Reverse Discordance Research Articles

Geochronological and geochemical effects of zircon chemical abrasion: insights from single-crystal stepwise dissolution experiments

Abstract. Chemical abrasion in hydrofluoric acid (HF) is routinely applied to zircon grains prior to U–Pb dating by isotope dilution thermal ionization mass spectrometry (ID-TIMS) to remove radiation-damaged portions of grains affected by Pb loss. Still, many chemically abraded datasets exhibit evidence of residual Pb loss. Here we test how the temperature and duration of chemical abrasion affect zircon U–Pb and trace element systematics in a series of 4 h, single-crystal stepwise dissolution experiments at 180 and 210 ∘C. Microtextural data for the zircon samples studied are presented in a companion paper by McKanna et al. (2023). We find that stepwise dissolution at 210 ∘C is more effective at eliminating material affected by open-system behavior and enriched in U, common Pb (Pbc), and light rare earth elements (LREEs); reduces the presence of leaching-induced artifacts that manifest as reverse discordance; and produces more consistent and concordant results in zircon from the three rocks studied. We estimate that stepwise dissolution in three 4 h steps is roughly equivalent to a single ∼ 8 h leaching step due to the insulating properties of the PTFE sleeve in the Parr pressure dissolution vessel, whereas traditionally labs utilize a single 12 h leaching step. We conclude that a single 8 h leaching step at 210 ∘C should remove Pb loss effects in the majority of zircon and that this can be used as an effective approach for routine analysis. Further, we calculate time-integrated alpha doses for leachates and residues from measured radionuclide concentrations to investigate (1) the alpha dose of the material dissolved under the two leaching conditions and (2) the apparent minimum alpha dose required for Pb loss susceptibility: ≥ 6×1017 α g−1.

Micro- to nanoscale constraints on metasomatic alterations of xenotime, inclusions of Th-, U- and Pb-phases and their geochronological implications (Ås pegmatite, Evje and Hornnes, S Norway)

Five crystals of pegmatitic xenotime from Ås II feldspar quarry (Evje, S Norway) were studied with comprehensive analytical methods from microscale to nanoscale with respect to fluid-mediated alterations and their geochronological implications. Xenotime crystals are strongly altered during dissolution-reprecipitation processes that resulted in the formation of (Th, U)-depleted xenotime subdomains with numerous microinclusions of (Th, U)-silicate, uraninite and minor galena. Remains of a primary xenotime (Xtm1) yielded LA-ICPMS UPb data characterized by a reverse discordance (from −10.5 to −2.6%) with a 208Pb/232Th mean age 988.4 ± 5.9 Ma (95% conf., MSWD = 0.75, n = 71), which is within uncertainty with a 207Pb/235U mean age 979.1 ± 5.0 Ma (95% conf., MSWD = 1.4, n = 22) yielded by data filtered to below ±5% discordance (i.e., from −5.0 to −2.6%). The altered xenotime domains (Xtm2) provided highly scattered dates, including 208Pb/232Th dates from 200 ± 14 to 2135 ± 120 Ma (n = 135). Discordant UPb data yielded upper intercept age 909 ± 16 Ma (MSWD = 7.9). EPMA Th-U-total Pb measurements and compositional characteristics of uraninite inclusions indicate three age populations of ca. 852–983, 594–687 and 37–101 Ma.TEM investigations revealed initial alterations within domains Xtm1, representing a primary xenotime, which progressed along parallel submicron- to nanoscale fractures. These fractures represent partially open grain boundaries, which are empty or are filled with secondary inclusions of (Th, U)-silicates, uraninite or coffinite. The submicron-sized inclusions are accompanied by subdomains of (Th, U)-depleted xenotime. Altered xenotime (Xtm2) is well crystalline in TEM imaging and electron diffraction patterns in contrast to primary unaltered xenotime domains (Xtm1) that demonstrate, in Raman spectra, moderate degree of radiation damage caused by U and Th decay. Secondary inclusions of Th and U phases are nanocrystalline or amorphous, which increases their potential for Pb-loss or accumulation of Pb in excess. Some of them contain nanoinclusions of Pb3O4 or metallic Pb, whereas <100 nm-sized inclusions of (Pb, Sb)-oxide formed in the altered xenotime.To summarize, this study provides important insights for our understanding of coupled dissolution-reprecipitation processes that affect xenotime and mobilization of released U, Th and Pb. Removal of highly mobile U in a fluid, and the presence of nanoinclusions of Pb3O4, (Pb, Sb)-oxide and metallic Pb have particular importance for xenotime dating and explain disturbance towards older ages. Nevertheless, both removal of U, Th and Pb from altered and recrystallized xenotime as well as presence of submicron- to nanoinclusions can result in age disturbance resulting in spread of dates along a concordia curve.

THE SPECIFIC URANIUM-LEAD ISOTOPIC SYSTEM OF MONAZITE FROM THE LEUCOCRATE GNEISS OF PRIAZOVIA

The uranium-lead isotopic system of monazites from leucocratic gneiss (Pryazovsky megablock) was studied and the age of their formation was determined. Monazites from leucocratic gneiss are characterized by significant discordance, which is obviously due to the radiogenic component of impurity lead. In our case, the reverse discordance is to a large extent obviously due to the addition of radiogenic lead, which was captured by monazite during crystallization and is a component of impurity (abnormal composition) of lead. Calculating the age of monazite using a concordia diagram (Arens-Wetherill method) gave the following results: at the upper intersection of the regression line, the age was 2029 ± 145 and 1106 ± 337 Ma, at the lower one MSWD = 35. Taking into account the rather large contents of ordinary lead in the monazite samples studied and the significant dependence of the isotopic age determination results on the adequately introduced correction for ordinary (impurity lead), the isotopic composition of which may not correspond to the isotopic composition of ordinary lead according to Stacey and Kramers, calculated the relationship between numerical age values obtained from the 207Pb/206Pb isotopic ratio and the 206Pb/204Pb ratio, which sufficiently characterizes the content of ordinary lead. Therefore, the numerical values of the age, calculated using different models, according to the obtained analytical data, are quite different: 2027 Ma, 2037 ± 18, 2029 ± 145, and 2099 Ma. In our opinion, the isotopic date, 2037 ± 18 Ma, calculated by the isotopic composition of lead, taking into account the errors in the determination of lead isotopic ratios, corresponds to the time of monazite crystallization to a greater extent. Within the margin of error, it coincides with the date of 2029 ± 145 Ma, obtained from the upper intersection of the discordia with the concordia in the coordinates of the Ahrens-Weatherill method.

Major Causes of Age Distortion in Uranium-Lead Isotopic Radiogeochronology

The causes of isotopic age distortion that may occur during the dating of endogenous geological processes (rocks) by the uranium-lead isotopic method are considered. Three groups of reasons are distinguished: mineralogical, geochemical and analytical. The main mineralogical reason for the distortion of the U-Pb isotopic age is the multistage crystallization of geochronometer minerals, which is manifested, for example, in zircon, in the anatomy of their crystals. It was concluded that in order to obtain reliable information about the time course of geological processes for complex crystals (primarily zircon), local uranium-lead isotope dating methods ("SHRIMP", LA-ICP-MS, etc.) should be used. The geochemical reasons include the discrepancy between the isotopic composition of impurity lead and the isotopic composition of corrective lead (abnormal isotopic composition of ordinary lead) and the polystage history of the development of the uranium-lead isotope system. It is noted that the most probable reason for the violation of the uranium-lead isotope system by zircons in the hypergenesis zone is the entrapment of uranium by defects in the crystal structure and cracks, and the predominant loss of uranium in monazite. At the same time, the loss of uranium by monazites depends on the composition of the acids. It is indicated that washing monazites in a weak solution of nitric acid leads to the appearance of a significant reverse discordance, while no loss of lead is observed. The same operation in a weak solution of hydrochloric acid leads to the preferential leaching of ordinary lead. For analytical reasons, the lowest accuracy of determining the prevalence of the 204Pb isotope (204Pb/206Pb ratio) is indicated. The impact of contamination of samples dated (method TIMS) by lead and uranium from reagents is considered. It is clear that the contamination of multi-grain samples (1-2 mlg) of minerals with uranium and lead from reagents with a modern isotopic composition, in a blank test of lead 10–9 g (the ratio of the mass of Pb of the sample to the mass of Pb from the reagents of 40 to 1) is not significant affects dating results (isotopic ratios of 207Pb/206Pb, 207Pb/235U and 206Pb/238U). A blank sample of uranium is usually 2 orders of magnitude smaller (10–11-10–12 g). A strong inverse relationship between the degree (proportion) of radiogenic lead contamination of radiogenic lead aliquots on the isotopic composition of lead and the calculated values of the lead content in the sample was revealed. When an aliquot for determining the content of uranium and lead is contaminated with ordinary lead from the reagents, the smallest distortion of the calculated value of the lead content occurs when the ratio of sample lead to tracer lead is 1:1, while a slightly smaller relative distortion of the lead content is noted with increasing age of the radiogenic lead of the samples.

U-Pb and Sm-Nd Evidence for Episodic Orogenic Gold Mineralization in the Kalgoorlie Gold Camp, Yilgarn Craton, Western Australia

Abstract Different genetic and timing models for gold mineralization in the Kalgoorlie gold camp (Yilgarn craton, Western Australia) suggest either broadly synchronous, late-stage mineralization related to metamorphic fluids at ca. 2640 Ma or a punctuated mineralization history from ca. 2675 to 2640 Ma with the involvement of early magmatic-hydrothermal systems (represented by the Fimiston, Hidden Secret, and Oroya gold-telluride lodes) and late metamorphic fluids (represented by the Mt. Charlotte gold stockwork veins). The results of U-Pb and Sm-Nd geochronological studies of zircon, apatite, and titanite from pre-ore dikes and syn-ore dikes constrain the absolute timing of mineralization and provide new evidence to this timing controversy. Emplacement ages constrained by U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon data are interpreted to be similar for both the pre-ore dikes (n = 10) and syn-ore dikes (n = 7) at ca. 2675 Ma. An inferred emplacement age of ca. 2675 Ma for the syn-ore dikes is supported by a Sm-Nd isochron age from apatite (laser ablation-inductively coupled plasma-mass spectrometry; LA-ICP-MS) of 2678 ± 15 Ma and by a U-Pb titanite age (LA-ICP-MS) of 2679 ± 6 Ma. The results of chemical abrasion-isotope dilution-thermal ionization mass spectrometry U-Pb zircon analysis from the pre- and syn-ore dikes are complicated by multistage Pb loss, reverse discordance, and potential inheritance. However, the data are compatible with the emplacement of Fimiston/Hidden Secret gold mineralization at ca. 2675 Ma and suggest a younger age for Oroya mineralization at ca. 2665 Ma. These results contrast with models for orogenic gold deposits that invoke broadly synchronous, late-stage mineralization related to metamorphic fluids at ca. 2640 Ma. The bulk of the Kalgoorlie gold camp’s estimated 2,300 t Au endowment was emplaced at ca. 2675 Ma as Fimiston/Hidden Secret Au mineralization. This early Au mineralization was deformed and overprinted twice by subordinate Au mineralization at ca. 2665 (Oroya mineralization) and ca. 2640 Ma (Mt. Charlotte mineralization). Gold mineralization in the Kalgoorlie gold camp was protracted in nature from ca. 2675 to 2640 Ma and reflects the interplay of early magmatic (Fimiston, Hidden Secret, Oroya) and late metamorphic (Mt. Charlotte) hydrothermal fluid systems in the formation of hybrid intrusion-related and metamorphic orebodies.

TS-Mnz – A new monazite age reference material for U-Th-Pb microanalysis

A single megacryst of Neoproterozoic monazite (“TS-Mnz”) was investigated by electron probe microanalysis (EPMA), Raman microspectroscopy, U-Pb dating by isotope dilution-thermal ionization mass spectrometry (ID-TIMS), and U-Th-Pb dating and trace element analysis by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The monazite contains both distinct unaltered domains that are either homogeneous or zoned, and altered domains with secondary inclusions of ThSiO4 along fractures. Altered domains can be easily recognized in backscattered electron images and avoided during microanalysis. The U-Pb ID-TIMS measurements in two laboratories revealed slight reverse discordance of U-Pb data and provided 207Pb/235U mean dates of 908.2 ± 1.5 Ma (95% conf., MSWD = 2.9) and 910.42 ± 0.34 Ma (2σ, MSWD = 1.0). Three individual LA-ICPMS sessions yielded 207Pb/235U mean dates of 899.8 ± 6.9 Ma (95% conf., MSWD = 0.10), 896.3 ± 6.5 Ma (95% conf., MSWD = 0.18) and 901.9 ± 2.0 Ma (95% conf., MSWD = 0.57), and 208Pb/232Th mean dates of 909.2 ± 5.8 Ma (95% conf., MSWD = 0.36), 915.1 ± 11 Ma (95% conf., MSWD = 0.31) and 909.9 ± 1.5 Ma (95% conf., MSWD = 0.56). The lower precision of the LA-ICPMS with respect to the ID-TIMS technique hides discrepancies between ID-TIMS 207Pb/235U dates as well as the reverse discordance, which is common for monazite. The discrepancies between ID-TIMS and LA-ICPMS data are interpreted as related to standardization and individual setup of analytical techniques; however, minor variance of discordance between individual analyzed spots also cannot be excluded. Further electron microprobe U-Th-total Pb dating provided dates of 914.6 ± 2.0 Ma (95% conf., MSWD = 0.99) and 922.9 ± 3.1 Ma (95% conf., MSWD = 4.3) for two fragments of TS-Mnz. The LA-ICPMS U-Th-Pb round-robin test employing TS-Mnz as a primary reference material and various monazite samples of known age as unknowns provided accurate U-Pb and Th-Pb dates within errors of the reference ages. To conclude, the TS-Mnz despite compositional variations reveals relatively reproducible U-Pb, Th-Pb and U-Th-total Pb dates and is proposed as a new age reference material for in-situ U-Th-Pb microanalysis with recommended reference mean 207Pb/235U and 208Pb/232Th ages of 910.42 ± 0.34 Ma (2σ) and 910.7 ± 1.3 Ma (95% conf.), respectively.

Radiogenic Pb mobilization induced by shock metamorphism of zircons in the Apollo 72255 Civet Cat norite clast

In situ U-Pb radiometric dating of zircons is regarded as one of the most widely used and reliable methods to acquire geochronologic ages. However, it has been recently reported that radiogenic Pb (Pb*) mobilization within zircon may, in some cases, cause inaccurate age determinations with no geological significance. Such Pb* mobilization can be caused by deformation, α-coil damage, fluid-assisted annealing, and recrystallization. In this study, we report an investigation of Pb* mobilization in shock metamorphosed lunar zircons. NanoSIMS (nanoscale secondary ion mass spectrometry) and IMS 1280HR ion microprobe dating, EBSD (electron backscatter diffraction) and CL (cathodoluminescence) mapping, and scanning ion imaging (SII) were applied to micro-zircon grains from the Apollo 72255 Civet Cat norite clast. Based on the large number of grains with similarities in internal zoning, habit and trace element geochemistry, and host mineral context, the Civet Cat norite zircons are interpreted to be primary, igneous grains. The chronology obtained for three consecutive surfaces (at different depths) by NanoSIMS, SII, and IMS 1280HR, respectively, indicates that the radiogenic Pb distribution of the Civet Cat norite zircons is heterogeneous among different polished or sputtering surfaces. Forty-two NanoSIMS U-Pb ages (beam size of 5 μm) are concordant on a Wetherill Concordia diagram, and their corresponding 207Pb/206Pb ages spread from 4015 Ma to 4459 Ma. More notably, the six oldest spots of the 42 define a concordant U-Pb age of 4460 ± 31 Ma (2σ, MSWD = 0.47, P = 0.92) and a weighted mean 207Pb/206Pb age of 4453 ± 34 Ma (MSWD = 0.056, P = 0.998). These dates are among the oldest in the lunar highland rocks. However, the 207Pb/206Pb ages of repolished surfaces of these zircons by IMS 1280HR (beam size of 5 μm) do not reproduce the NanoSIMS results (up to 300 Ma younger). The SII (spatial resolution of 2 μm) confirms a heterogeneous distribution of radiogenic Pb within single grains. The EBSD mapping of these zircon grains shows that they have 3–20° of cumulative lattice misorientation. It is proposed that shock-related deformation has facilitated Pb* migration after primordial crystallization. With currently available data, we cannot preclude the possibility that the large errors of the U-Pb ages obscure reverse discordance that would bias our oldest 207Pb/206Pb ages to older values. Conversely, our data could be explained by mixing of Pb-retention and Pb-loss nanodomains as seen in shocked terrestrial zircon such that U-Pb date of 4460 ± 31 Ma approximates the norite formation.

Comparison of LDL-C calculation by friedewald and martin/hopkins methods in 12,243 adults from the United States of America

Abstract Background Low-density lipoprotein cholesterol (LDL-C) is an established risk factor for cardiovascular disease (CVD), and a target for lipid-lowering therapy. LDL-C is typically not measured directly but is estimated using the Friedewald formula, which assumes a fixed factor for the ratio of triglycerides (TG) to very low-density lipoprotein cholesterol (VLDL-C). However this assumption is sometimes not valid. The Martin/Hopkins (M/H) formula estimates LDL-C using an adjustable factor for the TG:VLDL-C ratio and is expected to improve upon Friedewald when predicting measured LDL-C, and apolipoprotein B (ApoB), one molecule of which is associated with each LDL particle. Purpose We compared values of LDL-C calculated by the Friedewald and M/H methods with respect to their correlation with non-high density lipoprotein cholesterol (non-HDL-C) and ApoB, and their classification of individuals based upon attainment of the threshold value of 70 mg/dl (1.8 mmol/l) of LDL-C. This cut-point is a treatment target for individuals at high risk of CVD in the 2019 ESC guidelines for lipid modification, and a threshold for initiating statin therapy in the 2019 ACC/AHA guidelines. Methods In this analysis we included participants in the National Health and Nutrition Examination Survey (NHANES) from 2005–2016, age ≥18, &amp;lt;80 years who had measurements for total cholesterol (TC), TG and HDL-C. LDL-C was calculated using Friedewald and M/H. We correlated LDL-C (calculated using the two methods) with non-HDL-C and ApoB. We identified individuals with LDL-C &amp;lt;70 mg/dl using both methods. When LDL-C (Friedewald) was &amp;lt;70, but LDL-C (M/H) was &amp;gt;70, we classified these participants as discordant. Statistical analyses were performed in IBM SPSS for Windows v26. Results 12,243 individuals were included. 51.8% were female, mean (±SD) age was 45.5±17.4, 15.3% were treated with statins, ApoB was available for 2179 participants. Mean lipid concentrations (mg/dl) were: TC: 191.5±41.0, TG: 120.0±67.0, HDL-C: 54.1±15.7, LDL-C (Friedewald): 113.3±35.4; LDL-C (M/H): 114.9±35.2. In the whole population, LDL-C (M/H) was more strongly correlated than LDL-C (Friedewald) with ApoB (r=0.935 v 0.894) and non-HDL-C (r=0.981 v 0.944). In statin-treated participants, LDL-C (M/H) was also more strongly correlated with ApoB (r=0.951 v 0.914) and non-HDL-C (r=0.979 v 0.928). 1139 participants had LDL-C (Friedewald) &amp;lt;70 mg/dl. Of these, 206 individuals (18.1%) were discordant, having LDL-C (M/H) &amp;gt;70 mg/dl. Amongst statin-treated patients, 22.9% were discordant. Only 5.5% of individuals with LDL-C (M/H) &amp;lt;70 mg/dl showed reverse discordance (LDL-C (Friedewald) &amp;gt;70 mg/dl). Conclusions The M/H method of calculating LDL-C correlates more strongly with non-HDL-C and ApoB than Friedewald. Importantly the discordant results confirm previous observations that Friedewald underestimates LDL-C at low concentrations. This may result in under-use of lipid-lowering therapies. Funding Acknowledgement Type of funding source: None

Multimethod U–Pb baddeleyite dating: insights from the Spread Eagle Intrusive Complex and Cape St. Mary's sills, Newfoundland, Canada

Abstract. Baddeleyite (ZrO2) is widely used in U–Pb geochronology but analysis and age interpretation are often difficult, especially for samples which have experienced post-intrusive alteration and/or metamorphism. Here, we combine high spatial resolution (secondary ionization mass spectrometry, SIMS) and high-precision (isotope dilution thermal ionization mass spectrometry, ID-TIMS) analyses of baddeleyite from the Spread Eagle Intrusive Complex (SEIC) and Cape St. Mary's sills (CSMS) from Newfoundland. Literature data and our own detailed microtextural analysis suggest that at least seven different types of baddeleyite–zircon intergrowths can be distinguished in nature. These include secondary baddeleyite inclusions in altered zircon, previously unreported from low-grade rocks, and likely the first discovery of xenocrystic zircon inclusions mantled by baddeleyite. 207Pb∕206Pb baddeleyite dates from SIMS and ID-TIMS mostly overlap within uncertainties. However, some SIMS sessions of grain mounts show reverse discordance, suggesting that bias in the U ∕ Pb relative sensitivity calibration affected 206Pb∕238U dates, possibly due to crystal orientation effects, and/or alteration of baddeleyite crystals, which is indicated by unusually high common-Pb contents. ID-TIMS data for SEIC and CSMS single baddeleyite crystals reveal normal discordance as linear arrays with decreasing 206Pb∕238U dates, indicating that their discordance is dominated by recent Pb loss due to fast pathway diffusion or volume diffusion. Hence, 207Pb∕206Pb dates are more reliable than 206Pb∕238U dates even for Phanerozoic baddeleyite. Negative lower intercepts of baddeleyite discordia trends for ID-TIMS dates for SEIC and CSMS and direct correlations between ID-TIMS 207Pb∕206Pb dates and the degree of discordance may indicate preferential 206Pb loss, possibly due to 222Rn mobilization. In such cases, the most reliable crystallization ages are concordia upper intercept dates or weighted means of the least discordant 207Pb∕206Pb dates. We regard the best estimates of the intrusion ages to be 498.7±4.5 Ma (2σ; ID-TIMS upper intercept date for one SEIC dike) and 439.4±0.8 Ma (ID-TIMS weighted mean 207Pb∕206Pb date for one sill of CSMS). This first radiometric age for the SEIC is consistent with stratigraphic constraints and indicates a magmatic episode prior to opening of the Rheic Ocean. Sample SL18 of the Freetown Layered Complex (FLC), Sierra Leone, was investigated as an additional reference. For SL18, we report a revised 201.07±0.64 Ma intrusion age, based on a weighted mean 207Pb∕206Pb date of previous and new baddeleyite ID-TIMS data, agreeing well with corresponding SIMS data. Increasing discordance with decreasing crystal size in SL18 indicates that Pb loss affected baddeleyite rims more strongly than cores. Our SL18 results validate that the SIMS in situ approach, previously used for Precambrian and Paleozoic samples, is also suitable for Mesozoic baddeleyite.

A multi-technique evaluation of hydrothermal hematite U[sbnd]Pb isotope systematics: Implications for ore deposit geochronology

Hematite (α-Fe2O3) is a common iron-oxide mineral known to incorporate U into its crystal lattice at up to wt% concentrations and has been previously used to date ore formation within iron-oxide copper gold and banded iron formation deposits. However, there has been no detailed evaluation of the potential challenges this novel mineral geochronometer may present for accurate temporal interpretation. We report a multi-technique UPb geochronological study comprising laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), sensitive high-resolution ion microprobe (SHRIMP) and the first isotope dilution-thermal ionization mass spectrometry (ID-TIMS) procedure and analyses using an in-situ grain micro-sampling technique. We assess intra- and inter-sample data reproducibility, and examine the compatibility of spatial and analytical resolutions to texturally and compositionally diverse samples.Five samples of hydrothermal, primary, oscillatory-zoned hematite were analysed from distinct lithologies throughout the ~1.59 Ga Olympic Dam Cu-U-Au-Ag deposit, with typical U concentrations ranging between ~20 and 1000 ppm. LA-ICP-MS mapping of isotope distributions across mm-sized grains allows pinpointing of U-rich and relatively U/Pb isotopically ‘homogeneous’ domains. Micro-domains were extracted via laser-cut square shaped trenches (100 × 100 μm). UPb datum range in complexity, with concordant age domains measured by ID-TIMS indicating that hematite can retain a near-closed UPb system over ~1.6 Ga, while also demonstrating that younger Pb-loss events may be recorded. Significant reverse discordance is common in the data, likely to be a real feature due to both internal decoupling and mobility of Pb within a single grain and UPb fractionation during microbeam analysis. These effects will obscure the interpretation of real ages for Proterozoic samples where non-zero age Pb-loss events occur, and restricts the evaluation of common Pb mixing components. As a result, microbeam analyses may be biased younger or older in terms of 207Pb/206Pb dates in samples with complex histories. Microbeam datum compare favourably with high precision ID-TIMS ages. Although reproducible ID-TIMS UPb dates were generally not obtained, in the best-case example, a 207Pb/206Pb weighted mean age of 1589.91 ± 0.91 Ma (MSWD = 1.3, n = 5), within 0.2% of the Olympic Dam granite host rock age (1593.87 ± 0.21 Ma) was generated. All five hematite samples dated by LA-ICP-MS and SHRIMP yield weighted mean 207Pb/206Pb dates within 2% of the host granite age after small degrees of data rejection. The results demonstrate that with careful sample petrography, screening and data interpretation, hematite can be considered a very useful UPb mineral geochronometer, with potential application to all U-bearing Fe-oxide rich mineral systems.

Effects of thermal annealing and chemical abrasion on ca. 3.5 Ga metamict zircon and evidence for natural reverse discordance: Insights for U[sbnd]Pb LA-ICP-MS dating

We present a microstructural and UPb systematics study comparing pristine, thermally annealed (TA) and chemically abraded (CA) ~3500Ma zircon from a quartz-dioritic gneiss, with the aim to improve pre-analytical workflows for more accurate and precise LA-ICP-MS UPb dating of ancient zircon. Four zircon domains are identified: i) low- to medium-U concentric-oscillatory zoned cores, ii-iii) two porous high-U outer alteration domains, and iv) low-U narrow inward growing recrystallization rims. Raman spectroscopy on the pristine zircon reveals a positive correlation between increasing structural damage and U content. The porous high-U outer domains show a drastic increase in non-formula Ca above estimated amorphous fractions of ~0.8, which we ascribe to hydrothermal alteration of high-damage zircon characterized by percolating networks of amorphous areas. Upon treatment (TA, CA), hyper-spectral CL and Raman spectroscopy suggest structural recovery of point defects, but not full repair of high-damage amorphous areas. Calculated Raman radiation damage ages suggest that natural annealing affected all domains at ~500Ma, consistent with negligible laser ablation matrix effects comparing pristine and treated cores.We show that reverse discordance in pristine and TA cores is not an analytical artifact. High-U alteration domains are normal discordant and were partially reset at ca. 3410Ma. Upon CA, UPb discordance and scatter are reduced in the cores, yielding intercepts of 3503±14Ma and 560±550Ma (MSWD=1.0). The lower intercept matches the timing of Pb-loss recorded in the alteration domains and the Raman radiation damage age. We argue that short distance Pb redistribution within the cores, which led to reverse discordance, was controlled by UPb systematics recording both the crystallization age and the timing of Pb redistribution. Some excess Pb was redistributed from the partially re-set alteration domains into the cores, leading to mixed UPb systematics and further reverse discordance. Removal of the latter excess Pb upon CA suggests that radiogenic Pb from the alteration domains accumulated within distinct damage sites accessible for CA leaching.We conclude that standardization of pre-analytical treatment is not recommended; we propose to investigate the structural state of unknown specimen by means of Raman spectroscopy, in combination with commonly applied CL imaging. During LA-ICP-MS analyses, co-measurement of non-formula and selected trace elements is deemed highly valuable in detecting ablations for use in reliable UPb age determination.

Initial gold enrichment within a Neoarchean granite-greenstone belt: Evidence from ore-bearing and ore-barren samples in the Jiapigou deposits, NE China

The Jiapigou gold mineralization hosted in a Neoarchean granite-greenstone belt in the southern Jilin Province, at the easternmost part of the northern margin of the North China Craton (NCC), is one of the important gold deposits in China. However, its evolution, in particular the initial Au source and enrichment, have not been well constrained. The initial gold enrichment defined by Au concentrations of Neoarchean unaltered gneisses of this region, range from <0.005ppm to 0.001ppm, increasing to 0.006–0.031ppm Au for the ore-hosting Neoarchean altered mylonite gneisses. A combined study of zircon trace element, UPb dating, Hf isotope compositions, as well as whole rock chemical compositions was performed for dioritic gneisses from both ore-bearing (close to the ore body) and ore-barren zones (within the nearby rock body) with a view to constrain the gold initial enrichment. The dioritic gneisses have varied SiO2 contents (54.19–61.34wt.%), Al2O3 (13.06–16.75wt.%) with moderate to high K2O/Na2O ratios (0.7–1.3) and MgO (2.20–3.77%). Based on their chemical compositions, positive zircon εHf(t) values, and their formation ages of 2512±9Ma, we infer that these dioritic rocks were generated by partial melting of a juvenile source, most likely thickened mafic lower continental crust (LCC) with residual plagioclase associated with minor amphibole. The ore-bearing sample (H5) from a ductile shear zone has been extensively altered by fluids and is the sample containing zircons with blurred zoning and reverse discordance. Zircons from H5 have much higher radiogenic Pb but lower U and Pb contents than those in the ore-barren sample (H3), which contains zircons with oscillatory zoning and represents the basement rock. Based on the available petrological, geochronological and geochemical constraints, we propose the following model for Au initial enrichment within the Neoarchean granite-greenstone belt. At ca. 2.54Ga, associated with the accretionary processes in the proto-NCC, subduction-related fluids infiltrated the metasomatised subcontinental lithospheric mantle (SCLM), scavenging Au and Pb (mainly radiogenic Pb, henceforth Pb*) and triggering partial melting. The resulting basaltic magmas underplated the LCC and were re-melted at 2512±9Ma, when the onset of an extensional regime permitted the advection of heat and melts/fluids into the LCC. This generated voluminous dioritic magmas with juvenile Hf isotope signatures with little evidence of interaction with the mantle. Subsequent ca. 2505Ma re-melting of some granodioritic rocks led to the formation of potassic granite and residual fluids further enriched in Au and Pb*, both focused into ductile shear zones and other lithospheric weak zones. The fluids with anomalous UPb fractionation resulted in the rarely reported zircon reverse discordance, alteration and also enrichment in Au, as observed in sample H5 and H4, representing the initial stage of Au enrichment within the Neoarchean granite-greenstone belt.

ZNF277 microdeletions, specific language impairment and the meiotic mismatch methylation (3M) hypothesis.

We were intrigued by the pedigrees in the paper by Ceroni et al1 in the October 2014 issue that explored the association between ZNF277 microdeletions and specific language disorder (SLI). The authors make the case for ZNF277 microdeletions contributing to the risk of SLI while acknowledging that there was little evidence of co-segregation of these microdeletions with SLI in their discovery pedigree plus eight other families. The authors only discuss this discrepancy in terms of reduced penetrance, that is, unaffected siblings that carry the microdeletion, but say little about the reverse discordance in two of their three multiplex families, that is, where one of the two affected siblings has inherited the maternal microdeletion and the other has not. This reverse discordance has been reported in multiplex families with autistic spectrum disorder (ASD) in which a putative risk copy number variation (CNV) is segregating.2 This paper on ASD, and our finding of a maternal grandmother age effect in ASD prompted us to propose the meiosis mismatch methylation (3M) hypothesis.3 This states that, in female early meiosis I, the pairing of a chromosome carrying a microdeletion with a wild-type homologue increases the chance of abnormal methylation due to chromosome looping through misaligned pairing, such as would occur with silencing of a transposon.4 This in turn results in the functional silencing of the wild-type gene when transmitted to offspring. 3 M predicts that all offspring of a woman carrying a risk CNV are at risk of the condition even though only half inherit the CNV. Independently, around the time of formulating the 3 M hypothesis, an ASD family was reported in which one affected sibling had inherited a maternal deletion containing the oxytocin receptor gene (OXTR) and the other affected sibling had no deletion, but had epigenetic misregulation of this gene through aberrant gene silencing by DNA methylation.5 We suggest that it would be useful to examine at least the DNA methylation status of the maternally inherited ZNF277 allele in the siblings that have SLI but not the microdeletion.

Reply to Pembrey et al: 'ZNF277 microdeletions, specific language impairment and the meiotic mismatch methylation (3M) hypothesis'.

In a recent paper,1 we described a homozygous exonic microdeletion in ZNF277 in a girl with specific language impairment (SLI). This microdeletion was also identified in the heterozygous form in eight families of the SLI Consortium (SLIC) cohort and four families with ASD cases from the IMGSAC Cohort. We observed an increased allelic frequency of ZNF277 microdeletions in SLI probands (1.1%) compared with both ASD family members (0.3%) and unrelated controls (0.4%), suggesting that these microdeletions might be a risk factor for SLI. However, as the ZNF277 microdeletions showed incomplete segregation with the SLI phenotype, as they were also identified in unaffected family members and, in some cases, they were not inherited by the affected children (reverse discordance), we hypothesised that these CNVs might contribute to the SLI susceptibility in a complex manner, acting as a risk factor with a reduced penetrance.

Reliability of LA-ICP-MS U-Pb dating of zircons with high U concentrations: A case study from the U-bearing Douzhashan Granite in South China

Granite-hosted uranium ore deposits are the most important commercial U-producers in South China. These U-bearing granites generally contain high-U-concentration zircons. Zircons in the U-bearing Douzhashan Granite show black rims in cathodoluminescence images, which have very high U concentrations of up to >25,000ppm. The high-U black rim domains of zircons show similar REE patterns to those of low-U mantle domains with oscillatory zoning, but have higher REE concentrations and weaker positive Ce anomalies. The high-U black rim domains of zircons may have formed from late-stage evolved U-rich and H2O-rich magma. We carried out a SHRIMP U-Pb dating for these zircons and the results show that the high-U (12,007–26,706ppm) rim domains of the zircons always yield older 206Pb/238U ages (268±9Ma, n=5, MSWD=2.7) than the low-U (558–3667ppm) mantle domains (228±11Ma, n=4, MSWD=3.1), and both of these ages are older than the emplacement age (211±3Ma) of the U-barren Xiangchaoping Granite, which was intruded by the Douzhashan Granite. Hence, these results demonstrate a matrix effect associated with SHRIMP U-Pb analyses of high-U zircons. The reverse discordance for SHRIMP results of high-U zircons was produced by analytical artifact. However, our LA-ICP-MS analyses show that the black rim domains (12,840–26,018ppm U) of the zircons have the same U-Pb ages as the low-U mantle domains (1215–4075ppm U). Both domains yield a weighted mean 206Pb/238U age of 203±4Ma (n=13, MSWD=1.4), which is consistent with the 40Ar-39Ar age (207±4Ma) of muscovite from this granite. It thus indicates that LA-ICP-MS U-Pb analyses for high-U zircons (up to 26,000ppm U) show an insignificant matrix effect and likely yield more reliable U-Pb ages for the high-U zircons than SHRIMP analyses. In order to get reliable U-Pb ages using the LA-ICP-MS method for the high-U zircons, a pulse-analog cross calibration for the ICP-MS collector should be performed. Our precise U-Pb dating and geochemical studies indicate that the U-bearing Douzhashan Granite was emplaced during the late Indosinian stage and formed in a post-collisional setting of the Indosinian Orogeny. It is distinct from most U-bearing Granites in South China, which were largely emplaced during the early Indosinian stage and formed in a syn-collisional setting.

‘Reverse discordance’ between 68Ga-DOTA-NOC PET/CT and 177Lu-DOTA-TATE posttherapy scan

In this technical note, an unusual discordance between diagnostic and posttherapeutic scan resulting from the use of different somatostatin receptor ligands in two settings is described. Such observation, we believe, is multifactorial, but most importantly arises due to different receptor affinity profile of the ligands and different somatostatin receptor subtype expression in different tumors. It is important for the treating physician to be aware of this phenomenon that would aid in improving our understanding of complex ligand-receptor interactions in various somatostatin receptor-positive tumors with its possible implications for therapeutic decision making with radiolabeled somatostatin receptor analogues.

Comment on “Rheic Ocean ophiolitic remnants in southern Iberia questioned by SHRIMP U‐Pb zircon ages on the Beja‐Acebuches amphibolites” by A. Azor et al.

Comment on “Rheic Ocean ophiolitic remnants in southern Iberia questioned by SHRIMP U‐Pb zircon ages on the Beja‐Acebuches amphibolites” by A. Azor et al.

In situ LA-ICP-MS U–Pb dating of metavolcanics of Norrbotten, Sweden: Records of extended geological histories in complex titanite grains

In situ U–Pb dating of a variety of mineral phases is an important goal in petrology. This study reports data chiefly from titanite, but also from rutile and apatite, obtained using the laser ablation (LA)-ICP-MS methodology on polished thick sections in order to retain as much petrologic information as possible, and allowing trace element analyses from adjacent areas to the U–Pb analyses. The samples analysed come from Svecofennian intermediate to acid volcanic rocks of the Porphyry Group within the major iron ore province of Norrbotten, northern Sweden, where titanite is a common phase associated with the mineralisation. Using a laser beam of 30–45 μm in diameter, accurate and relatively precise (2–3% 2 σ on 206Pb/ 238U ratios) data can be obtained on titanite. Similar precision on Pb/U ratios can be obtained in rutile, but accuracy cannot be assessed because of the lack of well characterised standards. However, the data are a priori accurate as they are geologically reasonable. Apatite shows reverse discordance, which could be explained by a number of scenarios. However, the 207Pb/ 206Pb ages are fairly precise (1–3% 2 σ) and the calculated age is geologically reasonable, suggesting that the data may be accurate. The titanite grains studied show complex internal structures. Rare earth element (REE) analysis by LA-ICP-MS demonstrates that the core and rim zones are distinct and supports a model for two-stage evolution in two out of three samples, with the rims in these samples being enriched in total REE, and particularly in LREE in one sample, and displaying a positive Y-anomaly in contrast to the cores. In situ U–Pb analysis reveals that core zones from all samples retain distinct older ages of c.2050 Ma, whereas the reworked rims have either distinct younger or strongly reset ages. The older cores are interpreted as representing the first alteration/metamorphism of the volcanic pile. Titanite rims record a U–Pb age of 1870 ± 24 Ma (2 σ) at Luossavaara, and 1826 ± 15 Ma (2 σ) at the smaller Fe oxide–apatite body at Gruvberget. In both cases, the ages are consistent with previous age determinations from the Fe oxide–apatite deposits. A sample from Malmberget records a slightly different scenario, where U–Pb ages spread along concordia from c.1920 to 1708 Ma. We attribute this pattern to metamorphic reworking of primary metasomatic titanite. Rutile ages from inclusions within the core of Gruvberget titanite essentially mimic the ages obtained from the surrounding titanite and suggest that they have undergone the same history. Apatite inclusions within titanite at Malmberget record a younger age of 1584 ± 12 Ma (2 σ), which we attribute to later events known to have affected the area. These data indicate that the Porphyry Group volcanics must be at least c.2050 Ma in age, in contrast to previous age determinations from zircon which suggested a deposition age of c.1960 to 1880 Ma. The large grain size of the titanites studied, coupled with the ability for the first time to obtain in situ U–Pb ages from small areas, has allowed probing of this hitherto unrecognised event. Smaller zircon grain size, coupled with extremely saline hot, pressurised (high P– T) metasomatic fluids, relating to regional magnetite–apatite mineralisation in the period 1900 to 1800 Ma, may have resulted in resetting of zircon U–Pb systematics.

Stable disease and improved health-related quality of life (HRQoL) following fractionated low dose131I-metaiodobenzylguanidine (MIBG) therapy in metastatic paediatric paraganglioma: observation on false “reverse” discordance during pre-therapy work up and its implication for patient selection for high dose targeted therapy

The incidence of paraganglioma in the paediatric population is exceedingly rare, accounting for < 0.1% of childhood cancers. We report here the response and toxicity profile in a case of malignant paraganglioma which was treated with what is currently perceived as an unconventional and non-standard approach, using three consecutive low doses of 131I-MIBG (a cumulative dose of 11 647.6 MBq). The patient had a stable disease at the end of 42 months follow-up following the first treatment with 131I-MIBG. Excellent symptomatic and hormonal responses were observed. The only adverse effect was mild nausea in the first 24 h after therapy. In addition to the potentially primary end point of radiological and biochemical response measurement, we, in this paper, endeavoured to look for a quality of life evaluation for this form of treatment. Given the rarity of this condition, the experience gained by this therapeutic approach is intriguing from response and toxicity standpoints and may be extrapolated to malignant pheochromocytoma as well. An apparent "reverse discordance" between 131I-MIBG scintigraphy and 99Tc(m)-MDP bone scan encountered during pre-therapy work up is also described with possible explanations. This draws attention to an important clinical issue in selecting patients for high dose targeted therapy.

Diagnostic Significance of Positive I-123 BMIPP Despite Negative Stress Tl-201 Myocardial Imaging in Patients With Suspected Coronary Artery Disease

There is frequently discordance in the interpretation of myocardial single-photon emission computed tomography images using both rest iodine-123 15-(p-iodophenyl)-3-(R,S) methylpentadecanoic acid (BMIPP) and stress thallium-201 ((201)Tl); that is, negative BMIPP vs positive (201)Tl studies. However, little attention has been paid to reverse discordance (ie, positive BMIPP despite negative stress (201)Tl). In the present study 105 consecutive patients with suspected coronary artery disease (CAD) who underwent both rest BMIPP and stress (201)Tl imaging were evaluated. No patient with previous myocardial infarction was included. Scintigraphic studies revealed that BMIPP was in concordance with (201)Tl in 71 patients: both were positive in 38 and negative in 33, whereas both were discordant in 20 and reverse discordant in 14. In patients with reverse discordance, the pattern of BMIPP abnormality was segmental in 8 and spotty in 6; the former was related to coronary territory. Based on the BMIPP findings and clinical symptoms, 7 patients with positive BMIPP despite negative (201)Tl underwent coronary angiography: 3-vessel CAD was found in 3 patients, 2-vessel CAD in 1, coronary spasm in 2, and insignificant lesions in 1. In the remaining 7 patients, no cardiac event was observed during 2-year follow-up. In patients showing positive BMIPP despite negative (201)Tl, a segmental BMIPP perfusion abnormality indicates a high likelihood for CAD, despite normal stress (201)Tl imaging. Thus, adding BMIPP to (201)Tl imaging may help to better identify patients with significant CAD, including multivessel disease.