The LA‐ICP‐MS U‐(Th‐)Pb geochronology international community has defined new standards for the determination of U‐(Th‐)Pb ages. A new workflow defines the appropriate propagation of uncertainties for these data, identifying random and systematic components. Only data with uncertainties relating to random error should be used in weighted mean calculations of population ages; uncertainty components for systematic errors are propagated after this stage, preventing their erroneous reduction. Following this improved uncertainty propagation protocol, data can be compared at different uncertainty levels to better resolve age differences. New reference values for commonly used zircon, monazite and titanite reference materials are defined (based on ID‐TIMS) after removing corrections for common lead and the effects of excess ²³⁰Th. These values more accurately reflect the material sampled during the determination of calibration factors by LA‐ICP‐MS analysis. Recommendations are made to graphically represent data only with uncertainty ellipses at 2s and to submit or cite validation data with sample data when submitting data for publication. New data‐reporting standards are defined to help improve the peer‐review process. With these improvements, LA‐ICP‐MS U‐(Th‐)Pb data can be considered more robust, accurate, better documented and quantified, directly contributing to their improved scientific interpretation.

We have analysed sulphides, silicates, and chromites of the Merensky Reef for platinum-group elements (PGEs), Re and Au using laser ablation-inductively coupled plasma mass spectrometry and synthetic pyrrhotite standards annealed with known quantities of noble metals. Os, Ir and Ru reside in solid solution in pyrrhotite and pentlandite, Rh and part of the Reef’s Pd in pentlandite, whereas Pt, Au, Re and some Pd form discrete phases. Olivine and chromite, often suspected to carry Os, Ir and Ru, are PGE free. All phases analysed contain noble metals as discrete micro-inclusions with diameters typically <100 nm. Inclusions in sulphides commonly have the element combinations Os–Ir–Pt and Pt–Pd–Au. Inclusions in olivine and chromite are dominated by Pt ± Au–Pd. Few inclusion spectra can be related to discrete noble metal phases, and few inclusions have formed by sub-solidus exsolution. Rather, some PGE inclusions, notably those in olivine and chromite, are early-magmatic nuggets trapped when their host phases crystallized. We suggest that the silicate melt layer that preceded the Merensky Reef was PGE oversaturated at early cumulus times. Experiments combined with available sulphide–silicate partition coefficients suggest that a silicate melt in equilibrium with a sulphide melt containing the PGE spectrum of the Merensky ore would indeed be oversaturated with respect to the least soluble noble metals. Sulphide melt apparently played little role in enriching the noble metals in the Merensky Reef; rather, its role was to immobilize a pre-existing in situ stratiform PGE anomaly in the liquid-stratified magma chamber.

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.