The flow of surface-derived fluids through Alice Springs age middle-crustal ductile shear zones, Reynolds Range, central Australia

The south-east Reynolds Range, central Australia, is cut by steep north-west-trending Alice Springs age ( c. 334  Ma) shear zones that are up to hundreds of metres wide and several kilometres long with reverse senses of movement. Amphibolite facies (550–600  °C, 500–600  MPa) shear zones cut metapelites, while greenschist facies shear zones (420–535  °C, 400–650  MPa) cut metagranites. The sheared rocks commonly underwent metasomatism implying that the shear zones were the pathways of significant fluid flow. Altered granites within greenschist facies shear zones have gained Si and K but lost Ca and Na relative to their unsheared counterparts, suggesting that the fluid flowed down-temperature (and hence probably upward) through the shear zones. Time-integrated fluid fluxes calculated from silica addition are up to 2.1×10¹⁰ mol  m⁻² ( c. 4.2×10⁵  m³  m⁻²). Similar time-integrated fluid fluxes are also estimated from changes in K and Na. The sheared granitic rocks locally have δ¹⁸O values as low as 0 which is much lower than the δ¹⁸O values of the adjacent unsheared granites (7 to 9), implying that the fluid which flowed through these shear zones was derived from the surface. For the estimated time-integrated fluid fluxes, the fluids would be able to retain their isotopic signature for many tens to hundreds of kilometres. The flow of surface-derived fluids into the ductile middle crust, with subsequent expulsion upwards through the shear zones, may have been driven by seismic activity accompanying the Alice Springs deformation.     

Neodymium and Strontium Isotope Data for USGS Reference Materials BCR-1, BCR-2, BHVO-1, BHVO-2, AGV-1, AGV-2, GSP-1, GSP-2 and Eight MPI-DING Reference Glasses

We have measured ⁸⁷Sr/⁸⁶Sr and ¹⁴³ Nd/¹⁴⁴ Nd isotope ratios in different batches and aliquots of the new US Geological Survey (USGS) reference materials (RMs) BCR-2, BHVO-2, AGV-2 and GSP-2 and the original USGS RMs BCR-1, BHVO-1, AGV-1 and GSP-1 by thermal ionisation mass spectrometry. In addition, we also analysed the eight Max-Planck-Institut-Dingwell (MPI-DING) reference glasses. Nearly all isotope ratios obtained in the different aliquots and batches agree within uncertainty limits indicating excellent homogeneity of the USGS powders and the MPI-DING glasses. With the exception of GSP-2, the new USGS RMs are also indistinguishable from the ratios found in the original USGS RMs (⁸⁷Sr/⁸⁶Sr: 0.704960, 0.704958 (BCR-1, -2), 0.703436, 0.703435 (BHVO-1, -2), 0.703931, 0.703931 (AGV-1, -2); ¹⁴³ Nd/¹⁴⁴ Nd: 0.512629, 0.512633 (BCR-1, -2), 0.512957, 0.512957 (BHVO-1, -2); 0.512758, 0.512755 (AGV-1, -2)). This means that for normalisation purposes in Sr and Nd isotope geochemistry BCR-2, BHVO-2 and AGV-2 can well replace BCR-1, BHVO-1 and AGV-1 respectively.     

Strontium Isotope Analysis of Kimberlitic Groundmass Perovskite via LA-MC-ICP-MS

Strontium isotopic composition is a potentially powerful tracer in studies of kimberlitic rocks but the results from even the most carefully collected and stringently prepared bulk-rock samples are still hampered by contamination and alteration effects. Here we describe a LA-MC-ICP-MS technique which can obtain accurate, high precision Sr_i ratios from 50–150 μm kimberlitic groundmass perovskite without requiring time-consuming mineral separation procedures. Since perovskite is a robust magmatic phase with an extremely low Rb/Sr ratio, the effects of late-stage crustal contamination, post-emplacement alteration and age correction are minimised and results are more representative of primary melt compositions, while additionally preserving powerful grain-scale spatial and textural information. We demonstrate that the adopted protocol overcomes isobaric interferences from Kr⁺, Rb⁺, Er²⁺ and Yb²⁺, and that Ca dimers and Ca argides do not detectably affect the quality of ⁸⁷Sr/⁸⁶Sr ratios produced. To illustrate the utility of the technique, contrasting bulk-rock and in situ perovskite results from eleven Proterozoic kimberlites are documented.     

Refinements in BaSO4 to CO2 Preparation and δ18 O Calibration of the Sulfate Reference Materials NBS-127, IAEA SO-5 and IAEA SO-6

Refinements have been made to achieve over 99% yield in the conversion of CO to CO₂ in order to improve the reproducibility and accuracy of δ¹⁸ O measurements in sulfates. BaSO₄ (10-15 mg) was mixed with an identical amount of spectrographic-grade graphite and loaded into a Pt boat. The mixture was gradually heated to 1100 °C to reduce sulfate to CO and CO₂; the former gas was simultaneously converted to CO₂ by a glow discharge between Pt electrodes immersed in a magnetic field (produced by a pair of external neodymium magnets). A small memory effect was noticed during the analysis (less than 0.3‰ per 10‰ difference in δ¹⁸ O between two subsequently analysed samples). The memory effect, however, was suppressed by repetitive preparation of the same specimen. CO₂ produced in this way from sulfate reference samples was analysed on a dual inlet and triple collector mass spectrometer along with CO₂ equilibrated with VSMOW, GISP and SLAP water reference samples. To avoid large departures of measured isotope ratios from ¹⁸O/¹⁶O of the working calibrator we used CO₂ gas prepared from ocean water sulfate for this purpose. The calibrated δ¹⁸ O values (in ‰) obtained in this way for NBS-127, IAEA SO-5 and IAEA SO-6 reference materials were 8.73 ± 0.05, 12.20 ± 0.07 and -10.43 ± 0.12, respectively.     

Measurement of SIMS Instrumental Mass Fractionation of Pb Isotopes During Zircon Dating

An igneous zircon reference material (OG1) was characterised for U-Pb isotopes by ID-TIMS, and utilised to evaluate SIMS (SHRIMP) instrumental mass fractionation (IMF) of radiogenic Pb isotopes (²⁰⁷Pb*/²⁰⁶Pb*). The TIMS ²⁰⁷Pb*/²⁰⁶Pb* reference value for OG1 was 0.29907 ± 0.00011 (95% confidence limit), 3465.4 ± 0.6 Ma. The high ²⁰⁷Pb* (∼ 30 μg g⁻¹), negligible common Pb, and isotopic homogeneity permitted precise (± 1–2‰) ²⁰⁷Pb*/²⁰⁶Pb* measurements within the analytical sessions. External reproducibility of mean ²⁰⁷Pb*/²⁰⁶Pb* ratios between sessions was demonstrated for one instrument, yielding a mean IMF of +0.87 ± 0.49‰. The mean ²⁰⁷Pb*/²⁰⁶Pb* ratios between instruments were dispersed beyond uncertainties, with session IMF values from +3.6 ± 1.7‰ to −2.4 ± 1.3‰, and a grand mean IMF value (twenty-six sessions) of +0.70 ± 0.52‰, indicating a tendency towards elevated ²⁰⁷Pb*/²⁰⁶Pb*. The specific causes of variability in IMF are unclear, but generally reflect subtle differences in analytical conditions. The common practice in SIMS of assuming that IMF for Pb⁺ is insignificant could result in systematic age biases and underestimated uncertainties, of critical importance for precise correlation of Precambrian events. Nevertheless, a zircon RM such as OG1 can be readily incorporated into routine dating to improve ²⁰⁷Pb*/²⁰⁶Pb* accuracy and external reproducibility.     

Control on the emplacement of the andesite lava dome of the Soufriere Hills volcano, Montserrat by degassing-induced crystallization

Lava solidification is controlled by two mechanisms: external cooling and gas exsolution, the latter inducing crystallization due to increasing liquidus temperature. The andesite lava dome of the Soufriere Hills Volcano, Montserrat, is an extrusion dominated by crystallization caused by gas exsolution where cooling is unimportant in controlling emplacement. In the magma chamber the magma has an estimated viscosity of 7 × 10⁶ Pa s. During ascent, gas exsolution caused the magma to extrude in a highly crystalline state, with only 5–15% residual melt, viscosities in the range 10¹³–10¹⁴ Pa s and mechanical strength > 1 MPa. Deformation can be heterogeneous with extrusion along shear zones. Rheological stiffening in the upper conduit also causes large overpressures, shallow seismicity, and cyclic patterns of dome extrusion. Gas-rich porphyritic andesites tend to be the least mobile kind of lava, because transition from magma into hot crystalline material was reached during ascent.     

Effective Correction of Mass Bias for Rhenium Measurements by MC-ICP-MS

The geochemistry of Re-Os and the recent use of Re as a non-traditional stable isotope both need accurate and precise quantification of ¹⁸⁷Re/¹⁸⁵Re ratios. This paper reports rhenium isotopic data obtained from the analysis of a standard solution and geological samples by MC-ICP-MS. We show that measured isotopic ratios are modified by matrix effects that cannot be accounted for by the standard solution bracketing technique. The bias resulting from measurements on a spiked (¹⁸⁵Re-enriched) sample is shown to alter the apparent Re concentration by several percent. When spiking samples and calibrators with tungsten, simultaneous measurement of tungsten and rhenium isotopes compensates for the matrix-induced modification of mass bias. Rhenium and tungsten are shown to have different fractionation factors. This may be due to the fact that the two elements fractionate in a different but systematic way, or that the reference isotopic ratios used for elemental Re and W are incoherent with one another. The consistency of fractionation through time can be used to obtain an empirical relationship between W and Re measured ratios from a standard solution to obtain a sample's fractionation-corrected ¹⁸⁷Re/¹⁸⁵Re spiked ratio on samples containing pg g⁻¹ levels of Re, even if some matrix capable of affecting mass bias remains in the final solution.     

Diffusion-controlled corona reaction and overstepping of equilibrium in a garnet granulite, Yenisey Ridge, Siberia

Diffusion modelling is applied to layered garnet–pyroxene–quartz coronas, formed by a pressure-induced reaction between plagioclase and primary pyroxene in a metabasic granulite. The reconstructed reaction involves some change in composition of reactant minerals. The distribution of minerals between layers is satisfactorily explained by diffusion-controlled reaction with local equilibrium, in which the diffusion coefficient for Al was smaller than those for Fe, Mg and Ca by a factor of approximately four. Diffusion of Mg towards plagioclase implies a chemical-potential gradient for MgO component in a direction opposite to the changing Mg content of garnet; this is explained by the influence of Al₂O₃ on the chemical potential of the pyrope end-member. Grain-boundary diffusion is suggested to have operated, possibly with composition gradients different from those in the bulk minerals. Chemical-potential differences across the corona are estimated from the variation in garnet composition, enabling affinity (the free energy change driving the reaction) to be estimated as 6.9±1.8  kJ per 24-oxygen mole of garnet produced. This implies that the pressure for equilibrium among the minerals was overstepped by 1.4±0.4  kbar. The probable P–T conditions of reaction were in the range 650–790  °C, 8–10  kbar. Assuming a timescale of reaction between 10⁶ and 10⁸ years, estimated diffusion coefficients for Fe, Mg and Ca are in the range 9×10⁻²³ to 5×10⁻²⁰ m² s⁻¹. These are consistent with experimental values in the literature for solid-state diffusion, including grain-boundary diffusion.     

Determination of the Absolute Hydrogen Isotopic Ratio of V-SMOW and SLAP

By mixing ¹H₂O and ²H₂O, both with accurately known purity, samples were prepared with ²H/¹H ratios close to those of the international isotopic water standards: V-SMOW and SLAP. A mass spectrometrical comparison of these calibration samples with the actual water standards revealed:
²H/¹H of V-SMOW = (155.95 ± 0.08) × 10^−6
2H/¹H of SLAP = (89.12 ± 0.07) × 10⁻⁶
δ²H_V-SMOW(SLAP) =−428.5 ± 0.5 %     

Cycles of passive versus active diapirism recorded along an exposed salt wall

Although it has long been recognised that passive salt diapirism may encompass sub-ordinate cycles of active diapirism, where sedimentary overburden is periodically shed off the roof of the rising salt, there has been very little study of this process around exposed salt (halite) diapirs. However, the Late Miocene-Pliocene Sedom salt wall, on the western side of the Dead Sea Basin, presents an opportunity for detailed outcrop analysis of diapiric salt and the associated depositional and deformational record of its movement during both passive and active phases of diapirism. The sub-seismic scale record of diapirism includes sedimentary breccia horizons interpreted to reflect sediments being shed off the crest of the growing salt wall, together with exceptional preservation of rotated unconformities and growth faults. Areas of more pronounced dips directed towards the salt wall are capped by unconformities, and interpreted to represent withdrawal basins within the overburden that extend for at least 1500 m from the salt margin. Elsewhere, broad areas of upturn directed away from the salt extend for up to 1250 m and are marked by a sequence of rotated unconformities which are interpreted to bound halokinetic sequences. The margins of the salt wall are defined by steep extensional boundary faults that cut upturned strata, and have enabled rapid and active uplift of the salt since the Holocene. The Sedom salt wall therefore charts the transition from passive growth marked by withdrawal basins, growth faults and unconformities, to more active intrusion associated with major boundary faults that enable the rapid uplift of overburden deposited on top of the salt to ∼100 m above regional elevations in the past 43 ka. Individual cycles of passive and active diapirism occur over timescales of <30 ka, which is up to an order of magnitude less than typically suggested for other settings, and highlights the dynamic interplay between salt tectonics and sedimentation in an environment undergoing rapid fluctuations in water level.     

High Precision Determination of 48Ca/42Ca Ratio by TIMS for Ca Isotope Fractionation Studies

For studies of mass-dependent fractionation of calcium isotopes in natural materials, the ⁴⁸Ca/⁴²Ca ratio is a superior choice to the conventionally measured ⁴⁴Ca/⁴⁰Ca ratio for two important reasons. These are (1) mass-dependent fractionation can be determined free from the effects of inherited or ingrown radiogenic ⁴⁰Ca and (2) this ratio increases the spread of measured isotopic masses by 50%, resulting in statistically better resolution of fractionation, assuming similar precision. A third, though strictly technical, advantage is the inherent ability of a mass spectrometer to measure ratios close to unity (⁴⁸Ca/⁴²Ca) more precisely than very small or large ratios (⁴⁴Ca/⁴⁰Ca). However, because of the very low natural abundance of both ⁴⁸Ca and ⁴²Ca, their ratio has been very difficult to measure, the only attempt so far being on a high mass resolution MC-ICP-MS with a precision of 0.33%. We report here determination of the ⁴⁸Ca/⁴²Ca ratio by the more commonly available and user-friendly multi-collector TIMS using a ⁴³Ca-⁴⁶Ca double-spike, with a significantly better precision of 0.18% (2s). The ⁴⁸Ca/⁴⁰Ca or ⁴⁴Ca/⁴⁰Ca ratio can also be measured in the same mass spectrometer run to provide complementary information on any radiogenic component.     

San Quintín volcanic field, Baja California, Mexico: 'within-plate' magmatism following ridge subduction

The Holocene San Quintín volcanic province in northern Baja California comprises spinel-lherzolite-bearing alkali basalts. Trace element (La/Nb = 0.57–0.73; K/Rb = 402–479; La_N/Yb_N= 8.4, 9.9) and isotopic ratios (⁸⁷Sr/⁸⁶r = 0.70323–0.70352; ¹⁴³Nd/¹⁴⁴Nd = 0.512924–0.512996; ²⁰⁶Pb/²⁰⁴Pb = 19.108, 19.250; ²⁰⁷Pb/²⁰⁴Pb = 15.567, 15.589; ²⁰⁸Pb/²⁰⁴Pb = 38.82, 38.85) show that the lavas are compositionally indistinguishable from some ocean island, plume-associated basalts such as Hawaii and the Azores, and testify to an asthenospheric source for the magmas. The occurrence in Baja of such lavas may be related to the nature of the cessation of plate subduction beneath the peninsula; at present, San Quintín (and volcanic provinces to the north) are underlain by a 'no-slab window', whereas immediately to the south, remanent oceanic lithosphere may be preserved as a relict slab. This may act as a bamer to the upward passage of diapirs or magmas from the asthenosphere.     

Boron Isotopic Composition and Concentration of Ten Geological Reference Materials

We present boron isotope and concentration data from magmatic (komatiitic to rhyolitic) and sedimentary geological silicate and artificial glass reference materials that cover a wide spectrum of boron isotope compositions and boron concentrations. Boron isotope compositions were determined by TIMS (Cs₂BO₂⁺ -graphite and BO₂^- method) and boron concentrations by ICP-AES. Boron concentrations ranged from 7 to 159μ g^-1 and agree within 14% with published values. Based on replicate analyses of individually prepared sample aliquots an overall external reproducibility of better than 10% was determined. The obtained δ¹¹B values ranged from -12.6 to +13.6% and were reproducible within 1.1 % (2 RSD; excluding NTIMS) on the basis of individually prepared sample aliquots. The δ¹¹B values of JA-1 (+5.3%), JB-3 (+5.9%) and JR-2 (+2.9%) overlap the published data within analytical uncertainty. For the first time δ¹¹B values for the TB (-12.6%) and the MPI-DING glasses GOR-128-G (+13.6%), GOR-132-G (+7.1 %) and StHs6/80-G (-4.5%) are reported. The δ¹¹B values obtained by the Cs₂BO₂⁺ -graphite and the BO₂^- method as well as the majority of δ¹¹B values obtained using different sample preparation methods agree within analytical uncertainty. Therefore, we conclude that none of these analytical methods introduce any systematic error on the obtained δ¹¹B values.     

Quantitative study of the distributary braidplain of the Preboreal ice-contact Gardermoen delta complex, southeastern Norway

This raised delta structure is an ice-contact deltaic complex with a volume of c. 4.4.10⁹ m³, deposited c . 9500 yr BP in a shallow wide 'fjord' during the retreat of the Scandinavian ice cap. The delta plain lies at an altitude of 200–223 m. It aggraded c . 20 m above the contemporaneous sea level during a regional marine regression. The braidplain palaeochannel characteristics indicate a peak meltwater discharge of 7–9 10³ m³/s. Calculations based on a glacial ablation model indicate a mid-summer discharge of c . 5.5 10³ m³/s. However, the fluvial topset of the delta has an erosive base whose altitude decreases upstream and indicates stream incision by more the 6 m below the contemporaneous sea level. The deep scour is ascribed to episodic floods over the relatively short delta plain, which exceeded direct ablation-associated discharges. The depositional time-span of the delta is assessed to have been 70 years, calculated from coastal gradient and shoreline displacement curves. The average sedimentation rate of the delta is thereby inferred to have been extremely high, c . 6. 10⁷ m³/yr. The sedimentation is thought to reflect 'extreme' ice-margin conditions, where the sediment and water discharge was maximized by full-scale ablation, with simultaneous subglacial, englacial and supraglacial sediment and water supply. These conditions might further coincide with an abundant rainfall in the catchment area or the drainage of dammed waters, initiating episodic floods which eroded deep beneath sea level. As a whole, the study illustrates the hydrological conditions of proglacial sedimentation at the front of the rapidly retreating last Scandinavian ice cap.     

Sr isotope evolution of Maastrichtian seawater, determined from the chalk of Hemmoor, NW Germany

In 140 metres of Maastrichtian White Chalk (nannofossil chalk) exposed near Hemmoor, NW Germany, values of ⁸⁷Sr/⁸⁶Sr increase from 0.707760 in the Belemnella sumensis Zone (Lower Maastrichtian) at the base of the section (-54.5 m; referred to 0 m at a prominent marl, M900) to 0.707821 in the Belemnella baltica/danica Zone (Upper Maastrichtian) at the top of the section (+84.5 m). A plateau in ⁸⁷Sr/⁸⁶Sr occurs between -5m and +50m in the section, probably as a result of a very high rate of sedimentation in this interval. A belemnite and associated nannofossil chalk have similar ⁸⁷Sr/⁸⁶Sr values, suggesting that there has been little diagenetic alteration of the ⁸⁷Sr/⁸⁶Sr ratios in the chalk, which therefore preserves its original ⁸⁷Sr/⁸⁶Sr. Comparison of ⁸⁷Sr/⁸⁶Sr and nannofossil zonations for sequences at Bidart, France, and DSDP Sites reveals discordance and so possible diachronism of the basal boundaries of nannofossil Zones CC25B and CC25C.     

Dynamic sea-level change during the last deglaciation of northern Iceland

Rundgren, M., Ingólfsson, Ó., Björck, S., Jiang, H. & Haflioason, H. 1997 (September): Dynamic sea-level change during the last deglaciation of northern Iceland. Boreas , Vol. 26, pp. 201–215. Oslo. ISSN 0300–9483.
A detailed reconstruction of deglacial relative sea-level changes at the northern coast of Iceland, based on the litho- and biostratigraphy of lake basins, indicates an overall fall in relative sea level of about 45 m between 11300 and 9100 BP, corresponding to an isostatic rebound of 77 m. The overall regression was interrupted by two minor transgressions during the late Younger Dryas and in early Preboreal, and these were probably caused by a combination of expansions of local ice caps and readvances of the Icelandic inland ice-sheet margin. Maximum absolute uplift rates are recorded during the regressional phase between the two transgressions (10000–9850 BP), with a mean value of c . 15 cm ¹⁴C yr^-1 or 11–12 cm cal. yr^-1. Mean absolute uplift during the regressional phase following the second transgression (9700–9100 BP) was around 6 cm ¹⁴C yr^-1, corresponding to c . 3 cm cal. yr^-1, and relative sea level dropped below present-day sea level at 9000 BP.     

The last deglaciation of the Franz Victoria Trough, northern Barents Sea

A study of two piston cores and a 3.5 kHz seismic profile from the Franz Victoria Trough provides new stratigraphic, stable isotopic and foraminiferal AMS ¹⁴C data that help constrain the timing of ice-sheet retreat in the northern Barents Sea and the nature of the deglacial marine environment. Silty diamicton at the base of each core, interpreted as till or ice-marginal debris flow, suggests that the Barents ice sheet was grounded at the core sites (470 m water depth). Eight AMS ¹⁴C dates on sediment overlying the diamicton indicate that the ice sheet retreated from both core sites by 12.9 ka and that postglacial sedimentation began 10 ka ago. These dates, combined with a recently published ¹⁴C date from a nearby core, suggest that the Franz Victoria Trough may not have been deglaciated until c . 13 ka, 2000 years later than modeled ice-sheet reconstructions indicate. In the trough, oxygen isotopic ratios in planktonic foraminifera ^N. pachyderma (sinistral) were 0.5–0.750, lower during deglaciation than after, probably as a result of ice-sheet and/or iceberg melting. Foraminiferal assemblages suggest that Atlantic-derived intermediate water may have begun to penetrate the trough c . 13 ka ago.     

Zircon U–Pb age, trace element and Hf isotope composition of Kongling terrane in the Yangtze Craton: refining the timing of Palaeoproterozoic high-grade metamorphism

U–Pb age, trace element and Hf isotope compositions of zircon were analysed for a metasedimentary rock and two amphibolites from the Kongling terrane in the northern part of the Yangtze Craton. The zircon shows distinct morphological and chemical characteristics. Most zircon in an amphibolite shows oscillatory zoning, high Th/U and ¹⁷⁶Lu/¹⁷⁷Hf ratios, high formation temperature, high trace element contents, clear negative Eu anomaly, as well as HREE-enriched patterns, suggesting that it is igneous. The zircon yields a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2857 ± 8 Ma, representing the age of the magmatic protolith. The zircon in the other two samples is metamorphic. It has low Th/U ratios, low trace element concentrations, variable HREE contents (33.8 ≥ Lu_N≥2213; 14.7 ≤ Lu_N/Sm_N ≤ 354) and ¹⁷⁶Lu/¹⁷⁷Hf ratios (0.000030–0.001168). The data indicate that the zircon formed in the presence of garnet and under upper amphibolite facies conditions. The metamorphic zircon yields a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2010 ± 13 Ma. These results combined with previously obtained Palaeoproterozoic metamorphic ages suggest a c. 2.0 Ga Palaeoproterozoic collisional event in the Yangtze Craton, which may result from the assembly of the supercontinent Columbia. The zircon in two samples yields weighted mean two-stage Hf model ( T _DM2) ages of 3217 ± 110 and 2943 ± 50 Ma, respectively, indicating that their protoliths were mainly derived from Archean crust.     

The Use of Multiple Reference Samples for the Monitoring of Ion Microprobe Performance During Zircon 207Pb/206Pb Age Determinations

Data from a series of extended analytical sessions using a range of different zircon reference samples have been used to investigate the periodic derivation of abnormal ²⁰⁷Pb/²⁰⁶Pb ages during microbeam analysis%For the Canberra SHRIMP II, this phenomenon probably results from the presence of a signal at mass 204 that is alien to the Pb spectrum; isotopic fractionation is unlikely to be a significant contributor. In contrast, abnormal ²⁰⁷Pb/²⁰⁶Pb ages obtained from SHRIMP I at the same research centre require a different, but as yet unknown explanation. Assessment of the reasons for atypical ^207Pb/²⁰⁶Pb measurements and the means of correcting for them should therefore be independently assessed for individual analytical instruments. The use of reference samples with very different ages is an effective way of doing this, because older reference samples are more useful for documenting isotopic fractionation, whereas younger ones are more sensitive indicators of isobaric interference.