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Mafic rocks are the most common type of igneous rocks on Earth, however, constraining the crystallization age of mafic rocks can be challenging. Apatite is a common accessory phase in mafic rocks and is amenable to dating using the U–Pb system. However, the U–Pb system in apatite has a relatively low closure temperature (∼350°-550 °C) and is therefore prone to resetting by later thermal and metasomatic events. Here, a recently developed Lu Hf dating method using laser ablation reaction-cell mass spectrometry is applied to apatite from mafic rocks. The Lu Hf system in apatite has a higher closure temperature (∼650°-750 °C) compared to U–Pb, increasing the chances of obtaining primary crystallization ages. Furthermore, the laser-ablation method allows rapid data collection compared to traditional solution-based Lu Hf dating techniques. Four study areas were selected to compare the Lu Hf vs U–Pb systematics of apatite in mafic igneous rocks: the Paleoproterozoic Sudbury Igneous Complex (Canada), the Neoproterozoic Borborema Province (NE Brazil), the Paleoproterozoic Fennoscandian Shield (Finland), the Archean Yilgarn Craton and adjacent Mesoproterozoic Albany Fraser Orogen (Western Australia). For all analyzed samples that have apatite trace element compositions typical of an undisturbed primary mafic igneous lithology, the Lu Hf system retains primary igneous apatite crystallization ages, whereas the U–Pb system in the same grains often records isotopic disturbance or a cooling age. In few cases, the Lu Hf system has also been disturbed in response to recrystallization, however, such disturbance is readily detected with trace element data. Hence, this study demonstrates the potential of laser ablation apatite Lu Hf dating to obtain primary crystallization ages for otherwise difficult to date mafic rocks. • First study on Lu-Hf systematics for apatite from mafic rocks • U U-Pb system in apatite readily resets by thermal and metasomatic events • Lu Lu-Hf system is robust to volume diffusion < ∼660–730 °C, retaining primary crystallization ages. • Deformation leading to recrystallization 'resets' both U U-Pb and Lu Lu-Hf systems in apatite. • Apatite trace element data to differentiate isotopic disturbances related to recrystallisation versus volume diffusion [ABSTRACT FROM AUTHOR]

The zircon U-Pb system is one of the most robust geochronometers, but during an impact event individual crystals can be affected differently by the passage of the shock wave and impact generated heat. Unraveling the potentially complex thermal history recorded by zircon crystals that experienced variable levels of shock and heating, as well as additioanl pre- and post-impact thermal events, has been difficult using classical geochronological methods. The existing high-precision 40Ar/39Ar age constraints for the K-Pg Chicxulub event, and the previous U-Pb dating of the basement rocks from the impact site, make Chicxulub an ideal location to study impact-induced effects on the zircon U-Pb systematics and to evaluate potential 'memory effects' of pre-impact U-Pb signatures preserved within those individual zircon crystals. Recent IODP-ICDP drilling of the Chicxulub impact structure recovered 580 m of uplifted shocked granitoid and 130 m of melt and suevite, providing an unprecedented opportunity to study zircon crystals subjected to a range of shock pressures, thermal, and deformational histories. Zircon morphologies were classified using scanning electron microscopy (SEM) imaging and then samples were depth profiled using laser ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) to document the range of preserved age domains from rim-to-center within individual crystals. The results show U-Pb ages range from 66 to 472 Ma, which are consistent with both inherited Carboniferous and Late Paleozoic basement ages as well as Pb loss ages in response to the K-Pg impact event. While the bulk of the zircon grains preserve Paleozoic ages, high U (metamict) zones within fractured zircon crystals exhibited an age within uncertainty (66 ± 6.2 Ma) of the impact age (66.038 ± 0.049 Ma), indicating that inherited intragrain U-Pb kinetics and/or hydrothermal fluid flow may have controlled age resetting those zircon crystals rather than impact-induced shock and heating alone. Moreover, the calculated α-decay doses suggest that the zircon crystals experienced Stage 1 or early Stage 2 radiation damage accumulation. Therefore, we suggest that the lowered crystal annealing temperature in crystals that previoulsy experienced radiation damage make the zircon U-Pb clock either more susceptible to the relatively short heat pulse of the impact event, the moderate pressure and temperature conditions in the peak ring, and/or to hot-fluid flow in the long-lasting post impact hydrothermal system. • Shocked zircon crystals extracted from Chicxulub's peak ring rocks are analyzed via U-Pb LA-ICP-MS depth profiling. • U-Pb LA-ICP-MS depth profiling and the subsequent data export in 1 s increments allows to spatially resolve age trends. • Zircon U-Pb clock partially and possibly fully reset due to the impact event. • Younger U-Pb ages are preserved in fractured zircon with a high U [ppm] concent. or in zircon displaying granular textures. [ABSTRACT FROM AUTHOR]

Previous studies have shown that radiometric dating of fossils can be complicated by post-fossilisation alteration. A key challenge to obtaining geologically meaningful fossilisation dates in bioapatite is identifying regions that have remained relatively undisturbed since the time of fossilisation. In order to determine whether a geochemical indicator can be used to identify least-disturbed regions, we combine in situ trace element analysis and U-Pb geochronology of fossil teeth from the Arroyo Chijuillita Member, New Mexico, and the Dinosaur Park Formation, Alberta. The reliability of these U-Pb dates is supported by good agreement between the isotope dilution thermal ionization mass spectrometry (61.5 ± 1.6 Ma) and laser ablation ICP-MS (64.2 ± 2.4 Ma) dates for a targeted tooth region. Of fourteen teeth analyzed from these units, two record dates within uncertainty of the depositional age: one has a “diffusion” concave upward cross-sectional yttrium profile, the other a concave downward profile reflective of trace element loss. It was found for teeth from both units that smaller yttrium concentration ranges are correlated with younger U-Pb dates, likely the result of post-fossilisation alteration. A yttrium screen based on this relationship is proposed, and is found to be a better indicator of post-fossilisation open system behaviour in the samples analyzed than other geochemical features thought to be produced during fossilisation. This screen provides a new tool for identifying fossil material least disturbed since fossilisation, facilitating the use of bioapatite to date the time of diagenesis. [ABSTRACT FROM AUTHOR]

A study is conducted to supplement the uppermost Lower Jurassic–lowermost Cretaceous marine strontium isotope dataset and to present new statistical fits of the Middle–Late Jurassic seawater strontium isotope curve based on a numerical time scale and a detailed biostratigraphical zonal scheme. The use of the stratigraphical scheme allows reduction of dating errors related to uncertainty of numerical age determinations. The presented correlation charts enable direct calibration between strontium isotope stratigraphy and regional biostratigraphical frameworks. New strontium isotope data have been obtained from well-preserved Lower Bajocian, uppermost Callovian, Oxfordian, Kimmeridgian, and Upper Volgian belemnite rostra. The presented seawater 87 Sr/ 86 Sr curve is characterized by reliable 95% confidence limits (mean width of ± 0.000007), which take into account precision of dating of particular data points. A decrease of seawater 87 Sr/ 86 Sr ratio, from ca. 0.70730 to ca. 0.70683, is observed throughout the Middle Aalenian–Early Oxfordian (172.1–160.8 Ma ago). The steepest Phanerozoic fall of the ratio, with a rate of change of up to 0.00015 per 1 Ma, is recorded in the Bajocian segment of the strontium isotope curve. The Phanerozoic minimum of the marine 87 Sr/ 86 Sr ratio occurred at the Early–Middle Oxfordian transition. The 87 Sr/ 86 Sr ratio increased starting from the Middle Oxfordian till the end of the Jurassic (160.8–145.7 Ma) reaching a value of ca. 0.70720 at the Jurassic–Cretaceous transition. The Middle–earliest Late Jurassic decrease in seawater 87 Sr/ 86 Sr ratio is interpreted as a result of the increased hydrothermal activity of the seafloor during the breakup of Gondwana, and the formation of new Atlantic–Tethyan oceanic basins. The successive rise of the ratio is probably related to the decelerated hydrothermal venting of the oceanic crust although a partial increase in radiogenic strontium input from continental weathering cannot be excluded. [ABSTRACT FROM AUTHOR]

We studied the feasibility of using laser ablation inductively coupled plasma mass spectrometry (LA–ICPMS) for U-Pb dating of vertebrate and invertebrate apatite fossils. Two specimens of Carcharodon Carcharias from Pleistocene (2.58–0.01 Ma) sediments in Baja California peninsula, Mexico, give Gelasian ages of 2.0 ± 0.4 Ma and 2.0 ± 0.6 Ma which are consistent with the palaeontologic record reported for this species suggesting that the U-Pb system in the shark teeth remained closed until the present. Dentine data from an Oligocene (33.9–23.0 Ma) shark tooth, Otodus angustidens , from El Cien Formation, Mexico, give an anomalously young age of 19 ± 0.2 Ma with a low 207Pb/206Pb value of 0.65. This age is largely controlled by a sub-population of anomalously radiogenic data with low concentrations of U and Pb suggesting remobilization of Pb from high to low U domains. Regressing relatively non-radiogenic data from high U and Pb domains using a fixed average crustal value for initial common 207Pb/206Pb gives an age of 28.6 ± 0.2 Ma, which agrees with U-Pb zircon ages from tuff horizons in the sequence. Enamel from the same shark tooth is much lower in U and Pb and shows evidence for substantial mixing with radiogenic Pb from the dentine. Apatite shells from a brachiopod specimen, Lingula cobourgensis, and conodont fossils contain high U concentrations and significant proportions of radiogenic Pb. Some conodont specimens show high Th/U ratios combined with evidence of significant excess 206Pb. The amount of excess 206Pb requires a minimum Th partition coefficient of about 16,000 and likely much higher suggesting enrichment by ion exchange rather than recrystallization. In some cases conodonts with low Th/U can be precisely dated. A specimen of I. simulator gives an age of 302 ± 4 Ma which is consistent with its well constrained Gzhelian range (303.7–298.9 Ma). We conclude that processing LA–ICPMS data at the single cycle level allows more precise absolute dating of fossils. In some cases, separate regression of totally reset and partially reset domains can resolve ages of early and late diagenesis. [ABSTRACT FROM AUTHOR]

This study presents a new laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) based U-Pb geochronometric method for dating grossular-andradite (grandite) garnet. Grandite is a primary skarn mineral, therefore dating its growth directly dates hydrothermal activity. As zircon U-Pb geochronometry provides a high-resolution record of magmatic processes, grandite U-Pb dating has the potential to provide a complementary record for hydrothermal systems. This study characterizes four garnets of variable grossular-andradite content and age as potential reference materials for LA-ICPMS U-Pb geochronology: Willsboro Andradite (~ 1020 Ma, Adirondacks, USA), Red and Yellow Mali Grandite (~ 200 Ma, Southern Mali) and Lake Jaco Grossular (~ 35 Ma, Coahuila, Mexico). Isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U-Pb analyses of Willsboro andradite yield a mean 206 Pb/ 238 U date of 1022 ± 16 Ma. We use Willsboro Andradite as a primary reference material for LA-ICPMS U-Pb characterization of the two other garnets. The non-radiogenic Pb (Pbc) concentration in grandite is variable between specimens. Willsboro Andradite and Mali Grandites contain almost purely radiogenic Pb (Pb*), however Lake Jaco Grossular has a higher Pbc concentration comparatively and yields discordant U/Pb ratios. For specimens with highly variable Pb*/Pbc, linear regression is employed to derive a lower-intercept age. LA-ICPMS U-Pb dates for Yellow Mali Grandite (202 ± 2 Ma; 206 Pb/ 238 U weighted mean) and Lake Jaco Grossular (35 ± 2 Ma; lower intercept) agree with independent ID-TIMS U-Pb (202.0 ± 1.2 Ma) and (U-Th)/He dates (35 ± 5 Ma) derived for each specimen. The precision of LA-ICPMS U-Pb dates varies between 1–10% (2σ) for the analyzed garnets. Considering the short estimated lifespans of pluton-related hydrothermal systems (tens of thousands to a few million years), Neogene skarns present the best opportunity to test for and resolve separate episodes of garnet growth at these precision levels. As a performance test for the method, we present new U-Pb andradite data from a Late Miocene skarn system on Serifos Island, Greece. This garnet yields a lower-intercept age of 9.15 ± 0.36 Ma, in agreement with biotite Rb-Sr and zircon U-Pb dates from the causative pluton. [ABSTRACT FROM AUTHOR]

The extrusive component of the Karoo Large Igneous Province, the Drakensberg Continental Flood basalts of South Africa and Lesotho, is mainly composed of altered tholeiitic basalts without uraniferous mineral phases that are suitable for high-precision dating (e.g. baddeleyite or zircon), and thus dating is restricted to 40Ar/39Ar groundmass and plagioclase. Previous 40Ar/39Ar dates of plagioclase span from 178.9 ± 4.2 Ma to 189.2 ± 3.3 Ma, while most dates range between 180 and 183 Ma (Jourdan et al., 2007b ; Moulin et al., 2011 ; Moulin et al., 2017). Dating mafic rocks can be hindered by the presence of plagioclase xenocrysts (inherited 40Ar), and the ubiquitous presence of alteration phases. Separation protocols and acid leaching are used to minimize the effect of these issues, although previous 40Ar/39Ar studies of the Drakensburg lavas did not interpret their dates within a petrochronological context, and thus they are open to inaccuracy. We have used cathodoluminescence imaging, QEMSCAN analysis and optical petrology of thin sections to show that two distinct generations of plagioclase occur within the lavas, and at least two different phases of K-rich (adularia and K-rich clays) and Ca- alteration have occurred. 40Ar/39Ar dates have been obtained from some acid-leached and unleached aliquots of groundmass and both plagioclase generations. Our results show that i) large, cluster-forming plagioclase yield older 40Ar/39Ar dates and are interpreted to be xenocrystic, ii) apparently unaltered plagioclase in thin sections hosts secondary Ar reservoirs related to alteration, and iii) alteration and leaching can enhance the effects of 37,39Ar recoil in plagioclase. We conclude that the most accurate date for the Drakensburg lavas that applies to the entire thickness of the lava stacks is 184.4 ± 1.2 Ma, which was obtained from the matrix-forming plagioclase that contains the highest proportion of non-inherited radiogenic 40Ar, is least affected by secondary alteration and did not apparently suffer the effects of Ar isotope recoil. [ABSTRACT FROM AUTHOR]

Age determinations of rocks by in-situ Rb-Sr dating of minerals has the advantage of providing spatially resolved analysis without time-consuming preparation, but is hampered by isobaric and polyatomic interferences on Sr isotopes, especially 87Rb. LA-ICP-MS/MS using N 2 O as a reaction gas allows online chemical separation of 87Rb from 87Sr by measuring SrO+. Here, the method for in-situ Rb-Sr is optimized and applicability demonstrated for geological samples ranging from late Archean to 20 Ma in age. Rb-Sr isotopes of multiple reference materials and geological samples have been analyzed by LA-ICP-MS/MS to evaluate the method for dating rocks throughout the geological record. The quality of Sr isotopic data can be greatly improved by external calibration of mass bias. For internal calibration, better accuracy is obtained with the addition of H 2 as a trace molecular gas. The elemental fractionation of Rb and Sr during LA-ICP-MS/MS analysis varies for different reference materials. We find that the matrix of NIST SRM 610 renders it unsuitable for correction of 87Rb/86Sr. The Mica-Mg reference material is preferred for calibration of 87Rb/86Sr of micas despite showing significant Rb-Sr fractionation. In-situ Rb-Sr ages were determined for igneous samples ranging in age from 20 Ma to Mesoproterozoic, confirming accurate application of the method to samples of widely differing ages. The most accurate isochron ages were obtained with 87Rb/86Sr calibrated against Mica-Mg. Application to two metamorphic gneisses with late Archean protoliths from Labrador and Finland reveal thermal events at 1.66–1.70 Ga when they were part of a large Paleoproterozic active margin along the southern edge of Laurentia-Baltica. Data collected for minerals with differing closure temperatures for the Rb-Sr system allow detection of thermal events not recorded by other geochronological systems, indicating a huge potential of this technique not only for accurately dating samples, but also for providing previously inaccessible information on metamorphic histories. [ABSTRACT FROM AUTHOR]

The Re–Os isotopic heterogeneity of mantle-derived rocks has been well documented. However, it is unclear whether and how the Os isotopic heterogeneity of a mantle source can influence Re–Os isotopic dating and source tracing of mantle-derived melts. Here we report that replicate analyses of gram aliquots of single basaltic powders (one of reference material BHVO-2 and three of the Hatu basalts from the western Junggar region, China) show large variations in both Os concentrations and isotopic ratios. More importantly, these replicate data define good apparent Re–Os isochron correlations, yielding ages significantly older than their formation ages, and display remarkable correlations between the 187 Os/ 188 Os ratio and the reciprocal of the common Os concentration (i.e., 1/ 192 Os). This indicates that the initial Os isotopic composition in a basaltic magmatic system may be heterogeneous and cannot be homogenized by pulverizing. Theoretical and mathematical deduction demonstrates that the observed apparent Re–Os isochron correlations are the result of binary mixing without complete isotopic equilibrium at the time of formation, primarily due to limited diffusional exchange of Os isotopes between refractory Os-bearing inclusions and host minerals under mantle conditions. The regressed initial Os isotopic composition may bias the true value of the mantle source. Pulsed mantle melting and magma mixing during ascent are potential processes responsible for such initial Os isotopic heterogeneity in basaltic rock systems. Thus, when using the Re–Os isotopic system to date and to trace the source of basaltic rocks or other rocks with relatively low Os concentrations, it is necessary to consider whether the initial Os isotopic composition was heterogeneous or whether it had reached complete isotopic equilibrium. [ABSTRACT FROM AUTHOR]

Measurements of alpha-recoil-track densities in mica as a basis for geological dating depend on an etch model relating the number of tracks per unit volume ( N RT ) to the counted number of etched tracks per unit area ( ρ RT ). The model of Gögen and Wagner (2000) implies that ρ RT increases linearly with etch time ( t E ), so that N RT can be calculated from the slopes or intercepts of step-etch functions ρ RT ( t E ). This model rests on the assumption that the etch rate of the mica surface ( v V ) and the horizontal etch pit growth rate ( v H ) are both constant, in contradiction with experimental results and computer simulations of mineral dissolution. We present results of four different experiments aimed at relating etch pit densities to volumetric track densities. Step-etch data are vulnerable to observation-related artefacts at increasing t E and lack the resolution to confirm or refute the supposed linear increase of ρ RT with t E . The intercepts of regression lines fitted to step-etch data are imprecise and perhaps inaccurate. Intercept estimates based on mirror-image counts and (etch)–anneal–etch experiments indicate that v V increases during the initial etching stages. The (etch)–anneal–etch results show that no pre-etch is in fact required, implying that surface tracks are more resistant to annealing than tracks in the bulk of the mineral. Track-size measurements confirm that v H is also not constant but decreases with increasing etch-pit size. The results show that no recoil-track etch pit in phlogopite etched in 40% HF grows larger than ca. 7 μm and that the track-size distribution becomes quasi-invariant at > 6 min etching. This signifies that there exist accessible etching conditions at which the etch-pit size distribution becomes a fixed, distorted reflection of the size distribution of latent recoil tracks. It is not improbable that track addition and loss also cancel each other out in this equilibrium state, in which case both the etch-pit size distributions and densities ρ RT become independent of etch time. [ABSTRACT FROM AUTHOR]

Forty titanite grain fragments from 9 central uplift samples of metamorphosed anorthosite from the Manicouagan impact structure were dated by the (U–Th)/He technique. A (U–Th)/He central age of 207.1±6.4Ma (2 standard error (SE), n=40) has been determined. With 4 outlier ages removed the central age is refined to 208.9±5.1Ma (2 SE). Both of these ages are within error of the previously determined U–Pb zircon age of 214±1Ma (2σ) derived from the impact melt. Manicouagan's central uplift formed due to rapid elevation from ~7–10km depth as part of the modification stage of the impact process, which has facilitated the dating of its emplacement due to resulting rapid exhumation and cooling and closure of the (U–Th)/He system in titanite. Correlation with the previous U–Pb zircon 214±1Ma impact melt crystallization age indicates that the (U–Th)/He titanite dating technique offers a new approach to dating complex impact structures in the absence of viable melt sheets, or other melt products. The youngest ca. 195Ma (U–Th)/He dates preserved in some titanite fragments are synchronous with Early Jurassic, rift-induced lithospheric thinning and associated igneous activity that defines the Central Atlantic Magmatic Province (CAMP). The (U–Th)/He titanite data from Manicouagan indicate that the influence of this regional event may extend west of the previously proposed limit of CAMP activity. [ABSTRACT FROM AUTHOR]

U Pb dating of hydrogenic and fossil material can be a significant challenge because samples often contain low U levels and variable amounts of non-radiogenic Pb. Line scans using LA-ICPMS provide the widest range of ratios available for defining the radiogenic to common Pb mixing line, hence the best age estimate, but extracting this information requires processing many low-count data. Conventional least-squares regression of ratios based on such data can give inconsistent results. A new approach is introduced where data are regressed as count rates in a 3D space rather than ratios on a 2D plot. This is a maximum likelihood approach in that mixing planes covering a range of ages and common 207Pb/206Pb ratios (PbC) are compared to the data set and used to determine a relative probability density surface whose maximum should fix the best-fit age and common 207Pb/206Pb ratio. 95% confidence errors are determined from T and PbC values that cut off 2.5% of the area to the left and right of 2D plots generated by integration of the 3D plot over PbC and age, respectively. Results from a rugose coral fossil show that this approach gives age results similar to regressions of concordia data and more accurate than Tera-Wasserburg regressions on data from natural samples. Tests on randomly generated synthetic data do not show significant discrepancy between Isoplot ages but they do between values of PbC. The 3D approach is the only one that gives accurate results for both age and PbC. [ABSTRACT FROM AUTHOR]

Abstract: We evaluate the 190Pt–186Os decay system, measured by laser ablation MC-ICPMS, as a useful geochronometric tool for direct dating of platinum-group minerals (PGM) in large mafic/ultra-mafic intrusions by analysing mineralised rocks from the geochronologically well constrained Bushveld Complex, S. Africa. Mixed PGM (laurite, cooperite, laurite–platarsite, sperrylite and Pt–Fe alloy) from the Merensky Reef yield a Pt–Os isochron age of 1995±50Ma (2σ, n=48, MSWD=1.16, initial 186Os/188Os=0.119819±0.000006). While this is 59Myr younger than the U–Pb zircon age for the Merensky Reef (Scoates and Friedman, 2008) it is consistent with recent Re–Os ages for the Platreef. Considering the younger age recorded by both Re–Os and Pt–Os systems, we propose that the Pt–Os isochron age records a late, low temperature hydrothermal event affecting PGM and sulphides in the Platreef and Merensky Reef. Pt-rich phases may also yield single grain 190Pt–186Os model ages since initial 186Os/188Os can be well constrained. A Pt–Os model age of 2024Ma±101Ma was calculated for a Merensky cooperite; a Pt–Fe alloy from the Onverwacht pipe and a Tweefontein Hill sperrylite give Pt–Os model ages of 2113±106Ma and 2042±102Ma, respectively. A Pt–Os isochron derived from all Bushveld units gives an age of 2012±47Ma (2σ, 186Os/188Osi =0.119818±0.000006) with an MSWD of 1.19, hence PGE mineralisation in different stratigraphic horizons of the complex cannot be distinguished temporally using the Pt–Os chronometer. Nonetheless, the potential for the Pt–Os system in dating events in PGE mineralised systems is evident and the agreement of single grain model ages in high Pt/Os phases with the Pt–Os and Re–Os isochron ages indicates their usefulness in the geochronology of these systems. [Copyright &y& Elsevier]

Abstract: Re–Os geochronology of fractions composed of unsized, coarse, and fine molybdenite from a pod of unusual monazite–xenotime gneiss within a granulite facies paragneiss, Hudson Highlands, NY, yielded dates of 950.5±2.5, 953.8±2.6, and 941.2±2.6Ma, respectively. These dates are not recorded by co-existing zircon, monazite, or xenotime. SEM–BSE imagery of thin sections and separated grains reveals that most molybdenite grains are composed of core and rim plates that are approximately perpendicular. Rim material invaded cores, forming irregular contacts, probably reflecting dissolution/reprecipitation. EPMA and LA-ICP-MS analyses show that cores and rims have different trace element concentrations (for example, cores are relatively enriched in W). On the basis of inclusions of zircon with metamorphic overgrowths, we conclude that molybdenite cores and rims formed after high-grade regional metamorphism. The discovery of cores and rims in individual molybdenite grains is analogous to multi-component U-Pb geochronometers such as zircon, monazite, and titanite; thus, molybdenite should be carefully examined before dating to ensure that the requirement of age homogeneity is fulfilled. [Copyright &y& Elsevier]

Abstract: Argon system behavior in a young volcanic system has been investigated through analysis of alkali feldspar phenocrysts and glass separates in order to constrain the potential effects of excess Ar upon 40Ar–39Ar ages on feldspars and the crystal/melt partition coefficient. The importance of syn-and post-emplacement processes was investigated by sampling glass and pumice clasts derived from varying post-eruptive conditions, including welded sections, vapor-phase altered sections and heavily weathered portions of the ignimbrite. Excess 40Ar was detected in all glass samples at 1–10ppb level, yet despite this, step-heating of co-existing alkali feldspar revealed a total gas age of 0.644±0.038Ma compared with the generally accepted age of 0.668±0.004Ma for the timing of emplacement of the Arico Formation. This suggests that despite high concentrations of Ar dissolved in the glass at quenching, the alkali feldspar crystallizing from excess Ar-rich melt contain very little excess Ar. No correlation was observed between excess Ar in the glass and total gas age obtained from the co-existing alkali feldspar and a partition coefficient (KD) between feldspar and glass was constrained to be less than 10−3. A correlation between the concentrations of excess Ar and atmospheric derived 36Ar in the glass (manifested as a restricted range of 40Ar/36Ar up to ~320) suggests a uniform mechanism of incorporation of Ar from these reservoirs measured in all samples prior to eruption. In an assessment of post-eruptive factors, diffusional loss of Ar was not significant during the elevated temperatures (~ up to 670°C) associated with welding processes at the relevant timescales over which welding occurs. Since diffusive exchange occurs between crystals and melt over long periods in the magma chamber, ages determined on volcanic alkali feldspars reflect the eruption age as a result of the very low KD, but in systems containing excess Ar the ultimate accuracy of such ages will be limited by the actual value of KD. We observe that biotite, a mineral with a higher KD, yields anomalously old ages in such systems, an observation made previously for other volcanic systems. Diffusive loss of Ar from volcanic glass occurs on timescales approximating that of volcanic eruptions, suggesting that excess Ar measured in the glass separates is representative of a degassed state, and indicates even higher pre-eruptive Ar concentrations present in the magma chamber prior to eruption. Thus our measured KD is likely to be a maximum value. The significance of physical contamination of feldspar by glass is also highlighted and quantified demonstrating that less than 0.01% by weight of adhered phonolite glass was required to elevate a measured feldspar age. This is particularly significant for young samples where any such contamination effect would be magnified proportionally to the small amount of in-grown radiogenic Ar. [Copyright &y& Elsevier]

Abstract: The 147Sm–143Nd radioactive decay system has been widely used in the Earth sciences as a geochronometer and isotopic tracer. Recent studies have shown that laser ablation-multiple collector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) is capable of precise and rapid 143Nd/144Nd measurements with high spatial resolution, but its ability to measure 147Sm/144Nd accurately is still poorly constrained. A critical analytical issue is the fractionation effects between 147Sm and 144Nd in LA-MC-ICPMS, which must be carefully corrected to determine robust initial 143Nd/144Nd particularly for ancient geological samples. Here we show that LA-MC-ICPMS can produce substantial (>5%) 147Sm/144Nd fractionation during analysis of monazite and that the fractionation can be corrected with uncertainty of ~1% by using an independently defined 147Sm–143Nd isochron for an Archean monazite standard. In addition, we describe a method for improving the precision and accuracy of LA-MC-ICPMS isotopic measurements by calibration of Faraday amplifier response and isotopic ratio normalization using a synthetic standard. This method has been applied to one Neogene and two Archean monazites, which were also analyzed by isotope dilution-thermal ionization mass spectrometry (ID-TIMS). The LA-MC-ICPMS data were acquired from 16–37μm single spots. The LA-MC-ICPMS determinations of the 145Nd/144Nd for all samples and 143Nd/144Nd for the Neogene monazite compare well with the ID-TIMS results, indicating the robustness of our method to measure Nd isotopic ratios. For the Archean monazites, the method yields initial 143Nd/144Nd values identical to the ID-TIMS values with reproducibilities (2s.d.) of 39 and 58ppm. Analytical precisions of individual spot analyses for initial 143Nd/144Nd were calculated by taking into account the uncertainty in (i) the Nd isotopic normalization to the synthetic monazite, (ii) the correction factor for 147Sm/144Nd fractionation and (iii) the age, which results in 2σ of 45–87ppm. These results clearly demonstrate the capabilities of the LA-MC-ICPMS technique applied to monazite Sm–Nd isotope systematics for early crustal evolution studies. [Copyright &y& Elsevier]

Abstract: Perovskite, a common Th- and U-enriched accessory mineral crystallised from kimberlitic magmas, has long been thought to be an important geochronometer for dating the emplacement of kimberlite. However, it also contains variably high levels of common Pb, which makes it difficult to obtain a precise measurement of radiogenic Pb/U and Pb/Th isotopic compositions using microbeam techniques such as SIMS and LA-ICP-MS. We present calibration protocols for in situ U–Pb and Th–Pb age determination of kimberlitic perovskite using the large double-focusing Cameca IMS 1280. Linear relationships are found between ln(206Pb⁎+/U+) and ln(UO2 +/U+), and between ln(208Pb⁎+/Th+) and ln(ThO+/Th+), based on which the inter-element fractionation in unknown samples during SIMS analyses can be precisely calibrated against a perovskite standard. The well-characterized Ice River perovskite is chosen as the U–Pb and Th–Pb age standard in this study. The 204Pb-correction method was used to estimate the fraction of common Pb, which is consistent with the results obtained using the 207Pb-based correction method for the dated perovskites of Phanerozoic age. A Tazheran perovskite with unusually high U but rather low Th yielded a Concordia U–Pb age of 462.8±2.5Ma and a Th–Pb age of 462±4Ma. Two perovskite samples from the Iron Mountain kimberlite have identical Concordia U–Pb ages of 410.8±3.4Ma and 411.0±2.6Ma, which are consistent within errors with their corresponding Th–Pb ages of 409.2±7.2Ma and 412.3±3.3Ma, respectively. Two perovskite samples from the Wesselton Mine of South Africa yielded indistinguishable 206Pb/238U ages of 91.5±2.2Ma and 90.3±2.9Ma, and Th–Pb ages of 90.5±0.8Ma and 88.4±1.6Ma, respectively. Accuracy and precision of 1–2% (95% confidence level) for these measurements have been demonstrated by the consistency of their U–Pb and Th–Pb ages with the recommended U–Pb ages of previous works. [Copyright &y& Elsevier]

Abstract: Instrumental and spectral characteristics germane to chemical dating of monazite have been tested using the Cameca SX-100 at Rensselaer Polytechnic Institute. Statistical analysis demonstrates that, for trace element analysis, equal counting time on peak and background is required for optimal statistical precision, thus rendering impractical the procedure of fitting the entire spectrum to obtain background values. Energy shifts require shifting the detector voltage window between peak and background positions, and it is concluded that the differential auto PHA mode works optimally for this. Analyses of Pb-free phosphates, silicates, and oxides are used to measure spectral interferences with the PbMα peak and background positions. Backgrounds were modeled using both linear and exponential fits. It was found that the difference in background counts using the two fits varies with each of the five spectrometers examined, and that the high-pressure (3bar) detectors show larger differences in exponential vs. linear peak-minus-background (P-B) values than the low-pressure (1bar) detectors. In addition, every spectrometer requires a unique correction for every major element in monazite. An analytical protocol is presented that incorporates these results. This protocol was applied to several monazite standards to determine inter-spectrometer variability, and spectrometer reproducibility from session to session. It was found that the difference in composition (and age) between spectrometers on identical spots exceeds the 2 sigma standard error of the mean of composition (or age) on either spectrometer. This means that (a) additional sources of error beyond the counting statistics exist between spectrometers; (b) the precision of microprobe ages cannot be continuously improved by additional counting; and (c) the minimum realistic precision is on the order of ±2–3% for monazites with around 1500–2000ppm total Pb, or an additional absolute uncertainty of 20–50ppm Pb. [Copyright &y& Elsevier]

Abstract: This paper reports data related to the potential of the mineral monazite for reconstructing the low-temperature thermal histories of geological samples from confined fission-track length measurements. The data are derived from isothermal annealing experiments on 300 MeV 86Kr tracks. Simplified versions of the favoured annealing models for apatite are adequate for describing the dataset. A non-compositional model predicts that: (1) track accumulation sets in when a sample cools under ~100 °C; (2) a substantial reduction (~2.5 μm) takes place at ambient and lower temperatures on a geological timescale; (3) an additional reduction of similar magnitude (~2.5 μm) takes place during exposure for a geological time span to the annealing conditions in the earth''s upper crust; and (4) etching at elevated or room temperature is expected to affect the length of induced fission tracks but not that of fossil tracks. [Copyright &y& Elsevier]

Abstract: Kyanite–staurolite–garnet micaschists from the Plankogel unit, Saualpe–Koralpe region, Eastern European Alps, document a poly-metamorphic evolution. Textures indicate two stages of garnet growth, interpreted as (i) relics of an early, pre-Alpine metamorphic event and (ii) newly-grown garnet of Alpine age. The age of the first crystallisation event during prograde Permian metamorphim is constrained by Sm–Nd dating of the almandine-rich garnet core domains. Mineral–wr ages of nine high-Sm/Nd garnet single crystal core fractions from three distant localities (Laaken, SE Koralpe; Plankogel, NW Saualpe; Friesach, E Gurktal Alps) range between 275.0±2.7 and 264.1±3.6 Ma. The pooled mean regression age of just the garnet fractions is 269.6±6.2 Ma (MSWD=4.3; n =9), with an initial Nd isotope composition of εNd(270)=−8.6. Leaching experiments and whole rock analyses revealed slight Nd isotope disequilibrium between garnet porphyroblasts, REE-rich inclusions, and the matrix of the polyphase mineral assemblage. Kyanite inclusions in the inner and sillimanite in the outer parts of the garnet cores, decreasing grossular and increasing pyrope contents indicate variable pressure/temperature conditions during Permian garnet growth. Modelling of mineral assemblages in pressure–temperature pseudosections suggests that the first-generation, Permian garnet crystallized at about 0.46 GPa and 540 °C, whereas the eo-Alpine garnet documents distinctly higher pressures of 1.0 GPa and 600 °C. The data document regional prograde metamorphism in metapelites, at low or moderate pressure conditions, during the Middle Permian. Crustal-scale extensional processes and heat input from the mantle, induced by ongoing Paleotethys subduction and Neotethys opening in the course of the separation of Europe and Africa-related Adria were responsible for this low-P tectonothermal event. [Copyright &y& Elsevier]

Besides older granites and gneisses, the Thu¨ringer Wald in central Europe consists predominantly of altered Permian volcanic and pyroclastic rocks The alteration history of these volcanic rocks was investigated by K–Ar, 39ArK–40Ar, Rb–Sr, δ18O, δD and X-ray studies on feldspar and biotite separates. Alteration is accompanied by increases in the triclinicity and δ18O values (+14‰ to +19‰) of alkali feldspars, an increase in the δ18O values of low-potassium biotites (>+10‰) and in sericitized plagioclases (+12‰ to +19‰) and also the generation of apparent Mesozoic isotopic ages. In contrast, volcanic biotites with primary magmatic δ18O signatures still yield Early Permian ages, and biotites of the granitic basement give Carboniferous ages.The observed isotope changes of the volcanic feldspars indicate episodic fluid exchange with meteoric and formation brines (δ18O≤+5‰) at low temperatures (200 to 100 °C) during tectonic fracturing of the basement. As a result, sanidines were replaced by porous microclines which were subject to isotope exchange and Ar loss. Feldspars and biotites of the granitic wall rocks seem to be less affected by alteration processes, which may be due to their lower permeabilities.Plateau-shaped 40Ar/39Ar age spectra of volcanic alkali feldspar indicate complete resetting of the K–Ar system, whereas staircase spectra point to partial retention or repeated Ar loss. Because the feldspars represent mixtures of microcline and sanidine remnants, the apparent ages (260 to 160 Ma) represent maximum ages for the time of replacement. Rb–Sr feldspar model ages (180 Ma or 230 to 170 Ma, respectively) support the K–Ar data. The minimal 40Ar/39Ar spectra ages are concordant with both K–Ar and 40Ar/39Ar sericite model ages (160 to 150 Ma) of sericitized plagioclases. The model ages favour Jurassic episodic fluid reactions with alkali feldspars and plagioclases in the Thu¨ringer Wald. [Copyright &y& Elsevier]

Analytical errors calculated for individual spot 206Pb/238U measurements of zircon analyzed using high mass resolution secondary ion mass spectrometry (HR-SIMS, e.g., SHRIMP II) were assessed using natural zircon (z6266) and synthetic glass standards (NIST SRM 610). Evidence for U/Pb homogeneity of these materials includes new thermal ionization mass spectrometry (TIMS) U–Pb analyses of 22 fragments of z6266 zircon from two laboratories, which are identical within error and yield a weighted mean 206Pb/238U age of 559.0±0.2 Ma. TIMS U–Pb analyses of the SRM 610 yielded homogeneous 206Pb/238U=0.2566.Errors in HR-SIMS 206Pb/238U measurements are distinguished in increasing hierarchy as within-spot, within-session (internal), and external. Replicate SHRIMP measurements of 208Pb+/206Pb+ and 248[ThO]+/254[UO]+ in SRM 610 demonstrate that within-spot analytical errors are sufficient (i.e., MSWD''s∼1) to account for the dispersion of data. In contrast, it was not possible with SRM 610 to reproduce the 206Pb+/238U[Ox=0,1,2]+ ratios from spot-to-spot within the limits of the within-spot errors, after correcting the data for obvious systematic variations in the discrimination of the Pb and U isotopes. Identical findings were made following replicate 206Pb+/238U[Ox]+ analyses of z6266 zircon. That is, for both glass and zircon standards, there remained on average about ±1% (1σ) unaccounted variation per 206Pb+/238U[Ox]+ analysis (i.e., MSWDs≫1). Thus, unlike for 248[ThO]+/254[UO]+, the within-session errors in 206Pb+/238U[Ox]+ were necessarily higher than the within-spot errors.In contrast to 208Pb+/206Pb+ and 248[ThO]+/254[UO]+, the 206Pb+/238U[Ox]+ ratios in glass and zircon are significantly fractionated relative to accepted values, and it seems probable that this difference underlies the larger within-session errors for 206Pb+/238U[Ox]+. The usual methods (e.g., using 254[UO]+/238U+) to correct for the inherent discrimination are imperfect and sensitive to small differences in analytical conditions between spots. Matrix variations are ruled out, leaving the precise cause(s) of the additional variation in 206Pb+/238U[Ox]+ currently undetermined, and a hindrance to decreasing the uncertainties in spot 206Pb/238U measurements. The minimum within-session error for an individual 206Pb/238U measurement is the standard deviation (dispersion) of the discrimination-corrected 206Pb+/238U[Ox]+ values for a homogeneous reference material (e.g., SRM 610). The external error in 206Pb/238U additionally incorporates the standard deviation of the mean of the discrimination-corrected 206Pb+/238U[Ox]+ for a natural zircon standard. The analysis of 206Pb+/270[UO2]+ offers an improvement over measuring 206Pb+/238U+ because variable discrimination is reduced or eliminated with respect to 254[UO]+/238U+ and counting statistics are superior. [Copyright &y& Elsevier]