Laser Ablation MC-ICPMS Research Articles

Laser ablation MC-ICPMS U-Th and U-Th-Pb dating of Quaternary zircons from Jeju Island, Korea

BackgroundLaser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) emerged in the mid-1980s and rapidly became a crucial dating tool. The advent of femtosecond LA systems has substantially reduced volatility-dependent mass fractionation. This study showcases U-Th and U-Th-Pb dating results of Quaternary zircons collected from Jeju Island, Korea, utilizing an advanced femtosecond laser-connected multi-collector (MC)-ICPMS.FindingsZircon grains from trachyte samples near the Baeknokdam lake (JJ616-1), Yeongsil (JJ08-1), Chunwangsa (JJ09-1), and Oraidong (JJ09-3) provided weighted mean 238U-230Th ages of 28.7 ± 1.6 ka (n = 56/64, MSWD = 3.8), 81.8 ± 10.9 ka (n = 11/12, MSWD = 1.6), 92.6 ± 4.6 ka (n = 49/51, MSWD = 2.2), and 117.6 ± 8.2 ka (n = 48/50, MSWD = 3.2), respectively. The age determination for JJ08-1 zircon aligned well with the recommended value (82 ± 6 ka). Zircons from Sanbangsan (JJ615-1) and Wonmansa (JJ08-2) trachytes yielded common Pb and radioactive disequilibrium-corrected weighted mean 238U-206Pb ages of 785 ± 5 ka (n = 27/28, MSWD = 0.90) and 743 ± 8 ka (n = 28/30, MSWD = 0.79), respectively. The weighted mean 238U-206Pb ages of Penglai and 61.308 reference zircons were determined to be 4226 ± 21 ka (n = 22/25, MSWD = 3.8) and 2488 ± 20 ka (n = 19/20, MSWD = 1.8), respectively. These ages are concordant with the recommended values.ConclusionsOur data provides additional evidence of trachyte magmatism occurring in Jeju Island during the transitional period between the Early and Middle Pleistocene and the Late Pleistocene. The zircon samples analyzed in this study could serve as reference age data for Quaternary geochronology research.

Uranium-thorium isotope systematics of cold-seep carbonate and their constraints on geological methane leakage activities

Marine methane hydrates are a huge and dynamic carbon reservoir found mainly at the continental margins, and their stability might be affected by climate-associated pressure and temperature changes on the seafloor. Reconstructing the growth history of cold-seep carbonates, which formed during seafloor methane leakage, could help to constrain methane hydrate stability in the geological past. However, U-Th isotope systematics of these complex carbonate cements have not yet been systematically investigated on the micro-scale, leaving uncertainties in the U-series geochronology which is commonly applied to date the seep carbonates. In this study, we have developed multiple in-situ analytical methods, including U-Th isotope analysis by laser ablation MC-ICPMS, elemental concentration mapping by laser ablation ICPMS, as well as organic distribution mapping by Raman Spectroscopy, to provide insights into the U-series geochemistry and geochronology for the different types of cold-seep carbonates. Our result demonstrates that 238U and 232Th of these carbonates are dominantly derived from seawater and detrital particles, respectively. As the [230Th/232Th] (activity ratio) of micro-domains with high [232Th/238U] is negligibly affected by U decay, we have directly determined the initial [230Th/232Th] of the cold-seep carbonates which is 0.7 ± 0.1 (2 SD, n = 12). In general, U-series isotopes show closed-system behavior within our studied analytical precision in the interior of the seep cements, and the initial [230Th/232Th] derived from the isochron approach is consistent with the in-situ direct determination. A notable exception is a calcitic pipe sample with highly enriched U in its rim which has also experienced post-depositional mobilization. Our method is then applied to a large set of seep carbonate samples from the upper continental slope of the northern South China Sea (SCS). The obtained U-series ages provide evidence that cold seep was likely continuously active since at least ∼72 ka in the northern SCS, but the timing and duration of methane leakage vary between different sites within this region. While the upper continental slope methane hydrates are highly susceptible to destabilization under changing bottom water conditions, our extensive dating on seep carbonates from the SCS further indicates that bottom water P-T conditions could exert contrasting impacts on methane hydrate stability at different depths within the same geological setting. Overall, the in-situ U-series geochronological method developed in our study has the potential to date cold-seep carbonates efficiently and reliably, offering new opportunities to probe into the history of methane leakage in the late Quaternary.

In situ sulfur isotope analysis by laser ablation MC-ICPMS and a case study of the Erlihe Zn-Pb ore deposit, Qinling orogenic belt, Central China

Sulfur isotopic composition is useful for tracing sources of ore-forming materials. In situ sulfur isotope analysis by laser ablation coupled with multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) has an advantage over traditional bulk analysis in tracing sources of sulfur in an ore deposit that has complex mineral types and multiple ore-forming processes. However, in using this method, matrix effects exist widely for sulfides from ore deposits. In this work, matrix effects of different sulfides were examined by matrix-unmatched tests, in which the δ34S values of different types of sulfides were calibrated against a laboratory pyrite standard. The results show that the δ34S values of pyrrhotite, chalcopyrite, molybdenite, sphalerite, and arsenopyrite are in agreement with the reference values, suggesting that the matrix effect was negligible. However, a significant matrix effect occurred in the analyses of Ag2S and galena, leading to significant deviations in δ34S values of up to 1.2‰ and 3.2‰, respectively. The matrix effect occurred not only during the laser ablation stage but also during the transition of ions into the mass spectrometer, resulting in significant space charge effects caused by heavy-mass elements. Thus, pyrite can be used as an external bracketing standard for sulfides that do not have a heavy-mass matrix.Based on our matrix-effect study, the following available external standards were selected for the in situ sulfur isotope analysis of complex-type minerals from the Erlihe Zn-Pb deposit. The coarse-grained sphalerite grains (Sph1) and euhedral to subhedral pyrite grains (Py1) from massive ores have δ34S values ranging from 9.2‰ to 18.1‰. Such heavy sulfur isotopic compositions indicate the derivation of sulfur from the deposits of seawater sulfate, related to a thermochemical reducing process. Fine-grained sphalerite (Sph2) and pyrite (Py2) from vein and disseminated ores coexist with pyrrhotite and chalcopyrite, and have relatively light sulfur isotopic compositions, with δ34S values of about 7‰. The slightly positive δ34S values are similar to those of sulfides from sulfide-rich quartz veins, which contain δ34S values of about 4‰, and granodiorite rocks, with δ34S values of about 6‰. Based on previous studies, the light sulfur of the late-stage sulfides may have resulted from a metamorphic or magmatic hydrothermal event during the Indosinian orogeny. In situ sulfur analysis by LA-MC-ICPMS provides new evidence for sources of sulfur from each stage of mineralization in the Erlihe deposit, suggesting that this is a promising method for studying complex ore deposits.

Deciphering the zircon Hf isotope systematics of Eoarchean gneisses from Greenland: Implications for ancient crust-mantle differentiation and Pb isotope controversies

Abstract We report a Hf isotope investigation of zircons from four Eoarchaean orthogneisses from the Godthabsfjord region of southern West Greenland by laser ablation MC-ICPMS, to elucidate crust-mantle differentiation processes in the early Earth. Zircon crystals of all samples record a complex, multi-stage growth and disturbance history, and these discrete growth phases also exhibit disparate Lu-Hf isotope systematics. The oldest (3.84–3.82 Ga) zircon cores have tightly clustered 176Hf/177Hf ratios that are consistent with derivation of their tonalitic precursors from chondritic mantle at this time, with no evidence of input from older crustal or depleted mantle sources. Younger (3.67–3.62 Ga) zircon overgrowths have subchondritic Hf and plausibly grew from small fraction partial melts of the tonalitic host, involving variable dissolution of the older zircon cores. The Neoarchean (ca. 2.7 Ga) zircon component in some samples extends to significantly higher 176Hf/177Hf than the >3.65 Ga zircon, a feature that is interpreted to reflect addition of radiogenic Hf from the rock matrix during metamorphic zircon growth and recrystallisation at 2.7 Ga. The strongly positive eHf (3.82 Ga) values obtained by dissolution of GGU110999 zircons are interpreted to be an artifact of calculating eHf values at ages that are too old, and also from the inclusion of radiogenic younger domains in the analysed multi-grain fractions, rather than to a contribution from depleted Eoarchean mantle. Such data – from zircon grains with multiple age and isotopic components – should not be used to define the evolution of crust-mantle reservoirs. A re-interpretation of the existing Pb isotope data, incorporating the new Hf isotope constraints, posits that the protoliths to the Godthabsfjord gneisses were influenced by radiogenic Pb introduced as a fluid mobile component during recycling of a high-μ stagnant basaltic lid at ≥3.8 Ga. The destruction of this mafic protocrust, with attendant fluid release into chondritic mantle, may have been instrumental for the generation of stable Eoarchean tonalitic crust from ca. 3.8 Ga.

Sulfur and strontium isotopic study of epithermal mineralization: A case study from the SE Afar Rift, Djibouti

Epithermal mineralization was recently described in synrift volcanic rocks from the SE Afar Rift in Djibouti. To infer fluid sources in this system, coupled sulfur and strontium isotopic analyses were performed on mineralized samples from hydrothermal veins and stockwork zones. Mineralization includes gypsum and chalcedony, and/or quartz±carbonate containing gold and sulfides. Sulfur isotopic compositions of sulfides and sulfates were determined using Multicollector-Inductively Coupled Plasma Mass Spectrometer (MC ICPMS); Laser Ablation MC ICPMS; and Continuous Flow Elemental Analyzer-Isotope Ratio Mass Spectrometry while strontium isotopes were determined by Thermal Ionisation Mass Spectrometry (TIMS) methods. Sulfur isotopic composition of sulfides (mainly pyrite) varies from −9.2 to +6.6‰. These values are classically reported for hydrothermal sulfides resulting from the mixing of variable S sources such as seawater sulfate and magmatic source. In addition, individual pyrite grains within the same sample show both positive and negative δ34S values suggesting multi-phase mineralization processes. Coupled sulfur and strontium isotopic compositions of seven sulfate samples hosted in (i) gypsum mound, (ii) evaporites, (iii) stockwork veins in sediments or in volcanic rocks were also investigated to characterize the sources of Sr and S. The δ34S values of the sulfate ranged from −0.8 to +14.3‰ while the 87Sr/86Sr ranged from 0.70389 to 0.70639 consistent with variable mixing ratios of Sr and S derived from volcanic sources and seawater-derived sulfate. This study shows that, although precious-metal deposition is related to low-sulfidation epithermal environment, both acidic fluid of magmatic and saline origins contributed to the hydrothermal system in the SE Afar Rift.

Recycled metaigneous crustal sources for S- and I-type Variscan granitoids from the Spanish Central System batholith: Constraints from Hf isotope zircon composition

The Spanish Central System (SCS) is one of the largest granite batholiths in the European Variscan Belt. Zircons from five granitic intrusions from the eastern SCS have been separated and analysed for Hf isotopes by laser ablation MC-ICPMS, in order to evaluate the nature of granitic magma source. Two samples are cordierite-bearing S-type granites, the rest are amphibole-bearing I-type monzogranites. Hf-isotope composition of zircons from these granites defines a narrow range of negative ɛHf values (−1.1 to −5.8), typical of crustal protoliths. The within-sample variation of zircon Hf isotope composition is slightly above the analytical uncertainty (±1.5 ɛ units), suggesting the mixing of different magma batches during pluton assembly. The S- and I-type granites display the same range of initial ɛHf values, that is in agreement with previous data showing that both SCS granite types have similar (Sr, Nd, O, Pb) isotope signatures. These isotopic data suggest that they derived from similar sources, while the variation in peraluminosity could be related to different partial melting conditions.The presence of a Lower Ordovician inherited zircon population (478–462Ma), at least in I-type granites, suggests the involvement of Cambro–Ordovician orthogneisses as components in the source of some SCS granites. Inherited zircon population of similar age has also been identified in the SCS lower crustal granulites. The Hf-isotope composition of the analysed granite zircons is within that of zircons from the above granulites. Moreover, some evidence of a juvenile input at 560–595Ma is recorded by Hf-isotope data in zircons from both the SCS granites and granulite xenoliths. Hf-isotope signature, in conjunction with other chemical, isotopic (Sr, Nd, O, Pb) and geochronological evidences, suggests that SCS granites were probably derived from metaigneous lower crustal sources. The Variscan magmatism in central Spain is dominantly a crustal reworking event.

The 190Pt–186Os decay system applied to dating platinum-group element mineralization of the Bushveld Complex, South Africa

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.

High-temperature Si isotope fractionation between iron metal and silicate

The Si stable isotope fractionation between metal and silicate has been investigated experimentally at 1800, 2000, and 2200 °C. We find that there is a significant silicon stable isotope fractionation at high temperature between metal and silicate in agreement with Shahar et al. (2009). Further we find that this fractionation is insensitive to the structure and composition of the silicate as the fractionation between silicate melt and olivine is insignificant within the error of the analyses. The temperature-dependent silicon isotope fractionation is Δ 30Si silicate–metal = 7.45 ± 0.41 × 10 6/ T 2. We also demonstrate the viability of using laser ablation MC-ICPMS as a tool for measuring silicon isotope ratios in high pressure and temperature experiments.

Resetting, errorchrons and the meaning of canonical CAI initial 26Al/ 27Al values

The difference between the precise MC-ICPMS analyses of bulk calcium–aluminum-rich inclusion (CAI) fragments (e.g., Jacobsen et al., 2008) and supra-canonical values obtained by micro-analytical techniques, e.g., laser ablation MC-ICPMS (Young et al., 2005) and SIMS (Taylor et al., 2005), at face value seems to be problematic and therefore leads many to dismiss claims of the solar system ( 26Al/ 27Al) 0 greater than ~ 5 × 10 −5 as spurious. Here we use mass balance calculations to quantify the importance of open system isotopic exchange during CAI evolution and show that in situ supra-canonical, in-situ canonical, and bulk canonical measurements can all exist for an individual CAI. The calculations describe mechanisms of isotopic exchange that may have occurred early (100's ka) and late (~ 1.5 Ma) in the solar nebula and much later (> 10's Ma) on parent body planetesimals. A range of possible modal mineralogies is modeled in order to populate the compositional space defined by in situ and bulk CAI measurements. In support of these simulated data we describe in situ measurements of 27Al/ 24Mg, 25Mg/ 24Mg, and 26Mg/ 24Mg obtained by LA-MC-ICPMS comprising core-to-rim traverses across three CV3 CAIs. The CAIs exhibit distinctive Mg isotopic zoning profiles and varying abundances of daughter products of the short-lived nuclide 26Al that are consistent with varying amounts of open system isotope exchange.

In situ measurement of hafnium isotopes in rutile by LA–MC-ICPMS: Protocol and applications

This work presents a rigorous assessment of the accuracy and precision with which 176Hf/177Hf can be measured in rutile by laser ablation (LA) MC-ICPMS. For rutile with ≥40ppm Hf, we demonstrate that 176Hf/177Hf can be measured accurately and reproducibly with adequate precision for application to petrological problems. We present an analytical and data reduction protocol tailored to the specific challenges of measuring Hf isotope ratios in this low-Hf mineral. Precision and accuracy are optimised by interpolating between baselines measured every ~10 analyses. For many rutiles, the advantages of determining the Hf mass bias coefficient, βHf, during analysis are negated by the low precision with which it can be measured at low Hf contents. Hf mass bias is therefore monitored by regular analysis of a synthetic rutile doped with ~5000ppm Hf (SR-2) to facilitate an external mass bias correction. Because rutile contains negligible Yb, the Yb mass bias coefficient βYb must be inferred from βYb/βHf measured on zircon in the same session. To account for a systematic instrument bias of 0.5–1εHf units, rutile analyses are normalised to SR-2, for which 176Hf/177Hf has been determined by solution MC-ICPMS. In situ 176Hf/177Hf measurements for two natural rutile samples with 40–50ppm Hf are in excellent agreement with solution MC-ICPMS values. Ti/Hf exerts no influence on the accuracy of 176Hf/177Hf measurements on our LA–MC-ICPMS, as illustrated by indistinguishable 176Hf/177Hf measurements for a series of synthetic rutiles doped with varying amounts of Hf. Rutile and zircon from the Duria mantle peridotite (Central Alps), which formed at different parts of the P–T–time path, record contrasting Hf isotope signatures and emphasise the complementary nature of Hf isotope analysis of rutile and zircon.

Rapid U-series dating of young fossil corals by laser ablation MC-ICPMS

We demonstrate the utility of uranium-series age dating using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA MC-ICPMS) to ‘range find’ a large suite of fossil corals in order to select those for subsequent detailed and high resolution paleoclimatological analyses. The high abundance of unaltered and long-lived (100–200 years) microatolls on Kiritimati Island, Kiribati, offers the exciting prospect of constructing a millennial, if not continuous, coral paleoclimate record for the central Pacific spanning the period from the mid-Holocene to present. However, with a multitude of collected corals and Kiritimati sample sites, we required an efficient, cost-effective and reliable analytical method to optimize selection of quality coral belonging to a prescribed age. Two modern and 41 fossil microatoll corals from Kiritimati were analysed by the range-finding LA MC-ICPMS technique with a subset also analysed by high-precision solution MC-ICPMS and AMS radiocarbon to assess the reliability and accuracy of the laser ablation technique. Coral ages ranged from 6400 to 900 yr BP and laser ablation age precisions, constrained by the low 230Th count rate, ranged from 9 to 45% at the 2SE level. These ages generally agreed with ages measured by solution U-series and AMS 14C. However, conflicting results for ( 234U/ 238U) initial in samples analysed by LA- and solution-MC-ICPMS indicates that the laser ablation method may not be as suitable as previously thought for detecting subtle geochemical alteration. With 40–50 sample analyses per day, the laser ablation method offers a rapid, efficient and cost-effective means to histogram coral ages with multi-centennial accuracy, even at such low 230Th count rates typical for young Holocene samples. In addition to in situ corals, the LA MC-ICPMS technique could be further applied to corals recovered from drill core.

Hafnium isotope characterization of the GJ-1 zircon reference material by solution and laser-ablation MC-ICPMS

Twenty four splits from two different GJ-1 reference material zircon crystals were analyzed for their Lu and Hf concentrations, Lu/Hf and Hf isotopic compositions by solution ID-MC-ICPMS. Lutetium and Hf concentrations are variable between single splits ranging from 9.34 ppm to 14.3 ppm and 4976 ppm to 7719 ppm, respectively, with an average 176Lu/ 177Hf = 0.00025. Concentrations of both elements show a positive correlation, which argue for in-homogeneity of the zircon with respect to HREE and Hf, and however, do not affect the isotope composition. Solution data from two laboratories (Amsterdam and Münster) yield an average present day 176Hf/ 177Hf of 0.282000 ± 0.000005 (2 σ weighted mean), which we recommend as a reference and nomination value for laser-ablation Hf isotope analyses. Cl imaging shows no visible banding or zonation for CL-active elements. Laser-ablation Hf isotopic analyses yield homogeneous 176Hf/ 177Hf identical within analytical uncertainties with the value obtained by the solution work, which make these zircons suitable for Hf isotope determinations using LA-MC-ICPMS analyses. Notably, one laser-ablation analysis shows elevated Yb and Lu contents with a significantly lower 176Hf/ 177Hf, pointing to possible small scale in-homogeneity with elevated HREE contents.

Defining the southern margin of Avalonia in the Pontides: Geochronological data from the Late Proterozoic and Ordovician granitoids from NW Turkey

We provide new isotopic data from the Pontides, which substantiate the presence of a Cadomian basement with latest Proterozoic granitoids but also show the existence of Ordovician intrusives. Before the opening of the Black Sea, the Pontides formed a Palaeozoic orogenic belt at the margin of the East European Platform (Baltica) in an analogous position to the Variscan terranes in the central Europe. The Pontides consist of three terranes: The Strandja terrane has a Variscan basement with Carboniferous and Permian granitoids and an epicontinental Triassic to Jurassic sedimentary cover. The I.stanbul terrane has a Cadomian basement overlain by a sedimentary sequence of Ordovician to Carboniferous age. It underwent contractional deformation during the Late Carboniferous. The Sakarya terrane is characterized by Carboniferous high temperature metamorphism and plutonism. The boundary between the I.stanbul and Sakarya terranes forms a complex tectonic zone several tens of kilometres wide. The new age data come from this boundary zone in the Armutlu peninsula in northwest Turkey. Metagranitoids, which intrude amphibolites in the Armutlu peninsula, give latest Proterozoic and Ordovician U–Pb zircon laser ablation MC–ICP–MS and Pb–Pb evaporation ages. The latest Proterozoic (ca. 570 Ma) granitoids are similar in age to those reported previously from the basement of the I.stanbul terrane; they all form part of the widespread Pan-African granitoid plutonism on the margins of Gondwana. The Mid to Late Ordovician granitoids (460 Ma), on the other hand, probably have formed during the rifting of the I.stanbul terrane away from Gondwana during the opening of the Rheic ocean. In terms of the tectonic position, stratigraphy and geological evolution, the I.stanbul and Sakarya terranes are comparable to the Avalonia and the Armorican terrane assemblage in central Europe, respectively.

Problems in obtaining precise and accurate Sr isotope analysis from geological materials using laser ablation MC-ICPMS

This paper reviews the problems encountered in eleven studies of Sr isotope analysis using laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) in the period 1995–2006. This technique has been shown to have great potential, but the accuracy and precision are limited by: (1) large instrumental mass discrimination, (2) laser-induced isotopic and elemental fractionations and (3) molecular interferences. The most important isobaric interferences are Kr and Rb, whereas Ca dimer/argides and doubly charged rare earth elements (REE) are limited to sample materials which contain substantial amounts of these elements. With modern laser (193 nm) and MC-ICPMS equipment, minerals with >500 ppm Sr content can be analysed with a precision of better than 100 ppm and a spatial resolution (spot size) of approximately 100 μm. The LA MC-ICPMS analysis of 87Sr/86Sr of both carbonate material and plagioclase is successful in all reported studies, although the higher 84Sr/86Sr ratios do suggest in some cases an influence of Ca dimer and/or argides. High Rb/Sr (>0.01) materials have been successfully analysed by carefully measuring the 85Rb/87Rb in standard material and by applying the standard-sample bracketing method for accurate Rb corrections. However, published LA-MC-ICPMS data on clinopyroxene, apatite and sphene records differences when compared with 87Sr/86Sr measured by thermal ionisation mass spectrometry (TIMS) and solution MC-ICPMS. This suggests that further studies are required to ensure that the most optimal correction methods are applied for all isobaric interferences.

Astrophysics of CAI formation as revealed by silicon isotope LA-MC-ICPMS of an igneous CAI

Silicon isotope ratios of a typical CAI from the Leoville carbonaceous chondrite, obtained in situ by laser ablation MC-ICPMS, together with existing 25Mg/ 24Mg data, reveal a detailed picture of the astrophysical setting of CAI melting and subsequent heating. Models for the chemical and isotopic effects of evaporation of the molten CAI are used to produce a univariant relationship between P H 2 and time during melting. The result shows that this CAI was molten for a cumulative time of no more than 70 days and probably less than 15 days depending on temperature. The object could have been molten for an integrated time of just a few hours if isotope ratio zoning was eliminated after melting by high subsolidus temperatures (e.g., > 1300 K) for ∼ 500 yr. In all cases subsolidus heating sufficient to produce diffusion-limited isotope fractionation at the margin of the solidified CAI is required. These stable isotope data point to a two-stage history for this igneous CAI involving melting for a cumulative timescale of hours to months followed by subsolidus heating for years to hundreds of years. The thermobarometric history deduced from combining Si and Mg isotope ratio data implicates thermal processing in the disk, perhaps by passage through shockwaves, following melting. This study underscores the direct link between the meaning of stable isotope ratio zoning, or lack thereof, and the inferred astrophysical setting of melting and subsequent processing of CAIs.

Direct U-series dating of fossil human bone by laser ablation MC-ICPMS

The recent application of laser ablation ICP-MS to U-series dating allows the measurement of U and Th isotopes in tiny samples and at a high spatial resolution. This technique is ideal for Useries dating of valuable human fossils because it can be made virtually non-destructive, and because the spatial distribution of Useries isotopes is required to model uranium uptake in bone using the diffusion-adsorption model. We present results of recent dating studies on human fossils, including direct dating of the early anatomically modern human Omo Kibish 1.

In situ petrographic thin section U–Pb dating of zircon, monazite, and titanite using laser ablation–MC–ICP-MS

A laser ablation–multiple collector–inductively coupled plasma mass spectrometry (LA–MC–ICP-MS) analytical protocol is used to date accessory minerals (zircon, monazite, and titanite) at high spatial resolution (5–40 μm) using standard petrographic thin sections. The MC–ICP-MS instrument is equipped with a modified collector array containing a combination of Faraday buckets and multiple ion counters, which produces accurate and precise geochronological data using small sample volumes (pit depth ≤2 μm at 5 μm and ≤15 μm at 40 μm spot sizes). Standardization and normalization factors for the 206Pb/ 238U and 207Pb/ 235U values are calculated based on well-characterized external mineral standards previously dated by high precision ID-TIMS analysis. During an analytical session, the 2 σ relative standard deviation (i.e., external reproducibility) for the 206Pb/ 238U and 207Pb/ 235U values is ≤3%. The measured 207Pb/ 206Pb value is simultaneously corrected for instrumental mass bias by the aspiration of a Tl solution resulting in a 2 σ relative standard deviation of between ∼0.3 and 1%. The accuracy of the analytical protocol was verified on petrographic thin sections of several samples previously dated by ID-TIMS. The capacity of this new, ‘small volume’ in situ dating technique to provide contextual, relatively rapid and accurate age information is a substantial improvement in reconnaissance-style studies of geological areas with scarce geochronological age information.

In situ sulfur isotope analysis by laser ablation MC-ICPMS

A new method for in situ S isotopic analysis was tested using a laser ablation system together with a multi-collector (MC)-ICPMS. The method was tested for the analysis of pyrite, pyrrhotite, chalcopyrite and pentlandite using a large pyrite crystal as an in-house standard. Repeated measurements of Py, Pn, Po and Cpy provide an average internal precision of less than 0.1‰ (2 σ). The method was also applied to a pyrite-bearing orogenic Au deposit to display the ability of the method to resolve minor variations in δ 34S across growth zoning in pyrite.

LEAD ISOTOPE ANALYSIS OF ARCHAEOLOGICAL METALS BY MULTIPLE‐COLLECTOR INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY*

Lead isotope ratios in archaeological silver and copper were determined by MC–ICPMS using laser ablation and bulk dissolution without lead purification. Laser ablation results on high‐lead metals and bulk solution analyses on all samples agree within error of TIMS data, suggesting that problems from isobaric interferences and/or mass bias variations due to the presence of matrix elements are insignificant. Inaccurate laser ablation analyses on low‐lead copper reflect erroneous mass bias corrections from use of a non‐matrix matched standard. However, in most cases, silver and copper are analysable for lead isotopes by bulk dissolution or laser ablation MC–ICPMS with simplified sample preparation.