Ramón y Cajal” fellowship RYC2018-026335-I, funded by the Spanish Ministry of Science and Innovation (MCIN), the State Research Agency of Spain (AEI), and the European Social Fund (ESF)/10.13039/501100011033, as well as the DGICYT research project PID2020-118999GB-I00, funded by the Spanish Ministry of Science and Innovation (MCIN) and State Research Agency of Spain (AEI)/10.13039/501100011033.Peer reviewedPublisher PD

Deformation-related microstructures in magmatic zircon and implications for diffusion

An undeformed glomeroporphyritic andesite from Java, Indonesia, contains zoned plagioclase and amphibole glomerocrysts in a fine-grained groundmass and records a complex history of adcumulate formation and subsequent disaggregation by externally derived melts. A suite of xenocrystic zircon records Proterozoic and Archaean dates whilst a second population of zoned, euhedral, igneous zircon yields a SHRIMP crystallisation age of 9.3 0.2 Ma. Quantitative microstructural analysis (via electron backscatter diffraction - EBSD) show no deformation in the inherited xenocrysts, but intragrain orientation variations of up to c.30 in 80% of the young zircon population. These variations are typically accommodated by both progressive crystallographic bending and discrete low angle boundaries that overprint growth zoning. Dispersion of crystallographic orientations are dominantly by rotation about an axis parallel to the zircon c-axis [001], which is coincident with the dominant orientation of misorientation axes of adjacent analysis points in EBSD maps. Less common misorientation axes account for minor components of crystallographic dispersion. These observations are consistent with zircon deformation by dislocation creep and the formation of tilt and twist boundaries associated with the operation of {100} and {010} slip systems.The restriction of deformation microstructures to large glomerocrysts and the young magmatic zirconpopulation, and the absence of deformation within the host igneous rock and inherited zircon grains, indicatethat zircon deformation took place within a low-melt fraction (<5% melt), mid - lower crustal cumulate prior tofragmentation during magmatic disaggregation and entrainment of xenocrystic zircons during magmaticdecompression. Tectonic stresses within the compressional Sunda Arc at the time of magmatism are consideredto be the probable driver for low-strain deformation of the cumulate in the late stages of initial crystallisation.These results provide the first evidence of crystal plastic dislocation creep in zircon associated with magmatic crystallisation and indicate that the development of crystal-plastic microstructures in zircon is not restricted to high-strain rocks. Such microstructures have previously been shown to enhance bulk diffusion of trace elements (U, Th and REE) in zircon. The development of deformation microstructures, and therefore multiple diffusion pathways in zircon in the magmatic environment, has significant implications for the interpretation of geochemical data from igneous zircon and the trace element budgets of melts due to the potential enhancement of bulk diffusion and dissolution rates