Simon E. Jackson

Data acquisition and calculation of U–Pb isotopic analyses using laser ablation (single collector) inductively coupled plasma mass spectrometry

Zircon U/Pb geochronology using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) is rapidly being adopted in the earth sciences. Challenges in the accurate determination of ages and uncertainties, however, remain, and include finding the optimal ways to calculate and calibrate U/Pb and Pb/Pb measurements of unknowns. Of particular importance is the determination of U/Pb ratios that are complicated by laser-induced fractionation of U relative to Pb. In order to address these challenges and provide best practice recommendations, this study systematically explores and fully documents three algorithms for calculating U/Pb and Pb/Pb ratios (ratio of mean signals, mean of replicate ratios, and intercept of ratio vs. ablation time relationship), four different placements of calibration materials within a series of unknowns, and three different least-squares approaches for interpolation of drift correction factors (mean, linear, and quadratic). The lowest mean relative standard errors (2σ) for single uncalibrated laser analyses were obtained for the ratio of means and mean of ratios algorithms, and were ∼1% for 206Pb/238U and 207Pb/206Pb. The experimental uncertainty for the ratio of the mean and the mean of ratios algorithms was nearly three times higher for 206Pb/238U and 1.5 times higher for 207Pb/206Pb compared to that predicted from counting statistics. The experimental uncertainty for the intercept method, however, for 206Pb/238U was nearly six times higher than that predicted due to counting statistics. These results suggest that additional and unaccounted for sources of error are present. The best reproducibility (2σ) for both 206Pb/238U (∼2 to 3% 2RSD) and 207Pb/206Pb (∼1.5% 2RSD) was also obtained using the ratio of means and mean of ratios algorithms. The most precise fits to calibration materials placed in various positions within a ‘run’ of twenty data acquisitions were obtained using a quadratic fit to the data and when the calibration materials were closely spaced, as would be expected. There was no significant gain in accuracy, however, using a quadratic fit compared to other methods, suggesting that the more complex fits are simply following noise in the data. This clearly demonstrates that drift corrections based on mean or linear least squares are most appropriate for optimizing precision and accuracy for U/Pb dating of zircon using LA-ICP-MS. Close spacing of calibration materials between single or small groups of unknowns (6 or less) was shown to produce the most accurate results, demonstrating that long, continuous sequences of unknowns introduce significant deviation from the true age of the samples. The mean precision (2σ) of individual age analyses of the 1065 Ma 91500 zircon obtained in this study range from 16–38 Ma (1.5–3.5%, 2σ RSE), improving as more acquisitions of calibration materials are utilized. The accuracy was 1% or better.

Effect of oxygen in sample carrier gas on laser-induced elemental fractionation in U–Th–Pb zircon dating by laser ablation ICP-MS

Thermal breakdown of zircon during laser ablation sampling for ICP-MS analysis results in decoupling of Si from Zr, and Pb from U + Th, following the reaction ZrSiO4(U, Th, Pb) = ZrO2(U, Th) + SiO2(Pb). The rate of the laser-induced elemental fractionation can be linked to the concentration of oxygen in the ambient He sample carrier gas. Deposition of ZrO2 and SiO2 on the walls of the ablation craters is enhanced by the presence of oxygen during ablation, and the composition of the deposit is dominated by ZrO2. This study shows that even a small amount of oxygen (e.g., 500 ppm) added to the sample carrier gas can shift the U–Pb ages of zircon by as much as 9%, which is well beyond the typical analytical uncertainty of LA-ICP-MS zircon dating. Addition of up to 2000 ppm oxygen to the sample carrier also increases the instrument sensitivity by as much as 3 times for light elements and ca. 1.5 times for heavy elements. The increase in sensitivity is accompanied by an increase in the U and Th oxide formation, suggesting that the signal enhancement is not related to the concomitant M+ and MO+ ion formation, but rather to the improved atomization and ionization capabilities of the mixed gas ICP. The study shows that maintaining a constant level of O2 throughout the ablation cell over an analytical session at the time of analysis is a prerequisite for further improvements in the repeatability of U–Pb LA ICP-MS dating of zircon.

Methods for the determination of stable Te isotopes of minerals in the system Au–Ag–Te by MC-ICP-MS

The measurement of stable isotopes in ore- and ore-related minerals can provide insight into the geochemistry and formation of metal-bearing ore systems. Currently, there are few high-precision studies of the natural variability of stable tellurium isotopes, most of which are focused on meteorites and sulfides and are related to cosmogenesis; there are no modern studies on the variability of tellurium isotopes within native tellurium and tellurides from ore-forming systems. Tellurium is an element of interest due to its common association with gold in geologic systems, as well as its rarity in the Earths crust and increasing industrial demand for applications such as photovoltaics. This study presents a method by which tellurium can be sampled from Au–Ag tellurides and native tellurium, isolated by ion exchange chromatography, and analyzed for isotopic composition by multi collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS). Using a micromill, a sufficient mass of telluride or native Te sample can be extracted from coexisting ore and gangue minerals from ∼100 μm wide by ∼50 μm deep drilled holes. Acid digestion of micromilled samples and subsequent ion exchange chromatography isolated Te from matrix metals. The chromatography procedure has Te yields of 96% and produces no net fractionation of Te isotopes. MC-ICP-MS analyses were performed using two techniques, both of which employed the doping of samples using Cd to correct for instrumental mass bias. The first method was the introduction of 100 ppb Te-bearing solutions (with 100 ppb Cd) using a desolvating nebulizer (Aridus II). The second method involved solution nebulization of 2 ppm solutions of Te (with 1 ppm Cd) into the plasma of the MC-ICP-MS. Although the wet and dry methods produce statistically identical delta values, precision is increased using the wet method. The average uncertainty (two standard deviations of the mean) using the Aridus II is ±0.20‰ for 130/125Te (dry method), whereas that for the wet method is ±0.08‰. Repeated analyses of the Te standard over a period of ∼15 months by solution nebulization yielded an external precision of ±0.10‰ for 130/125Te. Natural, hypogene tellurides (calaverite, hessite, krennerite, and sylvanite) and native tellurium samples (n = 32) have a range of 1.64‰ in the isotope composition 130/125Te, demonstrating resolvable, disparate isotope ratios between samples from different areas and between samples from the same locality (e.g., Cripple Creek, Colorado). Fractionation of Te isotopes was caused by mass-dependent geological processes.

Investigating Age Resolution in Laser Ablation Geochronology: Workshop on Data Handling in LA‐ICP‐MSU‐Th‐Pb Geochronology; Vancouver, British Columbia, Canada, 12–13 July 2008

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) uranium-thorium-lead (U-Th-Pb) dating is an increasingly popular approach for determining the age of mineral grains and the timing of geological events. The spatial resolution offered by this technique allows detailed investigations of complex igneous and metamorphic processes, and the speed of data capture allows vast amounts of information to be gathered rapidly. Laser ablation U-Th-Pb dating is therefore becoming an increasingly influential technique to the geochronology community, providing cost-effective and ready access to age data for laboratories and end users worldwide. However, complications in acquiring, processing, and interpreting data can lead to inaccurate age information entering the literature. With the numbers of practitioners expanding rapidly, the need to standardize approaches and resolve difficulties (particularly involving the subjectivity in processing laser ablation U-Th-Pb data) is becoming important.

Effects of H2O- and O2-containing He carrier gases on the 206Pb/238U system bias and down-hole fractionation in LA-ICPMS of zircon

This paper deals with the effects the addition of H2O or O2 to He aerosol carrier gas has on 206Pb/238U ratios during laser ablation inductively-coupled plasma mass spectrometry (LA-ICPMS) of natural zircon and synthetic silicate glass. The following parameters were studied: (i) system bias defined as constant but LA-ICPMS system-dependent shifts in 206Pb/238U ratios, (ii) down-hole fractionation given by the slope of the 206Pb/238U ratio vs. LA time, and (iii) the sum of (i) and (ii) referred to as the overall elemental fractionation. To isolate and study changes of 206Pb/238U ratios arising independently from either LA sampling or ICPMS analysis of the generated aerosols, a diffusion-based gas exchange device was used. The supply of He + O2 caused relative changes in mean 206Pb/238U values and temporal gradients due to the LA process of less than 40%. By contrast, addition of H2O to the He carrier gas changed 206Pb/238U ratios by up to 150% through processes occurring during LA as well as ICPMS analysis. Here, the opposing temporal gradients of 206Pb/238U ratios canceled out at relative humidity levels close to the saturation vapor pressure under ambient lab conditions, i.e., 20 °C room temperature. As a consequence, changes in 206Pb/238U ratios caused by down-hole fractionation could be suppressed. However, the admixture of H2O also affected the system bias at the same time, resulting in increased offsets in 206Pb/238U ratios such that the accuracy of LA-ICPMS analyses worsened by up to 6% when applied to the age determination of zircon. Neither worsening nor significant improvements in the accuracy of 206Pb/238U ages were observed in the case of LA-ICPMS using He + O2 carrier gas mixtures.

Oxidation Conditions of Granitic Magmas Associated With Porphyry Copper Deposits in the Central Asian Orogenic Belt

Porphyry copper deposits are associated with oxidized felsic magmas (Sillitoe, 2010). Such oxidized magmas are considered to supply metals and S to ore deposits (Hedenquist and Lowenstern, 1994; Hattori and Keith, 2001; Cooke et al., 2005). Ce is 4+ in oxidized conditions and readily incorporated into zircon, which produces positive Ce anomalies. Previous studies show that the Ce/Ce ratios in zircon can be used as a proxy for oxygen fugacity of magmas (e.g., Trail et al., 2012). The ratios successfully discriminate fertile igneous rocks from barren rocks (Ballard et al. 2002; Liang et al., 2006; Qiu et al., 2013; Han et al., 2013). The Central Asian Orogenic Belt (CAOB) hosts porphyry-type deposits with significant range in size including largeand intermediatesize deposits (Figure 1). The CAOB, therefore, presents an opportunity to evaluate the relationship between the oxidation condition and metal-fertility of granitic magmas.