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Article

Zircon Internal Deformation and Its Effect on U-Pb Geochronology: A Case Study from the Himalayan High-Pressure Eclogites

by
Hafiz U. Rehman
Graduate School of Science and Engineering, Kagoshima University, Kagoshima 890-0065, Japan
Minerals 2024, 14(8), 742; https://doi.org/10.3390/min14080742
Submission received: 15 June 2024 / Revised: 16 July 2024 / Accepted: 22 July 2024 / Published: 24 July 2024
(This article belongs to the Section Mineral Geochemistry and Geochronology)
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Abstract

:
Zircon, with a chemical formula of ZrSiO4, is a widely used mineral for determining the crystallization age of igneous rocks. It is also used to constrain the timing of metamorphic events from its overgrowth or recrystallized domains. Furthermore, detrital zircon grains can provide information on the sedimentary provenance. Due to the trace amounts of uranium (parent) which decays into its daughter element (Pb), it is a prime geochronometer for the majority of magmatic and metamorphic rocks. With high-precision analytical instruments, such as TIMS, SIMS, and LA-ICP-MS, huge amounts of geochronological and trace element data from zircon have been generated around the globe to date. Target domains within zircon grains are analyzed to extract geochemical and geochronological records using spatially resolved techniques such as ion probes or laser ablation coupled with mass spectrometry. Before any such analysis, the zircon grains are examined for internal structures, growth zoning, and the presence of tiny inclusions. However, many researchers analyze multiple domains within single zircon grains for U-Pb isotope analysis with little regard for their internal structures, particularly crystallographic orientations. Hence, they may obtain mixed ages with variable discordance, leading to imprecise interpretation especially when the growth domains are not well-identified. Particularly, zircon grains that contain multi-growth domains or have local internal deformations within a single grain may not produce geologically meaningful age results if the analyses are conducted on mixed domains. This study presents a brief review on zircon geochronology, how to identify and visualize micro-deformations in metamorphic zircons through the EBSD analysis, and the effects of micro-deformation on age results. Examples from a case study conducted on zircons hosted in the Himalayan high-pressure eclogites are presented that show intra-grain plastically deformed domains and their effects on the corresponding age results.

1. Introduction

The history of U-Pb geochronology traces back to the nineteenth century when Williams [1] recognized the great potential of Uranium (U) as a radioactive element for geochronology. Rutherford and Soddy [2] applied radioactive isotope decay to rocks, followed by Holmes and Nier in the late 1930s, who applied the U-Pb dating method more precisely [3,4]. The zircon U-Pb method was used for the first time on Precambrian rocks from the Grenville Province, Canada [5]. The decay of 238U and 235U into 206Pb and 207Pb isotopes is significant enough to measure the Earth’s history, with a half-life of 4468 Myr for 238U to decay to 206Pb and 704 Myr for 235U to 207Pb [4]. Numerous studies discussed zircon textures, growth, geochemistry, U-Pb isotope system, and geochronology, and readers are referred to these classical works (e.g., [6,7,8,9,10,11,12,13,14,15]).
Researchers have put significant efforts into developing geochronological tools in the past century. As stated above, the initiation of the U-Pb isotope system for geochronology was tested after the discovery of Uranium, a potential radioactive element that decays to Pb [1]. The existing databases on zircon geochronology used for tracing the history of geological events have exceeded millions of analyses. The results help to understand regional tectonics, magmatic, and metamorphic events around the globe [16,17,18]. Common analytical techniques for zircon geochronology include different sample preparation processes followed by analyses with isotope dilution-thermal ionization mass spectrometry (ID-TIMS), ion microprobe known as the secondary ion mass spectrometry (SIMS), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Details on sample preparation and instrument operation are provided in the next section. However, due to the destructive nature of these analyses where individual zircon grains are partly or entirely destroyed, it is necessary to obtain all the necessary information regarding petrological, textural, and inclusion morphology from zircon grains before applying such destructive tools.
This study presents (1) an introduction to the common analytical methods used for zircon age dating, (2) an investigation of zircon samples for internal texture before applying geochronology tools, and (3) a short summary on geochronological results from magmatic and metamorphic zircon grains and the reason of higher discordance in some data-points. The final section (4) of the study reports internally deformed domains in zircons that were identified through the electron backscatter diffraction (EBSD) analyses. Later those grains were analyzed for U-Pb ages using an ion microprobe and the results showed that the internal deformation had affected the geochronological results.

2. Background

Recent progress in geochronological analytical techniques enabled geoscientists to produce reliable and precise age results through the Earth’s history. Today, the U-Pb isotope system is one of the most commonly used tools and is considered a precise dating method among geochronologists. It is beyond the scope of this article to compile a detailed history of geochronology; however, readers who are interested in the topic are referred to classical works presented by Davis [3] and Davis et al. [4]. Below, a brief description on common analytical methods for zircon geochronology is presented.
(1) 
Isotope-Dilution Thermal Ionization Mass Spectrometry (ID-TIMS)
In this technique, a bulk amount of zircon grains is dissolved into the solutions. The solutions are then dried on metal filaments and heated to evaporate the ions, followed by ion acceleration through the electric field with fixed energy levels. The mobilized ions are separated into specific masses with a magnetic field and then measured by the detector. This method involves a certain amount of sample mixed with a known amount of standard material (commonly termed as the “spike”) which is comprised of the same element but with a different isotopic composition from the unknown sample to be age-dated. The mixing of spike and unknown samples is termed as isotope dilution whereas the measurement method is referred to as ID-TIMS [3]. Although this method produces high precision analytical results, it requires a lengthy process for sample preparation and is time consuming. Therefore, in recent times, researchers prefer to use quick and in situ analytical methods such as those discussed below. In addition to this technique, the CA-ID-TIMS technique, abbreviated from chemical abrasion isotope dilution thermal ionization mass spectrometry involves the abrasion and removal of outer altered or hydrothermally affected domains from zircon that may have suffered Pb-loss [19]. The fresh domains are analyzed to obtain accurate geochronological results. This technique gained further accuracy and strength thanks to the efforts of researchers at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, who recently improved the technique of CA-ID-TIMS to produce quality geochronological results with procedural blanks reducing to less than a picogram level [20].
(2) 
Ion Microprobes known as the Secondary Ion Mass Spectrometry (SIMS)
Secondary ion mass spectrometry (SIMS) was implemented in 1982 at the Australian National University (ANU), where Compston et al. [21] and his student S. Clement used it for elemental abundances and isotope ratios (including the U-Pb age dating). In this method, sharp beams of oxygen ions are sputtered on the polished surface of zircon grains and secondary ion beams are produced. Those secondary ions of U and Pb are then measured in a mass spectrometer. This method needs a relatively smaller spot-size (20 to 30 μm) to ionize the U and Pb isotopes. A single analysis usually takes 30~40 min. The advantage of this technique is to obtain relatively quick results without the tedious job of isotope dilution that usually takes several days of labor. However, ion probe facilities are limited and have high operating costs. NanoSIMS is a type of ion microprobe that can analyze even smaller size samples (with an analysis spot of ~5 to 10 μm) and can achieve higher precision results but are costly too.
(3) 
Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)
This method is comparatively low cost and can achieve precise U-Pb age data along with trace/rare-earth element contents from a single analysis conducted on zircons. This method involves the use of laser beams which are sputtered on the sample surface. The ablated material is carried to the inductively coupled plasma mass spectrometer using helium and argon as carrier gases and the isotopes of desired elements (U and Pb in case of zircon age analysis) are measured (e.g., see details in [22]). Earlier models of LA-ICP-MS required laser beams to generate pits as large as 35 μm on the ablated grain to obtain useful geochronological results. However, recent models of LA-ICP-MS have improved to analyze samples with smaller spot sizes of ~25 μm. This technique is quick and takes less than 3 min to obtain geochronological result from a single spot analysis. However, to maintain precision and to obtain geologically meaningful ages, researchers commonly pool a large population of analyzed spots.

3. Zircon Age-Dating and the Effect of Micro-Deformation on Age

3.1. Petrological, Textural, and Internal Structural Identifications of Zircons

This study emerged from petrological, textural, and inclusion-related observations on zircons from the Himalayan eclogites exposed in the Kaghan Valley, Pakistan (Figure 1a,b). The U-Pb isotope analyses were performed on representative zircon grains to obtain age results. In addition, a literature survey on geochronology of zircon grains from various geological terrenes was also conducted. The survey provides ample evidence of data that are improperly interpreted or contain a large proportion of discordant age results (e.g., a few examples shown in Figure 2). The above results and observations strongly indicate that efforts are needed to avoid micro-inclusions and healed cracks, as well as mixed domains, before analyzing zircon grains for geochronological records. To identify and locate inclusions, cracks, and internal structures of zircons, optical and scanning electron microscopy (backscattered electron: BSE and cathodoluminescence: CL imaging) are commonly applied. Such textural and inclusion studies provide information to distinguish deformed or altered domains from pristine structures. Numerous articles have already discussed the importance of internal textures of zircons and their effects on geochemical signatures (e.g., [12,23,24,25,26,27,28,29]). Some of the above authors have found variable extents of discordancy in U-Pb ages; however, through the Lu-Hf isotope analysis on the same dated-spots, it was suggested that the majority of such zircons were formed during coeval magmatic event and the discordant age was due to Pb-loss resulted from the late-stage events that likely disturbed the U-Pb isotope system in the investigated zircon grains [25]. To overcome such problems, geochronologists generally conduct chemical abrasion on zircon grains to minimize or avoid altered domains [19,20], do depth profiling to find homogeneous domains [28], U-Pb signal plateau examinations to remove spiky signals, combine U-Pb age data with Raman spectroscopy to identify damaged or metamict domains [29], and the identification or filtering of erroneous peaks from U-Pb isotope or trace element signals before calculating the final age results. However, all the zircon analyses are destructive and before conducting any such techniques, it would be helpful to use non-destructive tools to identify homogenous, undisturbed, or pristine domains in zircons. Electron backscatter diffraction (EBSD) analysis is a useful nondestructive tool to extract additional crystal lattice information from zircon grains that are sometimes not visible through backscatter electron and cathodoluminescence imaging alone. Details on EBSD are discussed in the later section.
Here, a geological background and the petrological features of zircon grains found in the Himalayan eclogites are presented, followed by some evidence from the literature survey that shows discordant age results or possibly that the U-Pb age data were obtained from the analysis of mixed domains. The investigated zircon grains were hosted in the Himalayan eclogites which were formed by the India-Asia collision/subduction-related metamorphism during the Eocene (45 to 50 Ma). Protoliths of the Himalayan eclogites were considered as the basaltic protoliths that were emplaced on the northern margin of the Indian Plate during the Permian (260 to 279 Ma) [30,31] and references therein. These eclogites occur as sheets or lenses within the Higher Himalayan Crystalline rocks (Figure 1c). In this study, a few examples of zircon grains from the Himalayan eclogites which contain micro-deformed domains are presented. Further details of micro-deformed domains in zircon grains and NanoSIMS U-Pb age analysis are discussed in an earlier study [32]. The studied zircons yielded: (1) Permian age from the magmatic domains that represent the protolith emplacement age, (2) unclear or reset ages from the deformed domains, and (3) Eocene age from the metamorphic domains [32]. The authors [32], based on U-Pb age analysis on differently oriented domains within a single grain, interpreted the U-Pb age results to have been affected due to high-grade metamorphism which modified the intracrystalline structural, geochemical, and isotopic records in the zircon grains.

3.2. Importance of Non-Destructive EBSD Method in Identifying Pristine Domains in Zircons

Literature dealing with multiple U-Pb age spot analyses conducted on various domains of complex (magmatic, metamorphic, reset, or mixed domains) zircon grains provide evidence of discordance and large errors in the age values (e.g., a few examples displayed in Figure 2, and the literature contains numerous such examples). Data produced from complex zircon grains generally portray a large scatter in U-Pb ages as commonly seen in Concordia diagrams. Indeed, the author of this study does not claim that all the geochronological results are erroneous or meaningless, as the majority of the publications have statistically significant data to support the records of various geological events that the zircon grains preserve. However, 30 to 40% of the analytical data, and in some cases more than 50%, are geologically complex and difficult to interpret (e.g., data shown in Figure 2).
In this study, the importance of the prior investigation of the hidden features within the zircon grains (internal structures, deformed domains, healed cracks, etc.) is emphasized before carrying out geochemical or isotopic analysis on complex zircon grains. It is common practice that researchers pay attention to zircon internal structures, examine them via optical microscopes, backscatter electron (BSE) microscopy, and cathodoluminescence (CL) imaging systems. However, some of the healed fractures or hidden but deformed domains are not easily identifiable through the above-mentioned methods. EBSD analysis is a useful tool to extract information at micro- or even at nanoscales, because EBSD study can visualize deformed or complex domains within a single zircon grain. Once the internal crystal structure or micro-deformed domains in zircon grains are identified, it becomes easier to infer events from discordant U-Pb ages. Since the commonly applied LA-ICP-MS age-dating method needs considerable volume that has to be ablated (ca. 25~30 μm spot-size) on a single crystal which should, most probably, be homogeneous or should represent single record of a geological event.
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Figure 1. Simplified geological map (modified from [33]) of the study area showing a sketch of the Himalaya (a), a simplified geological map of the Indian and Asian plates with the Kohistan-Ladakh Island arc (b), and a geological map showing the occurrence of Himalayan high-pressure (HP) and ultrahigh-pressure (UHP) eclogites, Kaghan Valley, north Pakistan (c).
Figure 1. Simplified geological map (modified from [33]) of the study area showing a sketch of the Himalaya (a), a simplified geological map of the Indian and Asian plates with the Kohistan-Ladakh Island arc (b), and a geological map showing the occurrence of Himalayan high-pressure (HP) and ultrahigh-pressure (UHP) eclogites, Kaghan Valley, north Pakistan (c).
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Figure 2. Four examples showing a deviation of U-Pb age data from the Concordia lines reported from magmatic and metamorphic zircon grains. The plots shown in (a) and (b) display Wetherill Concordia plots and those shown in (c) and (d) display Terra-Wasserburg plots. In all the four examples, the U-Pb isotope analyses show large scatter and numerous analyses plot away from the Concordia lines, indicating Pb-loss, analyses conducted on mixed or complex domains, or ages reset due to secondary events. Data source for plot (a) is from Figure 7 in [34], for plot (b) from Figure 3 in [35], for plot (c) from Figure 5 in [36], and for plot (d) from Figure 3.3 in [37].
Figure 2. Four examples showing a deviation of U-Pb age data from the Concordia lines reported from magmatic and metamorphic zircon grains. The plots shown in (a) and (b) display Wetherill Concordia plots and those shown in (c) and (d) display Terra-Wasserburg plots. In all the four examples, the U-Pb isotope analyses show large scatter and numerous analyses plot away from the Concordia lines, indicating Pb-loss, analyses conducted on mixed or complex domains, or ages reset due to secondary events. Data source for plot (a) is from Figure 7 in [34], for plot (b) from Figure 3 in [35], for plot (c) from Figure 5 in [36], and for plot (d) from Figure 3.3 in [37].
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However, the large scatter on Concordia plots, in many cases (e.g., Figure 2) indicate that the analyses were either conducted on mixed domains or on deformed portions within the zircon grain that had potentially been reset. The issue of analyzing the inhomogeneous or mixed domains in zircon grains found in high-grade metamorphic rocks and their effect on geochronological results was discussed by Kröner et al. [38]. Other possibilities include Pb-loss due to secondary factors such as healed fractures, recrystallized domains, or diffusion-related chemical modifications (e.g., [39]). Although zircons with micro-deformations or reset-ages have been reported from several strongly deformed, highly sheared, or mylonitized rocks (e.g., [40,41,42,43,44,45,46,47,48,49,50,51]), in fact zircon grains found in unsheared or weakly foliated rocks such as those in the Himalayan eclogites [32] also proved to have plastically deformed domains with geologically reset or affected age data. The deformed domains in the investigated zircon grains were identified through EBSD analyses. Later, those structurally identified undeformed domains and crystallographically misoriented domains were analyzed using NanoSIMS with a smaller spot size (<10 μm), and the resulting age data showed a clear trend showing a strong link of deformation with the age-data (see Figure 5 in [32]). The studied zircon grains were found in Group I eclogites (Figure 1c). Group II eclogites (marked on Figure 1c) also contained numerous zircons, but their size was too small and a few grains, analyzed with EBSD, did not show internal deformation. Previous studies interpreted those eclogites to have formed by the transformation of mafic units (basaltic and gabbroic rocks) into metamorphic equivalents during the India-Asia collision and subsequent subduction of the Indian Plate slab [52,53,54,55,56,57,58,59,60].
The focus of this study is centered on the geochemical and geochronological records preserved in zircon grains, separated from Group I eclogites, that were primarily crystallized from the Permian Panjal Trap magmatism. Several zircon grains or domains in those grains were reset afterwards or developed overgrowth domains during eclogite facies metamorphism. Internal structures and crystallographic lattice orientation maps of representative zircon grains are presented to identify deformed domains. Furthermore, some examples from the published literature are briefly highlighted to shed light on the importance of zircon geochronology, sample and domain selection for geochemical analysis, deformational features, and the effects of micro-deformation on U-Pb ages.

4. Materials and Methods

The zircon grains discussed in this study are extracted from the Himalayan eclogites (Figure 1c). The geochronological data presented in this study were previously obtained from zircon grains extracted from Group I and Group II eclogites as reported in earlier publications [30,31,32]. Cathodoluminescence images of the zircon grains, displaying internal structures, were obtained using secondary electron microscopy. To identify the phases of the inclusions in zircons, energy dispersive spectral analysis and RAMAN microscopy were applied. Age data stated with representative zircon grains in Figures 3 and 4 were obtained though in situ analysis using the following three instruments: (1) high-resolution secondary ionization mass spectrometer (HR-SIMS), (2) laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and (3) NanoSIMS. For internal deformation within a single crystal or grain of zircon, crystallographic lattice orientation maps were obtained using the EBSD analysis. Specific details on analytical procedures are reported in past studies [32,33].
EBSD analyses were performed at Kagoshima University’s instrumental analytical center using the Hitachi High-Tech Scanning Electron Microscope (SEM SU-70) (Hitachi, Chiyoda, Tokyo) equipped with a field emission gun and an EDAX EBSD detector. EBSD data from the analyzed grains were acquired using the TSL-OIM Data Collection Software 5.3. Crystallographic orientation data from representative zircon grains were acquired at an accelerating voltage of 20 kV, a probe current of 28 µA, and a working distance of 25 mm. To avoid false signals from the diffracted electrons, careful background subtraction was performed through a charge-coupled device (CCD) camera. Before EBSD analysis, the samples were polished with 3 μm, 1 μm, and 0.3 μm size diamond paste, followed by SYTON colloidal silica gel polish. The diffraction patterns of the investigated zircon grains were detected on a phosphor screen (located in front of the 70° tilted sample in the sample chamber of SEM), and images were acquired through a high-resolution CCD camera. EBSD analysis of each zircon grain was conducted with scanning step-size of 1 and for some grains 2 μm. Data indexing was carried out using the TSL-OIM Data Collection Software 5.3. Crystallographic orientation, in the form of Euler angles (φ1, Φ, φ2), were recorded for each analyzed point in the investigated zircon grains. The diffraction patterns were then integrated on crystallographic orientation maps. Noise reduction (removal of wild spikes to five neighboring indexed points) and data cleaning was completed for 5 iterations. Gray-scale images, EBSD orientation maps, and misorientation maps of the analyzed zircon grains were exported as image files. Internal misorientation within the analyzed zircon grains were reproduced from the EBSD data using the MATLAB program and an open-source toolbox for quantitative textural analysis MTEX 5.7.0 “https://mtex-toolbox.github.io (accessed on 11 March 2024)” [61].

5. Results

The results presented here are based on the data produced from zircon grains from the Himalayan eclogites that were earlier discussed in detail elsewhere (e.g., [30,31,32]). In this contribution, some discussion related to the internal deformation in zircon grains and the effect on U-Pb age is reproduced from the previous study. In addition, EBSD analysis on several newly investigated zircon grains that also shows micro-deformation at grain-scale is presented.

5.1. Petrographic and Textural Features

Based on BSE and CL imaging, the studied zircon grains display two dominant types of internal structural morphologies. One group of grains found in the Group I eclogites, with relatively larger sizes in the range from 100 to 350 μm, displayed irregular zoning, contained abundant healed fractures, and at places showed diffused boundaries (Figure 3). Numerous inclusions including quartz, plagioclase, biotite, ilmenite, kyanite, and phengite were present in the studied zircon grains. In addition, a few grains also contained omphacite, garnet, and rutile. A second group of comparatively smaller zircon grains (50 to 100 μm), found in the Group II eclogites, show fir tree or sector zoning, and sharp boundaries with almost no cracks (Figure 4). The second variety of grains predominantly contained inclusions of quartz, ilmenite, omphacite, garnet, rutile, and rare coesite.
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Figure 3. Cathodoluminescence images of representative zircon grains from the Himalayan Group I eclogites displaying irregular morphologies, healed cracks, and diffused boundaries. These grains were analyzed by LA-ICP-MS [31] and the age data for each analyzed spot is shown. The majority of the analysis displays Permian (260~270 Ma) protolith ages; however, the relatively younger age values likely resulted due to Pb-loss or internal deformation. The scale bar under each grain is 50 μm.
Figure 3. Cathodoluminescence images of representative zircon grains from the Himalayan Group I eclogites displaying irregular morphologies, healed cracks, and diffused boundaries. These grains were analyzed by LA-ICP-MS [31] and the age data for each analyzed spot is shown. The majority of the analysis displays Permian (260~270 Ma) protolith ages; however, the relatively younger age values likely resulted due to Pb-loss or internal deformation. The scale bar under each grain is 50 μm.
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Figure 4. Cathodoluminescence images of representative zircon grains from the Himalayan Group II eclogites displaying metamorphic features (fir tree zoning). These grains were analyzed with HR-SIMS [30] and a few grains with lower contents of U and Pb were additionally analyzed with LA-ICP-MS [31]. Two zircon grains from this group were analyzed using EBSD but there was no internal deformation. Age-data represents the Eocene eclogite facies event in the Himalayan region. The scale bar under each grain is 50 μm.
Figure 4. Cathodoluminescence images of representative zircon grains from the Himalayan Group II eclogites displaying metamorphic features (fir tree zoning). These grains were analyzed with HR-SIMS [30] and a few grains with lower contents of U and Pb were additionally analyzed with LA-ICP-MS [31]. Two zircon grains from this group were analyzed using EBSD but there was no internal deformation. Age-data represents the Eocene eclogite facies event in the Himalayan region. The scale bar under each grain is 50 μm.
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5.2. EBSD Analyses

The EBSD analysis conducted on selected zircon grains in the Himalayan Group I eclogites revealed pristine as well as deformed domains within the studied grains [32]. The results from the homogenously oriented domains yielded concordant U-Pb age data that constrained timing of the magmatic crystallization of the eclogite protoliths, whereas some of the misoriented outer domains in the same grains showed reset or Pb-loss affected ages. The internal misorientation was relatively small (only several degrees different from the central or undeformed domains) but the slight variation in the crystal lattice of the deformed domains affected the geochronological clock (Figure 5, modified from [32]). In addition, a number of zircon grains were analyzed for EBSD and the results are consistent with those reported in the past study from the same eclogites [32]. The newly analyzed zircon grains also showed several degrees of internal misorientations at the grain scale (Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10).

6. Discussion

As can be seen in Figure 5, the zircon grains display several degrees of internal misorientations and, when the differently oriented domains were analyzed with NanoSIMS, some of the portions in those grains yielded ages that were likely affected by deformation or related events. In addition, several other zircon grains in the studied Himalayan Group II eclogites also display internal misorientations that were visualized through EBSD analysis (Figure 6, Figure 7, Figure 8 and Figure 9). A single grain shown in Figure 6 displays several healed cracks observed from the CL and SEM images (Figure 6a,b); however, EBSD phase and orientation maps (Figure 6c,d) elaborate additional information through the internal misorientation at grain scale. A pixel map for the internal misorientation from this grain (Figure 6d) shows subgrain boundary rotation that was not observable via CL or BSE imaging. Similarly, several other grains from the Himalayan eclogites showed identical features when investigated via EBSD analysis (see Figure 7, Figure 8 and Figure 9). The misorientation within the individual grains were plotted in the form of misorientation histograms (Figure 10). All the studied grains show local misorientation (several degrees) within the analyzed grains (Figure 7, Figure 8 and Figure 9). The above-mentioned pieces of evidence suggest that internal deformation at grain-scale may be a common feature in high-grade metamorphic rocks such as eclogites. Internal deformation in zircon grains in highly strained or sheared metamorphic rocks has been reported in a number of studies [40,41,42,43,44,45,46,47,48,49,50,51]. However, Himalayan eclogites lack penetrative deformation or foliation but contain internally deformed domains. This means that zircon grains can easily plastically deform under higher metamorphic pressure and temperature conditions. The large scatter in age data presented in Figure 2 could therefore indicate that zircon grains analyzed for U-Pb isotope ratios may have hidden micro-scale deformations that are not expected to affect age data. This study infers that different domains within the zircon grains may provide constraints on later deformational events that might have influenced the zircon. The discordant or misoriented portions in zircon grains could provide useful information; however, the identification of such domains is necessary, particularly when working with high-grade metamorphic rocks. Such micro-deformed domains are unlikely to be identified via commonly used methodologies. Moreover, the discordant age results in numerous studies can be interpreted for Pb-loss or secondary effects. However, if pristine domains are identified before destructive analyses, geochronologists may be able to get more precise and concordant age results. More detailed work on misoriented domains in zircons is needed to further clarify the effect of deformation on U-Pb ages and other geochemical signatures.

7. Conclusions

Zircon grains are commonly used to trace magmatic and metamorphic events. The geochronological and geochemical results obtained from zircons may be subject to alteration when the rocks hosting those zircon grains are deformed or recrystallized. The investigation of the internal structures of zircon grains, micro-inclusions in them, U-Pb isotope ratios, and trace element characteristics can be used to infer the geological history of rocks. However, looking at the published literature related to zircon U-Pb age data, many analyses show discordant values and plot away from the Concordia line. The discordant data are commonly interpreted to have been caused due to Pb-loss or secondary effects (e.g., hydrothermal alteration, resetting during late-stage deformation). The investigation of internal crystallographic structures of zircons through EBSD analyses can allow us to identify micro-scale deformed domains from those that are unaffected or undeformed. Analyses conducted on pristine domains can increase concordant age results. This study reports micro-scale deformation in zircon grains that are found in non-foliated rocks but contain internal misorientations. Plastic deformation in zircon grains can be interpreted to have resulted in the Himalayan eclogites under high pressure-temperature metamorphic conditions. Additional research is needed in similar rock types from other parts of the world.

Funding

This research was funded by JSPS Kakenhi No. #20K004135 to HUR.

Data Availability Statement

There is no specific research data associated with this article.

Acknowledgments

H.U.R. acknowledges support from students and colleagues at the Department of Earth Sciences, Kagoshima University, and other colleagues from various institutes (PML, Misasa, Okayama University, Japan, Institute of Earth Sciences, Academia Sinica, Taiwan, Atmosphere and Ocean Research Institute, The University of Tokyo, Montpelier University, France, and MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing, China) who helped during the HR-SIMS analysis, LA-ICP-MS analysis, NanoSIMS analysis, EBSD analysis, and useful discussions throughout. In addition, H.U.R. highly appreciate critical and constructive comments from four anonymous reviewers that helped in improving the quality of illustrations, results, and interpretations presented in this study. Bo Causer is also thanked for the language correction and proofing the entire text.

Conflicts of Interest

The author declares no conflict of interest.

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Figure 5. Representative zircon grains from the Himalayan Group I eclogites that show internal deformation and its effect on age data. Gray-scale images (a,c) and EBSD phase maps with crystallographic misorientations (b,d) of the same grains are shown. Gray-scale images are generated from the band contrast map data acquired at the time of EBSD analysis using MTEX (v.5.7.0.), a MATLAB toolbox. Scale bars along the vertical axes in (a) and (c) represent the degree of black and white tone on a scale of 20 to 200. The relatively larger holes in the gray-scale images display the pits formed due to HR-SIMS U-Pb analysis. Small circles on grains in (b,d) with digits are the NanoSIMS analysis. Domains displaying color variation (from blue to green to yellow) in the zircon grains indicate local deformation at the grain scale, evidenced from the difference in their crystallographic orientation with respect to the undeformed domain (blue). The U-Pb age values from the deformed domains were likely affected by deformation. The scale bars along the vertical axes (in (b,d)) show misorientation in degrees within the grain and with the surrounding phases. The investigated zircon grains showed internal misorientation up to 3 degrees. Age values for the analyzed spots are: Spot#7: 50 ± 21 Ma, 8: 182 ± 45 Ma, 9: 113 ± 28 Ma, 10: 95 ± 24 Ma, 35: 138 ± 25 Ma, 36: 145 ± 31 Ma, 37: 96 ± 35 Ma, 38: 61 ± 21 Ma, 39: 113 ± 30 Ma, 40: 94 ± 18 Ma, 41: 47 ± 27 Ma. For the second grain the age data for the analyzed spots are: 11: 120 ± 35 Ma, 12: 162 ± 46 Ma, 13: 180 ± 44 Ma. Age data are the same as reported in [32].
Figure 5. Representative zircon grains from the Himalayan Group I eclogites that show internal deformation and its effect on age data. Gray-scale images (a,c) and EBSD phase maps with crystallographic misorientations (b,d) of the same grains are shown. Gray-scale images are generated from the band contrast map data acquired at the time of EBSD analysis using MTEX (v.5.7.0.), a MATLAB toolbox. Scale bars along the vertical axes in (a) and (c) represent the degree of black and white tone on a scale of 20 to 200. The relatively larger holes in the gray-scale images display the pits formed due to HR-SIMS U-Pb analysis. Small circles on grains in (b,d) with digits are the NanoSIMS analysis. Domains displaying color variation (from blue to green to yellow) in the zircon grains indicate local deformation at the grain scale, evidenced from the difference in their crystallographic orientation with respect to the undeformed domain (blue). The U-Pb age values from the deformed domains were likely affected by deformation. The scale bars along the vertical axes (in (b,d)) show misorientation in degrees within the grain and with the surrounding phases. The investigated zircon grains showed internal misorientation up to 3 degrees. Age values for the analyzed spots are: Spot#7: 50 ± 21 Ma, 8: 182 ± 45 Ma, 9: 113 ± 28 Ma, 10: 95 ± 24 Ma, 35: 138 ± 25 Ma, 36: 145 ± 31 Ma, 37: 96 ± 35 Ma, 38: 61 ± 21 Ma, 39: 113 ± 30 Ma, 40: 94 ± 18 Ma, 41: 47 ± 27 Ma. For the second grain the age data for the analyzed spots are: 11: 120 ± 35 Ma, 12: 162 ± 46 Ma, 13: 180 ± 44 Ma. Age data are the same as reported in [32].
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Figure 6. Cathodoluminescence image (a), gray-scale image obtained from FE-SEM attached to the EBSD instrument (b), the EBSD orientation map (c), and image quality map displayed on the gray-scale image, displaying the internal misorientation (d). The color difference shows internal misorientation within the grain. The white line across the grain is for reference to draw the misorientation histogram shown in Figure 10a. The legend on the right side shows the details of analyzed points in the zircon grain and boundary rotation angles. The scale bar under the zircon grain is 100 μm.
Figure 6. Cathodoluminescence image (a), gray-scale image obtained from FE-SEM attached to the EBSD instrument (b), the EBSD orientation map (c), and image quality map displayed on the gray-scale image, displaying the internal misorientation (d). The color difference shows internal misorientation within the grain. The white line across the grain is for reference to draw the misorientation histogram shown in Figure 10a. The legend on the right side shows the details of analyzed points in the zircon grain and boundary rotation angles. The scale bar under the zircon grain is 100 μm.
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Figure 7. Cathodoluminescence image (a), gray-scale image obtained from FE-SEM attached to the EBSD instrument (b), the EBSD orientation map (c), and image quality map displayed on the gray-scale image, displaying the internal misorientation (d). The white line across the grain is used for the misorientation histogram in Figure 10b. Other details are the same as given in the caption of Figure 6. The scale bar under the zircon grain is 100 μm.
Figure 7. Cathodoluminescence image (a), gray-scale image obtained from FE-SEM attached to the EBSD instrument (b), the EBSD orientation map (c), and image quality map displayed on the gray-scale image, displaying the internal misorientation (d). The white line across the grain is used for the misorientation histogram in Figure 10b. Other details are the same as given in the caption of Figure 6. The scale bar under the zircon grain is 100 μm.
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Figure 8. Cathodoluminescence image (a), gray-scale image obtained from FE-SEM attached to the EBSD instrument (b), the EBSD orientation map (c), and image quality map displayed on the gray-scale image, displaying the internal misorientation (d). The white line across the grain is used for the misorientation histogram in Figure 10c. Other details are the same as given in the caption of Figure 6. The scale bar under the zircon grain is 100 μm.
Figure 8. Cathodoluminescence image (a), gray-scale image obtained from FE-SEM attached to the EBSD instrument (b), the EBSD orientation map (c), and image quality map displayed on the gray-scale image, displaying the internal misorientation (d). The white line across the grain is used for the misorientation histogram in Figure 10c. Other details are the same as given in the caption of Figure 6. The scale bar under the zircon grain is 100 μm.
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Figure 9. Cathodoluminescence image (a), gray-scale image (b), the crystallographic orientation map (c), and image quality map displayed on the gray-scale image (d). The white line across the grain is used for the misorientation histogram in Figure 10d. Other details are the same as given in the caption of Figure 6. The scale bar under the zircon grain is 100 μm.
Figure 9. Cathodoluminescence image (a), gray-scale image (b), the crystallographic orientation map (c), and image quality map displayed on the gray-scale image (d). The white line across the grain is used for the misorientation histogram in Figure 10d. Other details are the same as given in the caption of Figure 6. The scale bar under the zircon grain is 100 μm.
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Figure 10. Histograms displaying the misorientation angles in degrees (horizontal axes) against the number fractions (vertical axes) in the studied zircon grains (a–d). The histograms in (a–d) are plotted along the line shown on the zircon grains presented in Figure 6, Figure 7, Figure 8 and Figure 9, respectively. Scale bars under the zircon grains shown in (a,b,d) are 100 μm whereas in (c) it is 90 μm.
Figure 10. Histograms displaying the misorientation angles in degrees (horizontal axes) against the number fractions (vertical axes) in the studied zircon grains (a–d). The histograms in (a–d) are plotted along the line shown on the zircon grains presented in Figure 6, Figure 7, Figure 8 and Figure 9, respectively. Scale bars under the zircon grains shown in (a,b,d) are 100 μm whereas in (c) it is 90 μm.
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Rehman, H.U. Zircon Internal Deformation and Its Effect on U-Pb Geochronology: A Case Study from the Himalayan High-Pressure Eclogites. Minerals 2024, 14, 742. https://doi.org/10.3390/min14080742

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Rehman HU. Zircon Internal Deformation and Its Effect on U-Pb Geochronology: A Case Study from the Himalayan High-Pressure Eclogites. Minerals. 2024; 14(8):742. https://doi.org/10.3390/min14080742

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Rehman, Hafiz U. 2024. "Zircon Internal Deformation and Its Effect on U-Pb Geochronology: A Case Study from the Himalayan High-Pressure Eclogites" Minerals 14, no. 8: 742. https://doi.org/10.3390/min14080742

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