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Zircon-melt partitioning of Ce is sensitive to the redox conditions of magmas as zircon crystallizes, because Ce4+ is far more compatible than Ce3+ in zircon. This makes Ce in zircon an effective tool to constrain magma oxygen fugacity (f O 2). However, trivalent light rare earth element (LREE) concentrations in zircon analyses are easily inflated by inclusions and alteration, which makes estimation of the concentration of Ce3+ within the zircon structure very difficult. We propose new treatments of measured Ce concentrations in zircon and of zircon/melt Ce partitioning to improve estimates of Ce4+ and Ce3+ in zircon and the melt from which it grew. To minimize the LREE contamination effect that plagues zircon analysis, we propose a revised method to quantify Ce3+ and Ce4+ in zircon that extrapolates using a logarithmic function of ionic radius of the middle to heavy rare earth elements. We then present improved Ti-dependent zircon/melt partition coefficients for both Ce3+ and Ce4+ based on the lattice strain model to estimate Ce3+ and Ce4+ in the coexisting melt. Derived estimates of Ce4+ in both zircon and melt are robust; estimates of Ce3+ are less so. We then calculate Ce4+/Ce3+ of the melt using both Ce3+ melt obtained from our new partition coefficients and typical Ce total concentrations in melts based on the relatively narrow global measured range. The f O 2 values calculated using these estimated Ce4+/Ce3+ ratios and the model of Smythe and Brenan (2016) for analyzed zircons from a range of tectonic settings fall within plausible ranges. We suggest that our new approaches, particularly with respect to estimating Ce4+ concentrations, may be broadly useful for constraining magma oxidation state, especially for studies of detrital zircon whose provenance is unknown. • An alternative method of determining Ce3+ and Ce4+ in zircon and melt. • Improved zircon/melt partition coefficients for Ce3+ and Ce4+. • Contaminated zircon analyses can be screened out by La concentrations. • Modeled melt Ce4+/Ce3+ is capable of constraining magma oxidation states. [ABSTRACT FROM AUTHOR]

In-situ geochemical and isotopic analyses of zircon and monazite can provide compelling and detailed insights into the source and evolution of their hosting granite. Here we investigated the U-Th-Pb and Hf/Nd-O isotopic signatures of zircon and monazite in two petrological zones (i.e., fine- to medium-grained zone and medium-grained zone) of the Laojunshan granite pluton in the Qinling Orogen, central China, to better constrain their source nature and evolution processes. Co-existing magmatic zircon and monazite in both zones gave identical crystallization ages of ca. 113 Ma. In the fine- to medium-grained zone, both magmatic and xenocrystic zircons were identified, which respectively have ε Hf (t) values of −6.5 to −5.0 and − 17.4 to +5.1, and δ18O VSMOW values of +5.25‰ to +5.78‰ and + 5.17‰ to +9.15‰. The age distribution pattern and Hf O isotopic characteristics of the xenocrystic zircons are similar to those of detrital zircons from the wall rock, i.e., meta-sedimentary rocks in the Kuanping unit. Monazite grains in this zone have relatively low SiO 2 , ThO 2 but high CaO contents, dominated by cheralite-type substitution. These monazites have ε Nd (t) values of −5.8 to −4.6 and δ18O values of +3.79‰ to +5.66‰, except one grain (ε Nd (t) = −10.4 to −9.6, δ18O = +9.22‰ to +9.81‰), which is likely a xenocrystic crystal assimilated from wall rocks. The identical U Pb age but contrasting Nd O isotopic compositions between the xenocrystic monazite and the magmatic monazite grains indicate that the former might have experienced solid-state recrystallization process and completely lost radiogenic Pb. Both zircon and monazite geochemical and isotopic signatures consistently indicate that crustal assimilation occurred in the fine- to medium-grained granite zone. In contrast, in the medium-grained zone, only magmatic zircons and monazites were identified. The magmatic zircons have ε Hf (t) values of −3.2 to +2.7, and δ18O values of +4.65‰ to +6.36‰. The magmatic monazites are characterized by huttonite-type substitution with high SiO 2 , ThO 2 but low CaO contents. Their ε Nd (t) values vary from −5.7 to +1.6 (mostly < −4.4) and δ18O values from +4.65‰ to +6.36‰. Given that the isotopic features of zircons and monazites in this zone are consistent with their whole-rock isotope features and no inherited/xenocrystic grains were observed, the zircons and monazites should record the source isotope compositions. The presence of highly radiogenic Nd isotopes in monazite coupled with its normal mantle-like O isotopes suggest minor involvement of juvenile mantle-derived components in the magma source. Accordingly, we advocate combined study of zircon and monazite, which can better track granite source nature and evolution processes than individual mineral. • Zircon and monazite U Pb dating constrain the Laojunshan pluton formed at ca. 113 Ma. • Slight crustal contamination and magma injection have been effectively identified. • In-situ Hf/Nd-O isotopes can refine evolution processes and source nature of granite. [ABSTRACT FROM AUTHOR]

Two baddeleyite megacrysts were evaluated as potential reference materials (RMs) for SIMS oxygen isotope analysis, and utilized to understand and calibrate instrumental mass fractionation (IMF). A baddeleyite crystal (S0045) from the Phalaborwa carbonatite, South Africa has a mean δ 18 O VSMOW  = +4.6 ± 0.3‰ (range 0.75‰) measured using laser fluorination gas source mass spectrometry (LF-GMS) and one (S0069) from the Mogok metamorphic belt, Myanmar has δ 18 O VSMOW  = +22.2 ± 0.4‰ (range 0.89‰). SIMS standardization utilizing these inherently heterogeneous RMs is possible by analyzing a number of crystal fragments and utilizing one of them lying at the median of the range. Metamictization, lattice orientation, and chemical composition do not appear to be significant (<0.5‰) variables in matrix matching of RMs and unknowns. Propagation of errors while utilizing the imperfect RMs results in 10 μm diameter spot uncertainties of about ±0.3‰ (2σ). SIMS oxygen isotope analysis of co-crystalline zircon and baddeleyite from the 2.2 Ga Duck Lake sill (DLS) in the Northwest Territories, Canada, yield predominant δ 18 O VSMOW modes of +6.0‰ and +3.2‰, respectively. This difference is consistent with preserving high-temperature isotopic equilibrium between zircon and baddeleyite. DLS baddeleyite δ 18 O data as a whole are negatively skewed (to 0.0‰), and interpreted to reflect low temperature, open-system behaviour. Zircon δ 18 O are less affected, but also show hints of the same influences of secondary alteration and oxygen isotope exchange. [ABSTRACT FROM AUTHOR]

The full U-Pb isotopic systematics in a complex lunar zircon ‘Pomegranate’ from lunar impact breccia 73235 have been investigated by the development of a novel Secondary Ion Mass Spectrometry (SIMS) scanning ion imaging (SII) technique. This technique offers at least a four-fold increase in analytical spatial resolution over traditional SIMS analyses in zircon. Results from this study confirm the hypothesis that the Pomegranate zircon crystallized at 4.302 ± 0.013 Ga and experienced an impact that formed, U-enriched zircon around primary zircon cores at 4.184 ± 0.007 Ga (2σ, all uncertainties). The increase in spatial resolution offered by this technique has facilitated targeting of primary zircon that was previously inaccessible to conventional spot analyses. This approach has yielded results indicating that individual grains with a diffusive distance of less than ~ 4 μm have been reset to the young impact age, while individual grains with a diffusive distance larger than ~ 6 μm have retained the old crystallization age. Assuming a broad range in cooling rate of 0.5–50 °C/year, which has been observed in a suite of similar lunar breccias, a maximum localized temperature generated by the impact that reset small primary zircon and created new, high-U zircon is estimated to be between 1100 and 1280 °C. [ABSTRACT FROM AUTHOR]

Partial loss of radiogenic Pb and presence of older grains or cores (inheritance) are the main obstacles for accurate and precise U–Pb geochronology of magmatic zircons by ID-TIMS (isotope dilution thermal ionisation mass spectrometry). Sequential annealing and leaching (chemical abrasion, (Mattinson 2005)), restores crystallinity to zircons with low to medium radiation damage, and removes parts of the zircon with strongly radiation damaged crystal lattice, effectively eliminating Pb loss. The original version of chemical abrasion (Mattinson 2005) was designed for determination of precise 206 Pb– 238 U plateau ages of multi-grain zircon populations, free from inheritance, using multiple dissolution steps. However, the main application of chemical abrasion is to date zircon populations potentially affected by both Pb loss and inheritance, which is commonly achieved by analysis of single zircon grains. Chemical abrasion of single zircon grains involves only one high temperature leaching step, since stepwise leaching is rarely possible or practical due to the small sample size. Despite the widespread use of single zircon chemical abrasion, the optimal temperature and duration of annealing and single step leaching have not been reported thus far. Furthermore, the temperature and time durations of chemical abrasion (apparently an ad-hoc adaptation of the findings of multi-grain, multi-step leaching experiments) vary substantially between published geochronological studies, leaving the possibility of incomplete removal of Pb loss and hence a potential bias in comparison of the ages. Here we report the results of a systematic study aimed at determination of optimal conditions for chemical abrasion of single zircon grains to ensure complete removal of zones that were affected by Pb loss in a single high temperature leaching step. The tests were performed on three different zircon standards with various ages (~ 418 to 3465 Ma) using a three step partial dissolution technique. The optimal conditions for low to moderately radiation damaged zircons are annealing at 900 °C for 48 h and subsequent leaching at 190 °C or slightly higher in concentrated HF in a pressurized vessel for 15 h. Leaching at a lower temperature does not completely remove Pb loss. Annealing at higher temperature might be restoring crystallinity in the parts of zircon crystals that were affected by Pb loss. [ABSTRACT FROM AUTHOR]

This study aims to improve our understanding of the current limitations to high-precision U–Pb analysis of zircon by LA-ICP-MS by investigating the underlying causes of variation in ablation behaviour between different zircon matrices. Multiple factors such as: the degree of accumulated radiation damage; trace element composition; crystal colour; and crystallographic orientation are all systematically investigated. Due to the marked decrease in elastic moduli of natural zircon crystals with increasing radiation damage, the accumulation of this damage is the dominant factor controlling the rate of ablation for partially damaged to highly metamict zircon samples. There are slight differences, however, in ablation behaviour between highly crystalline matrices that cannot be attributed solely to differences in the degree of accumulated radiation damage. These differences are associated with structural weakening (i.e., decrease in elastic moduli and overall lower mechanical resistance) caused by an increasing degree of cation substitution in some of the zircon samples. Effects of crystallographic orientation and of crystal opacity (i.e., colour) on ablation behaviour are negligible compared to the combined influences of accumulated radiation damage and trace element substitution into the zircon structure. Experiments performed on natural and annealed zircon grains reveal that the reduction in ablation rates observed for the treated samples compared to the untreated grains is proportional to the degree of structural reconstitution achieved after annealing. Thermal annealing of natural zircon at temperatures > 1000 °C results in much more uniform ablation characteristics. This ‘homogenisation’ of ablation behaviour between zircon matrices produces a decrease in the laser-induced matrix effects and subsequent improvement in the accuracy of 206 Pb/ 238 U ratio determinations by LA-ICP-MS. [ABSTRACT FROM AUTHOR]

A persistent question in early Earth investigations is the depletion history of the mantle as recorded in the isotopic compositions of the oldest rocks and minerals. Here we present new, in situ, sensitive high resolution ion microprobe (SHRIMP) U–Th–Pb geochronology, and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) Lu–Hf tracing data for individual zircons extracted from the oldest rocks of the Antarctic and Australian continents. These are 3.88 and 3.85 Ga orthogneisses from the Gage Ridge and Mt. Sones in Enderby Land and a 3.73 Ga Meeberrie gneiss from the Yilgarn Craton. The zircons from each sample show complex U–Th–Pb systematics due to the mobile behaviour of Pb, resulting in several age populations with discordant to reversely discordant arrays on concordia plots. In contrast, measured 176 Hf/ 177 Hf compositions from the same analysis sites show narrowly defined compositional ranges, with 3 or fewer groups for each sample, reflecting the resistance of 176 Hf/ 177 Hf in zircons to disturbance even under high-temperature, granulite conditions. The initial 176 Hf/ 177 Hf calculated for the most primitive Hf population for each sample using the best age estimate yields near-chondritic weighted mean compositions of − 1.6 ± 0.5, − 1.6 ± 0.4 and − 0.6 ± 0.7 epsilon units (deviations in parts per 10,000 from a chondritic uniform reservoir (CHUR) reference) at 3877 ± 62, 3850 ± 50, and 3731 ± 4 Ma respectively. These results provide no evidence for significant Lu–Hf fractionation on the early Earth expected to result from processes including accretion, intra-mantle differentiation, or massive Hadean crustal growth. [ABSTRACT FROM AUTHOR]

A limiting factor in the accuracy and precision of U/Pb zircon dates is accurate correction for initial disequilibrium in the 238 U and 235 U decay chains. The longest-lived—and therefore most abundant—intermediate daughter product in the 235 U isotopic decay chain is 231 Pa (T 1/2 = 32.71 ka), and the partitioning behavior of Pa in zircon is not well constrained. Here we report high-precision thermal ionization mass spectrometry (TIMS) U–Pb zircon data from two samples from Ocean Drilling Program (ODP) Hole 735B, which show evidence for incorporation of excess 231 Pa during zircon crystallization. The most precise analyses from the two samples have consistent Th-corrected 206 Pb/ 238 U dates with weighted means of 11.9325 ± 0.0039 Ma (n = 9) and 11.920 ± 0.011 Ma (n = 4), but distinctly older 207 Pb/ 235 U dates that vary from 12.330 ± 0.048 Ma to 12.140 ± 0.044 Ma and 12.03 ± 0.24 to 12.40 ± 0.27 Ma, respectively. If the excess 207 Pb is due to variable initial excess 231 Pa, calculated initial ( 231 Pa)/( 235 U) activity ratios for the two samples range from 5.6 ± 1.0 to 9.6 ± 1.1 and 3.5 ± 5.2 to 11.4 ± 5.8. The data from the more precisely dated sample yields estimated D Pa zircon /D U zircon from 2.2–3.8 and 5.6–9.6, assuming ( 231 Pa)/( 235 U) of the melt equal to the global average of recently erupted mid-ocean ridge basaltic glasses or secular equilibrium, respectively. High precision ID-TIMS analyses from nine additional samples from Hole 735B and nearby Hole 1105A suggest similar partitioning. The lower range of D Pa zircon /D U zircon is consistent with ion microprobe measurements of 231 Pa in zircons from Holocene and Pleistocene rhyolitic eruptions ( Schmitt, 2007, 2011 ). The data suggest that 231 Pa is preferentially incorporated during zircon crystallization over a range of magmatic compositions, and excess initial 231 Pa may be more common in zircons than acknowledged. The degree of initial disequilibrium in the 235 U decay chain suggested by the data from this study, and other recent high precision datasets, leads to resolvable discordance in high precision dates of Cenozoic to Mesozoic zircons. Minor discordance in zircons of this age may therefore reflect initial excess 231 Pa and does not require either inheritance or Pb loss. [ABSTRACT FROM AUTHOR]

Abstract: In-situ analyses for oxygen isotope ratio and trace element concentration by ion microprobe of 3.3–3.7Ga zircons from rhyolite of Panorama Formation in the North Pole Dome, Pilbara Craton document intracrystalline patterns of variability, show compositional correlation, and provide metrics to assess alteration. Values of δ18O in the North Pole Dome (NPD) zircons average 5.78 ‰ with a tight range from 5.3 to 6.4 ‰ VSMOW. These values are similar to magmatic zircons throughout the Archean worldwide and there is no correlation of δ18O, Ur trace elements. SEM/CL examination shows no evidence of mineral inclusions in the SIMS analysis pits. In-situ analysis of Ca concentration is useful to assess the degree of alteration in zircon and retention of trace element compositions. Most zircon analyses (74 %) have elevated [Ca]≥120ppm, with values up to 1614ppm suggesting alteration, probably facilitated by radiation damage. Many high-Ca zircons show enrichment in middle–heavy REE (M–HREE), a peak near Dy, and concave-down trends in chondrite-normalized diagrams. The calculated Dy anomalies (Dy/Dy*) correlate with [Ca]. High Ca also correlates with P, Ti, Fe, Y and REE, and with cumulative (unannealed) α-doses from radiactive decay of Th and U (3.1 to 20×1015 events/mg). If not annealed, this α-dose could have caused radiation damage that exceeded the first percolation point. Radiation damage can also facilitate alteration of δ18O in zircon, but examples from other terranes show that alteration is heterogeneous and correlates to greater variability of δ18O within and among zircons. The tight range of oxygen isotope ratios in the NPD rhyolite zircons and their similarity to other Archean zircons, world-wide, suggests that the analyzed NPD zircons have retained original, magmatic oxygen isotope ratios. [Copyright &y& Elsevier]

U Pb zircon geochronology is one of the most widely used techniques in sedimentary provenance analysis. Unfortunately, the ability of this method to identify sediment sources is often degraded by sediment recycling and mixing of detritus from different source rocks sharing similar age signatures. These processes create non-unique zircon U Pb age signatures and thereby obscure the provenance signal. We here address this problem by combining detrital zircon U Pb geochronology with Raman spectroscopy. The position and width of the Raman signal in zircon scales with its degree of metamictization, which in turn is sensitive to temperature. Thus, combined U-Pb + Raman datasets encode information about the crystallization history of detrital zircons as well as their thermal history. Using three borehole samples from Mozambique as part of a source-to-sink study of interest for hydrocarbon exploration, we show that zircon populations with similar U Pb age distributions can exhibit different Raman signatures. The joint U-Pb + Raman analysis allowed us to identify three different annealing trends, which were linked to specific thermal events. Thus we were able to differentiate a dominant Pan-African U Pb age peak into several sub-populations and highlight the major effect of Karoo tectono-magmatic events. In our case study, we used Raman also as a means to systematically identify all zircon grains in heavy-mineral mounts, resulting in considerable time savings. Raman spectroscopy is a non-destructive and cost-effective method that is easily integrated in the zircon U Pb dating workflow to augment the resolution power of detrital zircon U Pb geochronology. • Raman spectroscopy supplements U Pb geochronological analysis of detrital zircon. • Thermal overprints revealed by the diverse metamict states of zircon crystals. • New approach allows discrimination of source rocks with same age of crustal growth. [ABSTRACT FROM AUTHOR]

Quantifying the distribution of granitic melt at all scales in mid- to lower crustal migmatitic terranes is critical to understand crustal melting processes, chemical differentiation of the crust and its rheological behavior during deformation. We propose a new method to determine the fraction of frozen granitic melt on a hand specimen scale based on the relative volumes of newly precipitated to total zircon (FPZ = F raction of newly P recipitated Z ircon) as obtained by image analysis on dated zircon cores and rims. Using the calculated Zr-solubility [Zr]sat in the melt at the inferred melting temperature and the Zr concentration in the bulk sample [Zr]bulk, the fraction of melt F melt can be determined through F melt = FPZ × [Zr]bulk / [Zr]sat. The such obtained F melt corresponds to the melt fraction in the hand specimen at the time the system closed for melt mobility. Thermodynamic modelling further allows estimation of H 2 O-contents required to maintain the melt fraction obtained from the melt-o-meter in a molten stage. The applicability of this method has been tested on eight migmatitic samples with peak temperatures between 725 and 925 °C. Most of the lower temperature migmatites (<750 °C) retained F melt of 0.15–0.25 (±0.03–0.05), a melt fraction below or at the static melt escape threshold. In contrast, most of the higher-temperature migmatites (>800 °C) retained F melt of 0.35–0.50 (±0.07–0.10). At these melt fractions, melt extraction and melt migration from and within the source should be efficient. Consequently, these samples are likely open-system migmatites affected by melt accumulation or depletion processes. The melt-o-meter requires that the rock types under consideration produced a granitic melt that remained zircon-saturated and is therefore restricted to migmatitic meta-sediments and meta-granitoids. When applied carefully, this melt-o-meter offers a new and powerful tool to not only quantify melt distribution but also evaluate the extent of melt mobility in migmatites. • New tool to quantify the amount of melt frozen in a hand sample of migmatite. • Size of zircon rims can be directly correlated with melt fraction in migmatites. • The melt-o-meter is independent from thermodynamic modelling. • May help to evaluate melt mobility and fluid-present vs. absent melting regime • Implications for large-scale distribution of the melt in the crust, hence rheology [ABSTRACT FROM AUTHOR]