Microtextures, geochemistry and geochronology of authigenic xenotime: constraining the cementation history of a Palaeoproterozoic metasedimentary sequence

Abstract

An ≈ 26 m thick unit of phosphatic sandstone and black shale (the Phosphatic Unit) in the Palaeoproterozoic Mount Barren Group of south‐western Australia contains abundant authigenic xenotime crystals showing well‐preserved diagenetic textures. Despite extensive regional deformation and thermal metamorphism, the peak of which occurred at ≈ 1205 Ma, the Phosphatic Unit was preserved as a low‐strain envelope because of its pre‐compaction carbonate and phosphate cementation. In situ U–Pb geochronology of xenotime reveals four discrete age populations at 1693 ± 4, 1645 ± 3, 1578 ± 10 and 1481 ± 21 Ma. When integrated with petrography, the age data place a timeframe on: (i) sediment deposition; (ii) phosphogenesis; (iii) diagenetic cement infilling; (iv) diagenetic pyrite formation; (v) secondary porosity generation; (vi) hydrocarbon migration; (vii) burial compaction; and (viii) hydrothermal alteration, up until peak thermal metamorphism. Xenotime growth at ≈ 1693 Ma occurred prior to compaction, whereas xenotime growth at ≈ 1645 Ma occurred during burial. Xenotime growth at ≈ 1580 Ma and at ≈ 1480 Ma appears to be the far‐field record of thermotectonic events associated with intracontinental extension and magmatism recorded elsewhere in Australia. Geochemical analysis, integrated with geochronology, shows a systematic increase in MREE/HREE in xenotime crystals with decreasing age and with increasing stratigraphic depth. Coupled with a decrease in xenotime abundance and age with depth, it suggests that: (i) the main focus of porosity infilling was at the top of the Phosphatic Unit and progressed downwards over the > 200 Myr period of porosity infilling, and (ii) the changes in xenotime REE chemistry may be due to an influx of MREE from increasing amounts of dissolved apatite or changes, with respect to REE solubility, in the physiochemical nature of the fluids with burial depth.

Characterization of Xenotime From Datas (Brazil) as a Potential Reference Material for In Situ U‐Pb Geochronology

“…Several authors have used the xenotime composition in order to distinguish its origin (e.g., Kositcin et al, ; Lan et al, ; McNaughton & Rasmussen, ; Rasmussen et al, ; Tartèse et al, ; Vallini et al, ). By comparing the composition of XN01–XN05 (Table ) against other xenotimes, we observed a higher correlation between the Datas xenotime and hydrothermal xenotime studied by Lan et al () and Kositcin et al (), even though there are some overlaps between the hydrothermal data from the literature and some metamorphic and igneous xenotime data (Figure ).…”

In situ SHRIMP U–Pb dating of monazite integrated with petrology and textures: Does bulk composition control whether monazite forms in low-Ca pelitic rocks during amphibolite facies metamorphism?

“…where in the Mount Barren Group (Vallini et al, 2002(Vallini et al, , 2005unpub. data), did not survive prograde amphibolite facies metamorphism.…”

Helium Diffusion in Natural Xenotime

“…Xenotime is commonly dated using in situ methods including electron microprobe (Hetherington et al, 2008;Švecová, et al, 2016) ion microprobe (Aleinikoff, Hayes, et al, 2012;Aleinikoff, Slack, et al, 2012;Aleinikoff et al, 2015;Fletcher et al, 2004), and laser microprobe (Grand'Homme et al, 2016;Liu et al, 2011) techniques in addition to traditional isotope dilution methods (Hawkins & Bowring, 1997). Xenotime U/Pb dates, often paired with compositional analyses, are used to resolve crystallization, metamorphic, and diagenetic ages in rocks (Aleinikoff & Grauch, 1990;Harrison et al, 2002;Hetherington et al, 2008;Vallini et al, 2002Vallini et al, , 2005 and to constrain the timing of hydrothermal and mineralization events in ore deposits (Aleinikoff, Hayes, et al, 2012;Aleinikoff, Slack, et al, 2012;Aleinikoff, Grauch, et al, 2012;Grand'Homme et al, 2016;Švecová, et al, 2016).…”