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Abstract: Bulk composition and specific reaction history among common silicate minerals have been proposed as controls on monazite growth in metapelitic rocks during amphibolite facies metamorphism. It has also been implied that monazite that formed during greenschist facies metamorphism may be preserved unchanged under upper amphibolite facies conditions. If correct, this would make the interpretation of monazite ages in polymetamorphic rocks exceedingly difficult, because isotopic dates could vary significantly in rocks that have experienced identical metamorphic conditions but differ only slightly in whole-rock composition. Low-Ca pelitic schists from the Mount Barren Group in southwestern Australia display a range of whole-rock compositions in AFM space and different peak mineral assemblages resulting from amphibolite facies metamorphism (∼8kb, 650°C). In this study, we test whether bulk composition controls the formation of monazite through geochronology and textural evidence linking monazite growth with deformation and peak metamorphism. X-ray element mapping of monazite from the metapelitic rocks reveals concentric zoning in many grains with compositionally distinct cores and rims. In situ SHRIMP U–Pb geochronology of monazite yields two 207Pb/206Pb age populations. The cores, and texturally early monazite, give an age of 1209±10Ma, interpreted to record prograde metamorphism, whereas the rims and “late” monazite define a single population of 1186±6Ma, which is considered the likely age of peak thermal metamorphism. The growth of monazite was widespread in low-Ca pelitic schists representing a broad range of compositions in AFM space, indicating that variations in bulk composition in AFM space did not control the formation of monazite during amphibolite facies metamorphism in the Mount Barren Group. [Copyright &y& Elsevier]

Zircon U-Pb geochronology is vital for providing age constraints for the calibration of the geologic timescale. The presence of zircon grains that yield concordant ages younger than the host sedimentary rock is often ignored, attributed to young outliers and possible contaminants. Using in situ U-Pb dating techniques, which analyze zircon crystals drilled directly from polished thin sections, two tuff beds from the Brockman Iron Formation, Pilbara Craton, Australia, yielded weighted mean 207Pb/206Pb ages of 2466 ± 6 Ma and 2469 ± 5 Ma (95% confidence level), consistent with independent age constraints. The tuffs also contain zircons that gave younger, yet concordant, U-Pb dates (2450–2270 Ma) as well as some with highly discordant dates. The "young" zircons have low or negligible common Pb, and display magmatic-type euhedral outlines, oscillatory zonation and Th/U ratios, indistinguishable from syn-depositional zircons, and cannot be readily dismissed on the grounds of high discordance or common-Pb contents. The "young" zircon dates are statistical outliers that overlap with the age of hydrothermal xenotime in deformed, bedding-parallel veins in shale that underlies the iron formation, which yields an age of 2383 ± 4 Ma (95% confidence level). The xenotime in overpressure veins provides independent evidence for hydrothermal fluid flow broadly coeval with isotopic resetting of "young" zircon crystals. Although SEM-CL imaging found no evidence for alteration, element mapping shows that most zircon grains have been altered by fluid infiltration, which proceeded along fractures or radiation-damaged growth zones leading to elevated Fe, Ca and Y contents in the altered domains. Total and partial Pb loss caused by hydrothermal alteration can explain the young concordant and discordant ages, respectively. These results have implications for the use of U-Pb zircon geochronology, particularly in detrital zircon studies, which routinely use the age of the youngest zircon grains to constrain the age of deposition. The erosion and re-deposition or in situ growth of young zircons in low-grade metasedimentary rocks may lead to erroneously young depositional ages. [ABSTRACT FROM AUTHOR]

Uranium-lead geochronology of zircon in felsic tuffs plays a key role in the development of chronostratigraphic frameworks for sedimentary successions. This is particularly true for Precambrian basins, where biostratigraphy is of limited use for age determination or stratigraphic correlation. Although zircon is widely used to date tuffs, it can yield null results due to the presence of xenocrystic populations or isotopic resetting. Other potential U-Pb chronometers, such as monazite, which also crystallize from felsic magmas, are largely unreported from felsic tuff layers. Consequently, there has been a lack of studies to evaluate the potential of monazite for U-Pb isotope geochronology of tuffs. We present results of in situ U-Pb ion microprobe dating of monazite that co-exists with zircon in felsic tuffs from two Precambrian basins: the Earaheedy Basin (Frere Formation) in Western Australia and the Bushveld Basin (Lephala Formation) in southern Africa. We show that 207 Pb/ 206 Pb ages from monazite in felsic tuffs from the Frere Formation (1891 ± 9 Ma) and the Lephala Formation (2307 ± 9 Ma) are identical to zircon ages from the respective tuffs. Also, monazite in the Bushveld tuff preserves additional information, including a chronological fingerprint of the volcanic source (2.4–2.5 Ga xenocrysts) and the timing of low-grade metamorphism (2.16 Ga overgrowths). The ca. 2.16 Ga low-grade metamorphism identified in the Bushveld Basin is reminiscent of that widely recorded in the Pilbara Craton, Western Australian, lending support to arguments for a connection between the Kaapvaal and Pilbara cratons. Igneous and low-grade metamorphic monazites are readily distinguished based on texture and chemistry. Our study demonstrates that monazite can be used to accurately date felsic tuffs, offering a powerful tool for constraining depositional and metamorphic histories of Precambrian sedimentary basins. [ABSTRACT FROM AUTHOR]

Abstract: In situ U–Pb isotopic measurements were carried out by ion microprobe on the Zr-rich accessory minerals zirconolite [CaZrTi2O7], tranquillityite [Fe8 2+(ZrY)2Ti3Si3O24] and baddeleyite [ZrO2] in low-K, high-Ti mare basalt 10047 collected during the Apollo 11 mission. The analysed minerals are concentrated in pockets of late-stage mesostasis that comprises an intergrowth of silica, barian K-feldspar and Si–Al–K glass, from a phaneritic, subophitic, basalt comprising mainly pyroxene, plagioclase, ilmenite, cristobalite and troilite. Most Zr-rich minerals are unaltered, however, some tranquillityite is replaced by a complex intergrowth of zirconolite, baddeleyite, ilmenite and fayalite, suggesting that the mineral became unstable during crystallization. Several baddeleyite crystals have also undergone alteration to secondary zircon. Zirconolite was analysed in thin section 10047,11 and tranquillityite and baddeleyite in 10047,227, using a ∼6μm primary ion beam. Both zirconolite and tranquillityite have significant U and low initial Pb contents, and are highly suitable for Pb/Pb dating. Fifteen analyses of zirconolite give a 207Pb/206Pb age of 3708±7Ma (207Pb/206Pb:204Pb/206Pb isochron; 95% confidence, including renormalisation of ratios) and twenty-five analyses of tranquillityite give 3710±6Ma. The 207Pb/206Pb dates are consistent with each other and refine results from an earlier study. Baddeleyite data were less precise, mainly due to lower secondary ionisation efficiency. Our results show that zirconolite and tranquillityite can provide precise isotopic dates and, given their presence in other samples, they represent important U–Pb chronometers for refining lunar geology. [Copyright &y& Elsevier]

Abstract: In situ U–Pb dating of monazite and xenotime in sedimentary rocks from the mid-Archean Soanesville Group in the Pilbara Craton, yields ages for provenance, diagenesis and multiple low-grade metamorphic events. Detrital monazite and xenotime grains give dates >3250Ma, whereas diagenetic xenotime provides a new minimum age of 3190±10Ma for deposition of the basal Soanesville Group, previously constrained between ∼3235Ma and ∼2955Ma. Metamorphic monazite provides evidence for three episodes of growth: at 2.88, 2.16 and 1.65Ga. Element mapping of monazite for La, Sm, Y and Th reveals distinct cores and rims in some crystals that were used to guide the placement of analytical spots during in situ U–Pb dating by sensitive high-resolution ion microprobe (SHRIMP). Specifically, La and Sm distributions closely correlate with different generations of monazite. The presence of two generations in single monazite crystals highlights the need for characterizing mineral chemistry prior to geochronology. It also shows the importance of using in situ dating techniques rather than methods that rely on the analysis of entire, potentially multi-aged, crystals. The ages recorded by metamorphic monazite span more than one billion years and are interpreted to record cryptic tectonothermal events within the craton. The 2.88Ga age coincides with a phase of regional deformation, metamorphism and gold mineralization along a major crustal lineament, whereas the most common monazite age population (at 2.16Ga) corresponds with the migration of a foreland fold-and-thrust belt across the craton. The youngest age (1.65Ga) coincides with an episode of tectonic reworking in the Capricorn Orogen along the southern Pilbara margin. The prolonged history of monazite growth may, in part, relate to channelized fluid flow during reactivation of long-lived N- to NE-trending crustal structures that transect the craton. Despite repeated episodes of metamorphism, the isotopic system in each generation of monazite remained unperturbed, yielding precise dates. The ability of monazite to record three separate events, and in some instances two events in a single crystal, distinguishes it from most other low-temperature mineral chronometers, which are readily reset during metamorphic overprinting. Low-temperature monazite geochronology can provide a detailed isotopic history of cryptic thermal events and reveal the temporal and spatial patterns of far-field fluid flow related to tectonic processes. The previously unrecognized history of crustal fluid flow in the Pilbara Craton has implications for chemical, mineralogical and isotopic studies seeking to understand conditions on the early Earth. [Copyright &y& Elsevier]

Abstract: A population of oscillatory zoned, igneous zircon grains in a Javanese andesite contains fluid and mineral inclusions (up to 10 μm across) trapped during zircon growth. Orientation contrast imaging and orientation mapping by electron backscatter diffraction reveal that crystal-plastic deformation overprints growth zoning and has localized around 1–10 μm pores and inclusions. Cumulative crystallographic misorientation of up to 25° around pores and inclusions in zircon is predominantly accommodated by low-angle (<5°) orientation boundaries, with few free dislocations in subgrain interiors. Low-angle boundaries are curved, with multiple orientation segments at the sub-micrometer scale. Misorientation axes associated with the most common boundaries align with the zircon c-axis and are consistent with dislocation creep dominated by <100>(010) slip. A distinctly different population of sub-micron pores is present along subgrain boundaries and their triple junctions. These are interpreted to have formed as a geometric consequence of dislocation interaction during crystal-plasticity. Dislocation creep microstructures are spatially related to differences in cathodoluminescence spectra that indicate variations in the abundance of CL-active rare earth elements. The extent of the modification suggests deformation-related fast-pathway diffusion distances that are over five orders of magnitude greater than expected for volume diffusion. This enhanced diffusion is interpreted to represent a combination of fast-diffusion pathways associated with creep cavitation, dislocations and along low-angle boundaries. These new data indicate that ductile deformation localised around inclusions can provide fast pathways for geochemical exchange. These pathways may provide links to the zircon grain boundary, thus negating the widely held assumption that inclusions in fracture-free zircon are geochemically armoured once they are physically enclosed. [Copyright &y& Elsevier]