Paul J. Sylvester

A simple method for the precise determination of ≥ 40 trace elements in geological samples by ICPMS using enriched isotope internal standardisation

The combination of enriched isotopes and conventional elemental internal standards permits the precise determination of > 40 trace elements by ICPMS in a broad spectrum of geological matrixes. Enriched isotopes expand the suite of available reference isotopes spaced through the mass spectrum, so that the complex mass-dependent variations in sensitivity encountered during ICPMS analysis can be monitored and deconvolved. The method we have developed is straightforward, entailing simple sample preparation, instrument calibration, and data reduction procedures, as well as providing extended element coverage, improved precision, and both time and cost benefits compared to alternative analytical strategies. Analytical precision near or better than 1% RSD (relative standard deviation) is achieved for most elements with mass > 80 amu and between 1% and 4% (RSD) for elements with mass 80 amu and < 10 ng g−1 to 1 μg g−1 for elements with mass < 80 amu). The subtle geochemical differences which can be resolved using this method are demonstrated by analyses of Nb, Ta, Zr, and Hf in magmas from ocean islands and subduction zones. These data reveal significant departures from chondritic Zr/Hf and Nb/Ta values, and systematic trends which are consistent with greater incompatibility of Zr relative to Hf and also of Nb relative to Ta during melting of the upper mantle. The occurrence of significantly subchondritic Zr/Hf and Nb/Ta ratios in Nb-poor subduction zone magmas, supports the notion that the depletion of high-field strength elements in subduction magmas is due to their removal from the mantle wedge by prior melting events.

U–Pb dating of detrital zircons for sediment provenance studies—a comparison of laser ablation ICPMS and SIMS techniques

Abstract New developments in U–Pb dating of zircons by laser ablation (LA) ICPMS are described and, for the first time, a direct comparison of detrital zircons dated by LA ICPMS and SIMS methods is presented. True real-time mass bias correction is made by aspirating a Tl/U tracer at the same time as laser ablation. The method is similar to that described in Horn et al. (2000) , except that enriched 233U rather than 235U is used in the tracer solution. Correction for laser-induced Pb/U elemental fractionation is based on a mathematical treatment of time-resolved data that is independent of laser ablation characteristics and does not require external standardisation. Internal corrections for mass bias and elemental fractionation eliminate the effects of variable sample matrix on isotopic ratios and improve the accuracy of U–Pb dating by laser ablation ICPMS. With the proper error propagation, the precision of U–Pb age determinations is only slightly worse than SIMS-based ion probe dating. However, LA ICPMS is capable of much more rapid analysis of the large number of zircons required for sediment provenance studies. There is excellent agreement between concordant laser ablation ICPMS and SIMS analyses of detrital zircons extracted from lower Silurian metasandstone from the Ulven Group (Skarfjell Formation) in the west Norwegian Caledonian nappes. Both LA ICPMS and SIMS U–Pb zircon ages indicate that sedimentary detritus of the Ulven Group was supplied from a terrain containing zircons of Archean, Proterozoic and early Ordovician age.

Present Trends and the Future of Zircon in Geochronology: Laser Ablation ICPMS

In situ U-Th-Pb geochronology was born some two decades ago with the introduction and development of high-resolution secondary ion mass spectrometry (SIMS or SHRIMP [Sensitive High Mass Resolution Ion MicroProbe]; Compston et al. 1984, Williams 1998, Compston 1999, Davis et al.; this volume, Ireland and Williams, this volume). This technique clearly demonstrated the existence of age heterogeneities within the single crystals of zircon and other accessory phases and therefore the need for high-spatial resolution (tens to hundreds of cubic micrometers) geochronological data. In situ dating by ion probe is capable of achieving an analytical precision that is only an order of magnitude worse than the conventional isotope dilution-thermal ionization mass spectrometry (ID-TIMS) dating technique. It has the advantage, however, of more readily identifying concordant portions of grains, does not require chemical treatment of samples prior to the analysis, is essentially nondestructive, and can achieve greater sample throughputs. A major obstacle to the wider use of ion probe dating has always been the high cost of instrumentation and hence relative scarcity of suitably equipped geologic laboratories. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) emerged in 1985 and rapidly became an important analytical tool for trace element determinations in geological samples (Jackson et al. 1992). It was soon realized that the large variations in radiogenic Pb and Pb/ U isotopic ratios found in nature could be resolved by ICPMS techniques and, when coupled to a laser, ICPMS could be used as a dating tool similar to the ion probe. The pioneering work of Feng et al. (1993), Fryer et al. (1993), Hirata and Nesbitt (1995) and Jackson et al. (1996) illustrated the potential usefulness of laser sampling for in situ dating by ICPMS particularly well. However, these studies and others that followed also revealed the major …

Trace element analysis of scheelite by excimer laser ablation-inductively coupled plasma-mass spectrometry (ELA-ICP-MS) using a synthetic silicate glass standard

Abstract Concentrations of W and trace elements in scheelites (CaWO 4 ) associated with Archean lode-gold deposits in Western Australia were determiined by ArF (193 run) excimer laser ablation-ICP-MS with external calibration against a silicate glass standard reference material, NIST 610. The excimer laser beam drilled well-defined pits in both scheelite and silicate glass. With internal standardization against Ca alone, W measurements for the scheelites fall within 5% of the concentrations expected from electron microprobe measurements. A matrix effect between scheelite and silicate glass is apparent in the behaviour of W during; laser ablation: W is progressively fractionated from Ca in the glass but not in the scheelite. Proper data reduction, therefore, requires use of the early maximum count rates for W and Ca rather than the mean count rates during ablation. W isotopic ratios measured in the scheelites and an in-house generated silicate glass, ANU 252, are both within ∼2.5% of the accepted values for the natural ratios and are reproducible to ∼1%, if data for the first ∼11 s of ablation are excluded. Using the same data reduction techniques employed for W, concentrations of Sr, Y, Mo, REE and Pb, present at ppm levels in the scheelites, were measured with a precision of 4% or less. Measurements on Th and U, present at the 5–10 ppb level, and P, Mn, Nb and Ta are less precise (∼5–40%) and concentrations of Rb, Zr, Ba, Sn, Hf, TI and Bi are, for the most part, below detection limits. For Sr, Sm and Nd, replicate ELA-ICP-MS measurements made on 80-μm wide spots in scheelite largely encompass the concentrations determined by ID-TIMS on bulk samples. REE patterns determined by ELA-ICP-MS for the scheelites vary smoothly as a function of atomic number. Most of the patterns are hump-shaped but others are rather flat except for positive Eu anomalies. This suggests that the hydrothermal fluids that formed the scheelites did not have a common composition and source.

Solubilities of Pt and Rh in a haplobasaltic silicate melt at 1300°C

The solubilities of Platinum (Pt) and Rhodium (Rh) in a haplobasaltic melt (anorthite-diopside eutectic composition) have been determined experimentally by using the mechanically assisted equilibration technique at 1300°C, as a function of oxygen fugacity (10 212 , fO2 # 1 bar), imposed by CO-CO2, N2-H2-H2O, Ar-O2, and air gas mixtures. Samples were analyzed by sample nebulization (SN) inductively coupled plasma-mass spectrometry and, using some of these samples as standards, also by laser ablation (LA) inductively coupled plasma-mass spectrometry. The latter is a true microanalytical technique that allows small-scale sample heterogeneity to be detected. At each oxygen fugacity step, a time-series of samples was taken, to demonstrate that the solubilities converge on a constant value. In addition, solubilities were measured after both increasing and decreasing the imposed fO2. The results fall into three groups, according to oxygen fugacity. At high fO 2s, (fO2

Chemical and phase composition of particles produced by laser ablation of silicate glass and zircon—implications for elemental fractionation during ICP-MS analysis

10 22 bars), samples are homogenous at all sampling scales. Both Pt and Rh predominantly dissolve in the silicate melt as 21 species, with some evidence for Pt 41 and Rh 31 at the highest fO2s studied (air and pure O2). From these data, we obtained the following expressions for the solubilities of Pt and Rh: Pt/ppb 5 2100(fO2) 1 10980(fO2) 1/2 Rh/ppb 5 68630(fO2) 3/4 1 31460(fO2) 1/2 At fO2 , 10 25 bars, the true solubilities of Pt and Rh appear to be obscured by Pt-Rh micronuggets, which remain suspended in the melt despite stirring on time scales of 10 3 h, resulting in samples that are heterogenous on the laser sampling scale. Samples at intermediate fO2 (10 22 to 10 25 bars) are affected by the micronugget problem on the sampling scale of the conventional SN-inductively coupled plasma mass spectrometry, but these can be filtered out by analyzing on the laser sampling scale. Copyright

The solubility of rhenium in silicate melts: Implications for the geochemical properties of rhenium at high temperatures

The chemical and phase compositions of particles produced by laser ablation (266 nm Nd:YAG) of silicate NIST glasses and zircon were studied by SIMS and HR-TEM techniques. The data suggest that the formation of phases of different mineralogy and/or chemical composition from the original sample at the ablation site can result in elemental fractionation (non-stoichiometric sampling) in material delivered to the ICP-MS for quantitative analysis. Evidence of the element fractionation is preserved in chemically zoned ejecta deposited around the ablation pit. The chemical composition and mineralogy of particles varies with particle size so that the efficiency of transport of particles also plays a role in elemental fractionation. During the first 250 pulses in a typical ablation experiment using a 266 nm laser, particle sizes are mainly <2.5 μm; thereafter they decrease to <0.3 μm. Pb and U are fractionated significantly during the ablation of both silicate glass and zircon. During the ablation of glass, both micron-sized, melt-derived, spherical particles, and nm-sized, condensate-derived particle clusters, are produced; the very smallest particles (<0.04 μm) have anomalously high Pb/U ratios. For zircon, both larger (0.2–0.5 μm) spherical particles and agglomerates of smaller (∼0.005 μm) particles produced by ablation are mixtures of amorphous and crystalline materials, probably zircon, baddeleyite (ZrO2) and SiO2. Evidence for thermal decomposition of zircon to baddeleyite and SiO2 is preserved in the wall of the ablation pit, and may lead to the commonly observed increase in Pb/U recorded during laser ablation ICP-MS analysis. It follows that a matrix-matched external calibration is essential for achieving highly precise and accurate laser (266 nm wavelength) ablation ICP-MS analysis of Pb and U in silicate samples.

Melt solidification and late-stage evaporation in the evolution of a FUN inclusion from the Vigarano C3V chondrite

The solubility of rhenium (Re) in a haplobasaltic melt (anorthite-diopside eutectic composition) has been experimentally determined using the mechanically assisted equilibration technique at 1400°C as a function of oxygen fugacity (10−12 < fO2 ≤ 10−7 bar), imposed by CO-CO2 gas mixtures. Samples were analysed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). This is a true microanalytical technique, which allows small-scale sample heterogeneity to be detected, while providing a limit of detection of 2 ppb Re. Time-resolved LA-ICP-MS spectra revealed the presence of suboptically sized micronuggets of Re in all samples, which, because they are present at the 0.5 to 10 ppm level, dominate the true solubilities of Re (<1 ppm at the conditions of the experiment) in bulk analyses of the samples. Nevertheless, the micronuggets could be filtered out from the time-resolved spectra to reveal accurate values of the true Re solubility. A number of time series of samples were taken at constant fO2 to demonstrate that the solubilities converge to a constant value. In addition, solubilities were measured after increasing and decreasing the imposed fO2. The results show that Re dissolves in the silicate melt as ReO2 (Re4+) and ReO3 (Re6+) species, with the latter predominating at typical terrestrial upper-mantle oxygen fugacities. The total solubility of Re is described by the following expression (fO2 in bars): [Re/ppb] = 9.7(±1.9) × 109 (fO2) + 4.2 (±0.3) × 1014 (fO2)1.5Assuming an activity coefficient for Re in Fe-rich metal of 1, this gives a value of DRemet/sil of 5 × 1010 at log fO2 = IW-2, appropriate for metal-silicate partitioning in an homogenously accreting Earth. Thus, Re is indeed very highly siderophile, and the mantle’s abundance cannot be explained by homogenous accretion.

Detrital zircon from the jack hills and mount narryer, Western Australia : Evidence for diverse >4.0 Ga source rocks

Vigarano 1623-5 is a forsterite-bearing refractory inclusion in which oxygen, magnesium, and silicon show large degrees of mass-dependent isotopic fractionation. The core of 1623-5 consists of forsteritic olivine, fassaite, melilite (Ak89), and spinel, all of which show isotopic mass fractionation, averaging ~30.6%./amu for magnesium and ~10.0%./amu for silicon; it is enclosed in a mantle consisting of aluminous melilite (Ak16–60), spinel, perovskite, and hibonite. Hibonite and spinel in the mantle are enriched in heavy isotopes of magnesium by 8–15%./amu relative to the core. 1623-5 originally formed by solidification of a magnesium-rich melt. The endemic isotopic mass fractionation may have been caused by evaporation of that melt or may reflect an earlier evaporation event affecting the reservoir from which the interior of 1623-5 formed. Later flash remelting of the outer part of the core, and volatilization of SiO2 and MgO from the melt so produced, caused formation of a mantle that is in isotopic and petrologic disequilibrium with the core.

Refractory inclusions from the Leoville, Efremovka, and Vigarano C3V chondrites - Major element differences between Types A and B, and extraordinary refractory siderophile element compositions

Geochronological, geochemical, and morphological analysis of detrital zircon in the Jack Hills and Mount Narryer metasedimentary belts, Western Australia, indicates the grains were derived from diverse rocks, including >4000 Ma sources that predate the oldest known terrestrial rock. In three metaconglomerate layers in the western part of the Jack Hills, 4200–3800 Ma zircon makes up 14% of the population, 3800–3600 Ma grains form only 2%, and 3550–3250 Ma zircon is dominant with a significant peak at 3380 Ma (U‐Pb ages and trace element concentrations were obtained by laser‐ablation microprobe inductively coupled plasma mass spectrometry). These grains are interpreted as being derived from similar rock types because they are indistinguishable in U concentration (50–200 ppm), internal zoning (both oscillatory and sector zoning within the same grain), and morphology (subequant fragments of grains). We conclude that a previously proposed evolved granitic source is unlikely because the zircon differs significantly in U concentration, internal zoning, and morphology from zircon in typical Archean granitic rocks, such as the 3730–3300 Ma granitic gneisses that surround the Jack Hills belt. More likely sources were intermediate composition plutonic rocks that were distal or perhaps destroyed or removed from the region during Neoarchean tectonism. In contrast, detrital zircon in quartzites and metaconglomerates at Mount Narryer appears to have been derived from granite based on elongate prismatic morphology, fine oscillatory zoning, relatively high U concentration (100–600 ppm), and xenotime and monazite inclusions. Ages are also different: 4200–3800 Ma zircon makes up only 3% of the Mount Narryer population (most grains are 4200–4100 Ma), 3800–3600 Ma zircon forms 31%, and peaks are at 3650, 3600, and 3500 Ma. Trace element concentrations are broadly similar, except Mount Narryer zircon generally has higher U, smaller Ce and Eu anomalies, and lower Nb/Ta. Mount Narryer zircon is interpreted as having local granitic sources because the <3800 Ma grains closely match the age and nature of zircon in the surrounding granitic gneisses, which may include a minor, currently undiscovered 4200–4100 Ma component. The diversity of ancient zircon suggests that Earth’s crust was heterogeneous by 4200 Ma, having already differentiated into granitic and intermediate components, as is the case in modern continents.