Geochemical characterisation of xenotime formation environments using U-Th
Introduction
Xenotime (YPO4) is a trace mineral in many rock types (Rasmussen, 2005). Its significance far outweighs its low abundance and small grain size for two main reasons: 1) it forms in many common rock types from a range of high- to low-temperature processes, and 2) it is an excellent mineral for U/Pb geochronology, particularly in-situ geochronology where petrography is pivotal to age interpretations. As such, petrographically characterised xenotime is an underused but important mineral in the determination of the fourth dimension (i.e. time) in resolving the 4D evolution of terranes with mixed rock assemblages.
Xenotime and its sister REE-phosphate mineral, monazite, are commonly found in fractionated pegmatites, where they can form large (i.e. > 1 cm) crystals. However, in addition to pegmatites and other fractionated and/or alkaline igneous rocks, xenotime also forms as an early to late diagenetic overgrowth on detrital zircon grains in siliciclastic sediments, during hydrothermal mineralisation, and as a low- to high-grade metamorphic mineral in metasedimentary rocks (see Table 1).
Although the occurrence of xenotime is relatively common and widespread in many common rocks, the size of most non-igneous xenotime grains is typically < 10 μm and its low abundance combine to make it largely invisible in routine optical petrography studies. Its recognition usually relies on targeted scanning electron microscopic inspection. Further, in-situ geochronology at the scale of xenotime occurrences is currently constrained to those grains which are ~ 10 μm in size or larger. Smaller analysis sizes by ion microprobes are feasible (c.f. monazite; Stern et al., 2005), but require high U-contents to produce usable analytical precisions. This size constraint is a balance between decreasing age precision with smaller areas of analysis, particularly for hydrothermal/metamorphic xenotime which generally have low U-contents, and the availability of enough, sufficiently large, grains for analysis. Further, the depth of analysis increases from ion microprobe, electron microprobe to LA-ICPMS analysis (laser ablation-inductively coupled plasma mass spectrometer), thus constraining the application of the latter to analysing xenotime grains which may be thinner than ~ 10 μm.
This study will focus on xenotime documented in previous studies where it is primarily used for geochronology. Such studies provide the best definition of the xenotime origin, deduced from petrography and age data, and routinely provide the U- and Th-contents of the xenotime, but generally no other trace element data. A few studies have published additional geochemical data to complement interpretations of the xenotime formation environment. Kositcin et al. (2003) identified that the REEs (Rare Earth Elements) and U-Th contents of xenotime may distinguish igneous and diagenetic from hydrothermal growth, features recently confirmed by Aleinikoff et al. (2015). Zi et al. (2015) also observed different U-Th characteristics between detrital (presumed igneous) and hydrothermal xenotime in metasedimentary rocks, and between different generations of hydrothermal xenotime in the same sedimentary samples. Vallini et al. (2005) documented four phases of diagenetic/hydrothermal xenotime cement infill of pore spaces within metasandstones of the Mt. Barren Group, Western Australia, using petrography and in-situ geochronology. The REE compositions of the four xenotime generations showed a systematic increase in Mid-REE/Heavy-REE (i.e. Gd/Yb) with decreasing age and increasing depth in a phosphatic sandstone unit. Kositcin et al. (2003), Aleinikoff et al., 2012b, Aleinikoff et al., 2012c, Aleinikoff et al., 2015 and Lan et al. (2013) all noted the presence of a significant negative Eu-anomaly in igneous, and most detrital (presumed igneous) xenotime grains, and the general absence of a Eu-anomaly in diagenetic and hydrothermal xenotime. Rasmussen et al. (2007c) used mid-HREE maps of xenotime grains to explain growth complexities and noted high Fe in analyses of xenotime related to iron ores, which was attributed to fine hematite inclusions.
Because most published geochronology data for xenotime, in which the xenotime origin has been deduced, present U- and Th-content data, this study seeks to assess xenotime U- and Th-contents as a potential discriminator of xenotime formation environment. There are insufficient data to more thoroughly review other geochemical discriminators of formation environment at this stage.
Section snippets
Petrography of xenotime
Examples of xenotime formed in different environments are shown in Fig. 1, Fig. 2, Fig. 3. Xenotime formed in metasedimentary rocks are described generally by Rasmussen, 1996, Rasmussen, 2005 and all formation environments are described specifically in publications listed in Table 1. Metamorphic xenotime (Fig. 1) is identified from its age, within the 4D context of the sample, and textural relationships to relict and metamorphic minerals and fabrics. Xenotime may form at very low metamorphic
Methodology
Reliable published xenotime geochronology studies, where the relevant data are available, are compiled and categorised by xenotime formation environment (Table 1). We have declined to use geochemical data where the age information related to chemical analyses is lacking, given the fine scale growth histories and complexities which are observed in some studies (e.g. Vallini et al., 2005). This also excludes much work on xenotime on fractionated granites/pegmatites where the distinction between
Discrimination of xenotime formation environment
Within most rocks, the petrography of xenotime grains provides the initial interpretation of their formation environment. However, for detrital grains in metasedimentary rocks and rocks with geological overprint complexities, primary formation environments are more obscure, and may be clarified by the addition of geochemical discrimination.
In general, the geochemical data for xenotime from all environments of formation overlap to a large extent on a U vs U/Th diagram (Fig. 20). However, a
Acknowledgements
This compilation is based on many published xenotime geochronology studies, some of which would not have been possible without the technical expertise and thoroughness of Ian Fletcher, who is thanked sincerely. The authors acknowledge the many students and colleagues whose published work and countless discussions on xenotime shaped the data interpretations discussed herein. Reviews from Tsuyoshi Ilzuka and an anonymous reviewer are greatly appreciated. NM acknowledges the support of the John de
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Authigenic monazite and xenotime Pb-Pb/U-Pb dating of siliciclastic sedimentary rocks
2022, Earth-Science ReviewsCitation Excerpt :Distinguishing diagenetic from hydrothermal xenotime is not straightforward because they both display pyramidal or irregular overgrowths onto detrital zircons and possess indistinguishable U, Th and REE compositions (Kositcin et al., 2003; Vallini et al., 2005, 2006; Lan et al., 2013; Shi et al., 2015; McNaughton and Rasmussen, 2018). Such chemical resemblance points to a gradual rather than abrupt transition from diagenetic to hydrothermal fluids (Lan et al., 2013; McNaughton and Rasmussen, 2018). In this situation petrographic observation will be the dominant tool to determine the origin.
SIMS Pb-Pb dating of phosphates in the Proterozoic strata of SE North China Craton: Constraints on eukaryote evolution
2022, Precambrian ResearchCitation Excerpt :Morphology, texture and geochemistry have been utilized to discriminate xenotime of different origins. Such methods can easily discriminate detrital xenotime and monazite because of their typically negative Eu anomalies and rounded to subrounded morphology, but it is difficult to distinguish diagenetic and hydrothermal origins because of their similar morphology and mineral chemistry (Kositcin et al., 2003; Vallini et al., 2005, 2006; Lan et al., 2013; Shi et al., 2015; McNaughton and Rasmussen, 2018; Zi et al., 2018). Diagenetic/hydrothermal xenotime commonly overgrow onto detrital zircon displaying an irregular morphology and HREE enrichment with insignificant negative Eu anomaly, whereas diagenetic/hydrothermal monazite commonly occur independently showing an irregular morphology and LREE enrichment with insignificant negative Eu anomaly (Kositcin et al., 2003; Rasmussen et al., 2004, 2010; Vallini et al., 2005, 2006; Lan et al., 2013; Zi et al., 2015; McNaughton and Rasmussen, 2018).
Pb fractionation. Interpretation of radiometric ages sometimes encounters difficulties because of diverse origins of phosphates including diagenetic, hydrothermal, and detrital origins. An integrated petrographic and geochemical study can aid in telling apart diagenetic xenotime/monazite from their hydrothermal and detrital counterparts, thereby correctly applying them to constrain the depositional age of sedimentary rocks.