Fragments of cassiterite grains were crushed under acetone in a pre-cleaned agate pestle and mortar and transferred to a PFA beaker. The acetone was then evaporated and the powdered material was rinsed first in H2O, then with 4 M HNO3 before being refluxed in aqua regia at 120 ∘C overnight with the aim of dissolving sulfide and Fe oxide inclusions that were exposed within the powdered cassiterite grains (e.g. Gulson and Jones, 1992). Samples were rinsed in 4 M HNO3 and H2O. In the case of samples that underwent an additional step of HF leaching, samples were refluxed at 120 ∘C overnight in ∼ 300–400 µL 29 M HF and trace HNO3 on a hotplate. The leached cassiterite was then rinsed in 4 M HNO3 and H2O. Sub-aliquots of leached cassiterite material that visually approximated the material contained in a ∼ 500 µm sided cube of cassiterite were then transferred into 500 µL PFA microcapsules (Parrish, 1987) that had undergone several steps of overnight cleaning in 9 M HBr within the Parr vessels, in addition to normal HF and HCl cleaning routines. Samples were briefly (∼ 30 min) refluxed (hotplate, 120 ∘C) and rinsed in 1 M HBr, leaving a minimal amount of acid in the capsule, whilst at the same time ensuring no sample material was removed before being spiked with ∼ 0.005 g of EARTHTIME 535 tracer (Condon et al., 2015). Approximately 450 µL of 9 M HBr (Romil ultra-purity acid – UPA – analysed to have < 0.04 pg mL-1 of Pbc) was added before placing in the oven for five nights at 210 ∘C within a high-pressure digestion vessel. Following decomposition, the solution was dried on a hotplate at 120 ∘C and converted to a chloride by the addition of 400 µL of 3 M HCl and placed in the oven for a further 12 h at 180 ∘C using a high-pressure vessel.

One noteworthy observation made over the course of cleaning the PFA capsules in 9M HBr overnight at 230 ∘C was that the apparent Pbc was found to have doubled relative to the previous cleaning stage at 210 ∘C, and the equivalent amount of in-house 208Pb tracer produced very poor beam intensities. This was interpreted as a result of organic compounds released by the reaction of the HBr with the PFA capsule. Despite potential benefits from more rapid decomposition at 230 ∘C, lower temperatures are recommended.

A two-stage column chemistry procedure was determined following investigations to isolate Pb and U from matrix elements using 0.05 mL of pre-cleaned AG 1-x8 200–400 mesh resin (BIO-RAD, CA, USA) and a homogeneous solution of dissolved cassiterite equating to a ∼ 500 µm length cube grain (or 0.875 mg at an assumed density of ∼ 7 g cm-3). In the first stage a modification of the HCl-based anion exchange chromatography typically used for zircons (Krogh, 1973) was used. The sample was loaded onto the columns in 3 M HCl, before further 3 M HCl being added in several stages of 30 µL and then several stages of 130 µL 3 M HCl. These washes contained the majority of eluted Nb, Ti and Th. The Pb was eluted in three stages of 150 µL 6 M HCl, collected and dried down with ∼ 10 µL 0.03 M H3PO4. The U was eluted using three steps of 150 µL of H2O, followed by one step of 150 µL of 1 M HCl. As the U fraction was found to contain almost all eluted Fe and Sn, a further U clean-up stage was required. The U-bearing solution was dried and re-dissolved in 300 µL 8 M HNO3 before being loaded onto columns, again containing 0.05 mL of pre-cleaned AG 1-x8 resin. Columns were washed in three stages of 350 µL 8 M HNO3, followed by two steps of 350 µL 8 M HCl. The U was eluted using two steps of 200 µL 0.2 M HCl, collected and dried down with ∼ 10 µL 0.03 M H3PO4.

For steps after crushing in a pestle and mortar, the Pbc procedural blank was found to be on the order of ∼ 0.5 pg. The procedural blank will be subject to laboratory variations, and the efficacy with which any material abraded during crushing within a pestle and mortar can be removed prior to decomposition within HBr. The effects of HF leaching prior to decomposition are evaluated below; the lowest total sample and laboratory Pbc of ∼ 1.5 pg, including any pestle and mortar contributions, indicates that crushing is a minor contributor to post-HF leaching. Crucially, the component of Pbc contributed by the method is 1 to 4 orders of magnitude less than the amount of Pb* of the materials examined within this study, and the initial Pbc within the cassiterite is likely 1 or 2 orders of magnitude greater than that from the laboratory.

The Pb and U of a given sample were independently loaded on a zone-refined Re filament in 1.5 µL of silica gel matrix (Gerstenberger and Haase, 1997). Isotope ratio measurements were made using a Thermo Triton TIMS at the British Geological Survey following typical methods described by Tapster et al. (2016). Raw U and Pb data were filtered using the Tripoli software programme (Bowring et al., 2011). Data reduction and uncertainty propagation used a modified Excel spreadsheet (Schmitz and Schoene, 2007) with the U decay constants of Jaffey et al. (1971), the 238U / 235U ratio of Hiess et al. (2012) and the decay constants of Cheng et al. (2000) for 230Th. Concordia diagrams and regressions were constructed using the Excel add-in ISOPLOT 4.15 (Ludwig, 2008), and initial disequilibrium corrections utilised the IsoplotR package (Vermeesch, 2018)

The SPG-IV cassiterite is taken from the Pitkäranta ore district, Russian Karelia, and was utilised by Neymark et al. (2018) as a cassiterite reference material for deriving the fractionation factor of analytical sessions. The age of the cassiterite based on geological constraints and zircon dating of associated magmatism was inferred to be between 1546.7 and 1537.9 Ma (Amelin et al., 1997). Neymark et al. (2018) presented an LA-ICP-MS 207Pb* / 206Pb* weighted-mean date of 1542.7±1.5 Ma and a date of 1539.8±3.5 Ma for three partially dissolved, reversely discordant ID-TIMS analyses (N. Rizvanova, written communication, 2017, in Neymark et al., 2018).

The free regression of three tightly clustered ID-TIMS analyses (Fig. 4) of spatially independent aliquots of the SPG-IV cassiterite yields a lower intercept of 1535.9±5.5 Ma (mean squared weighted deviation or MSWD = 2.0). As utilised by Neymark et al. (2018) for dates that cluster close to concordia, anchoring the common Pb component of the T–W isochron to the mean 207Pb / 206Pb value of 11 galena analyses from the ore system (1.0104±0.0062) (Larin et al., 1990) generates a lower intercept date of 1536.6±1.0 (MSWD = 1.02) (Fig. 4). The total Pbc amounts for these analyses range from ∼5 to 15 pg, and 206Pb / 204Pb ranges from ∼ 12 000 to ∼ 23 000.

Comparison of the ID-TIMS T–W lower intercept dates with the LA-ICP-MS Pb-Pb isochron dates used to derive the fractionation factor (Neymark et al., 2018) indicates an absolute offset ∼ 0.4 % older for the LA-ICP-MS Pb–Pb isochron data. The equivalent ID-TIMS 204Pb / 206Pb–207Pb / 206Pb isochron date for this study yields 1540.9±3.6 Ma, which is broadly in concert with that derived by the LA-ICP-MS data (Neymark et al., 2018).

The Jian-1 cassiterite is derived from the Jiangxi W–Sn district, South China (Neymark et al., 2018). Previously, an LA-ICP-MS cassiterite weighted-mean 206Pb / 238U date of 159.5±1.5 Ma (n=31; MSWD = 0.4) (Zhang et al., 2017) was reported for the deposit, superseded by a weighted-mean 206Pb / 238U date of 156.55±0.36 Ma (n=40; MSWD = 1.4) (Neymark et al., 2018). In both studies, data points overlapping concordia within uncertainty are interpreted as concordant or free of Pbc.

Four individual aliquots of the Jian-1 cassiterite were analysed by ID-TIMS. All data are discordant on the T–W plot (Fig. 5) and are interpreted as containing a component of initial common Pb. The total Pbc amounts for these analyses range from ∼ 1.5 to 14 pg, and their 206Pb / 204Pb ranges from ∼ 140 to ∼ 5300. The three data points with the greatest 206Pb / 204Pb form a lower intercept of 155.05±0.20 Ma (MSWD = 2.1), in agreement with the lower intercept formed by all four data points of 154.969±0.082 Ma (MSWD = 1.4).

Comparison with LA-ICP-MS data (Fig. 5) indicates that the coarser-scale sampling of ID-TIMS analyses identified domains with lower Pb* / Pbc relative to the previously published LA-ICP-MS data (Neymark et al., 2018) and therefore provides additional spread and an isochron on the T–W plot that seemingly was not permitted by the LA-ICP-MS data. Despite this, the isochron overlaps within uncertainty with the majority of previously published LA-ICP-MS data for Jian-1 (Neymark et al., 2018), indicating that the accuracy of the measurements by microbeam was relatively robust at the 1 %–3 % precision of single data points. However, the ID-TIMS lower intercept date of 154.969±0.082 Ma is ∼ 1 % younger than the LA-ICP-MS weighted-mean 206Pb / 238U date of 156.55±0.36 Ma (n=40; MSWD = 1.4) (Neymark et al., 2018). This offset would likely be reduced further if a fractionation factor was renormalised to the ∼ 0.4 % younger ID-TIMS intercept date of SPG-IV than was assumed by Neymark et al. (2018). However, this would still not fully account for the offset on the weighted mean at the ±0.23 % 2σ precision stated for Jian-1 (Neymark et al., 2018).

The Cligga Head W–Sn greisen deposit is located at the central–northern periphery of the post-Variscan, early Permian, SW England Cornubian Batholith, UK. Key features of the local geology (Fig. 6; Hall, 1971; Jackson et al., 1977; Moore and Jackson, 1977) are a porphyritic granite stock that intruded and locally thermally metamorphosed Devonian pelitic and psammitic meta-sediments at the contacts at about 1 kbar (Hall, 1971). The westerly extent of the granite has been eroded by the coastline. The main granite stock is cross-cut by a complex of sheeted quartz–muscovite (< 1 % topaz and fluorite) greisen-bordered veins that predominantly contain quartz, in addition to tourmaline, chlorite, fluorite, cassiterite–stannite–arsenopyrite, wolframite and minor Cu sulfides. The cassiterite-bearing greisen-bordered veins extend across the contact into the meta-sediments.

Rhyolite porphyry dykes intruded ∼ 150 m south of the stock. The absence of W–Sn mineralisation and greisenisation of the dykes, in addition to reported xenoliths of the granite greisen within the dykes (Reid et al., 1906), indicates that they occurred after the W–Sn mineralisation, although they contain disseminated chalcopyrite and supergene derivatives (Moore and Jackson, 1977). Previous geochronology of muscovite from the greisened zones indicated a date of ∼ 280 Ma for the magmatic–hydrothermal system (Fig. 6). Two LA-ICP-MS U–Pb ages were presented (Moscati and Neymark, 2019) for cassiterite from Cligga Head: a TW lower intercept age of 287.9±2.6 Ma (n=59; MSWD = 2.2; initial 207Pb / 206Pb value of 0.790±0.034) or an anchored isochron with a lower intercept of 289.2±2.5 Ma (MSWD = 2.4). The weighted-average Pbc-corrected 206Pb / 238U date is 288.4±3.0 Ma (n=56, MSWD = 1.3). A second sample yielded lower intercept dates of 287.2±4.8 Ma (n=40; MSWD = 2.1; initial 207Pb / 206Pb value of 0.839±0.013) and a Pbc-corrected weighted-average 206Pb / 238U date of 285.2±4.2 Ma (n=38, MSWD = 1.3).

Samples of the granite porphyry stock and the rhyolite porphyry dyke (Fig. 6) were analysed by zircon CA-ID-TIMS U–Pb in order to provide constraints on the timing of cassiterite mineralisation. Zircon was analysed with the ET535 tracer, and methods followed Tapster et al. (2016) using the same decay constants and 238U / 235U as the cassiterite analyses. Data are corrected for initial 230Th disequilibrium using a Th / U (melt) of 1.43 derived from a mean of granitic magmatism in the Cornubian Batholith (Simons et al., 2016). All data are contained within the Supplement.

The 10 analyses of zircon tip fragments from the host granite stock yield a range in 206Pb / 238U CA-ID-TIMS dates of ∼ 290 to ∼ 285 Ma. The youngest four dates form a statistically acceptable weighted mean of 285.163±0.097 Ma (MSWD = 1.01; n=4), which is interpreted as the emplacement age (Fig. 6).

The eight analyses of zircon tip fragments from the porphyritic dyke, which intrudes to the SE of the stock, yield 206Pb / 238U dates of ∼ 290 to ∼ 282 Ma. The youngest date, whilst overlapping with concordia within its relatively low single data-point uncertainty, demonstrates high discordancy (∼ 11 %) and does not form a single population weighted mean with any other dates; it is therefore attributed to residual Pb loss and rejected from the age interpretation. The youngest dates that form a statistically acceptable weighted-mean date yield an age of 283.21±0.032 Ma (MSWD = 0.01; n=3) taken as the timing of emplacement (Fig. 6).

The dates for magmatic events at Cligga Head provide the minimum and maximum constraints on the absolute timing of the magmatic–hydrothermal greisen system and the true “age” of cassiterite that is independent of the cassiterite isotopic dates. They define a possible window for the “true” age of cassiterite of 1.95±0.33 Myr, or about 0.7 % of the ∼ 285 Ma absolute age.

Cassiterite was sampled from two locations at Cligga Head (Fig. 6). From the first locality within the granite stock a single crystal (C1) was used within the decomposition experiments. In the first experiment a single part of the crystal was powdered into a single “parental” aliquot and divided into eight approximately equal parts. We evaluated the effect of not leaching four of these cassiterite fractions in 29 M HF relative to leaching, as described above and used in all of the other experiments (Fig. 7). We then evaluated intra-crystal variation in U–Pb systematics by analysing an additional three, spatially independent, parts of the same C1 crystal. To evaluate the inter-sample variation, fragments from four independent crystals were analysed from a second vein (C2) containing multiple cassiterite crystals that was sampled from within the meta-sediments. Cassiterite ID-TIMS methods followed those described above (Sect. 5). All cassiterite U–Pb data are contained within the Supplement.

The range of ID-TIMS experiments described above permits us to evaluate the U–Pb systematics of cassiterite at high precision and within the context of independent geochronological constraints derived from zircon CA-ID-TIMS dating within a well-constrained geological model. There are three key features of the dataset.

The lower intercept date formed by a single population of HF-leached cassiterite sub-aliquots of the same parental material (C1.1) indicates a binary mixing line between an initial Pb and a radiogenic Pb source. The lower intercept yields a precise date that is consistent with the indirect age constraints on cassiterite U / Pb*. The effects of any initial isotopic disequilibrium have to be considered when deriving a sample age and uncertainty (see below).

Open-system behaviour with respect to Pb (Pb loss) would shift data points to the right (higher U / Pb) of the C1.1 (or similar) isochron. Estimated uranium concentrations (see the cassiterite ID-TIMS U–Pb data table in the Supplement) are similar between cassiterites from the two veins and as such there is no apparent reason why uranium mobility would be manifest to a greater extent between different veins. We consider the most likely explanation to be that the over-dispersion results from variation in the initial Pbc of the cassiterite to more “radiogenic” initial 207Pb / 206Pb values than ∼ 0.88 as defined by the C1.1 isochron, potentially as low as ∼ 0.56. The majority of documented 207Pb / 206Pb values from feldspars in granites and fluid inclusions in the Cornubian Batholith are ∼ 0.8–0.9 (Hampton and Taylor, 1983; Wayne et al., 1996), with only one magmatic feldspar analysis yielding a 207Pb / 206Pb of ∼ 0.55 (Hampton and Taylor, 1983). It is unlikely that these low values reflect the primary input signature from the magmatic volatiles. Initial Pb values with 207Pb / 206Pb this low are uncommon but are present in a global array of vein carbonates (Roberts et al., 2020). When the nature of cassiterite-bearing greisen systems, such as Cligga Head, is taken into account, the ability to generate atypical Pbc isotopic compositions is less surprising. Greisenisation at the margins of the cassiterite-bearing veins represents extensive, destructive, fluid–rock interaction and elemental exchange during alteration of host rocks by high temperatures of ∼ 450–300 ∘C and highly acidic, fluorine-rich (HF) fluids (Burt, 1981; Codeço et al., 2017; Lecumberri-Sanchez et al., 2017). As at Cligga Head, W–Sn greisen-bordered vein magmatic–hydrothermal deposits are commonly hosted by, or are proximal to, thick continentally derived sedimentary packages (Lecumberri-Sanchez et al., 2017) that likely represent the source of Sn enrichment prior to magma genesis (Romer and Kroner, 2016). These sediments will contain older U- and Pb*-rich continental mineral detritus (e.g. zircon, monazite) that can be extensively leached by the hot, F-rich, acidic ore-forming fluid, generating significant spatial and temporal variations in the Pbc isotopic composition incorporated into the cassiterite lattice over the lifetime of single crystal precipitation and the magmatic hydrothermal system as a whole. It is perhaps significant that the cassiterite that appears to contain the lowest initial Pbc component originates from a vein bordered by greisened meta-sediment rather than being hosted within the granitic stock.

The high-precision U–Pb lower intercept cassiterite date (C1.1; Figs. 7 and 8) comes with the caveat that it is not corrected for potential initial isotopic disequilibrium effects within the 238U decay chain. In zircon geochronology a deficit in 206Pb due to 230Th disequilibrium (Schärer, 1984) can be reasonably corrected using a reasonable assumption of the mineral–melt partition coefficient or an estimate of melt Th / U, as well as an estimate of mineral Th / U, typically based on assumed 208Pb / 232Th–206Pb / 238U concordance for zircon ID-TIMS data. Cassiterite typically has extremely low 232Th relative to U (Th / U = <10-4; Neymark et al., 2018). This geochemical feature may be derived from the preferential partitioning of U into ore fluids and/or the preferential partitioning of U upon the precipitation of cassiterite, leading to a deviation from secular equilibrium. Both processes will require a significant correction for a deficiency in 206Pb due to low 230Th / 238U activity in the initial cassiterite. Alternatively, this geochemical feature could simply be explained by processes that do not preclude secular equilibrium with regards to 230Th upon cassiterite crystallisation. These include low 232Th / 238U magmatic ore fluid sources or the contribution of localised U-rich components to fluids that are in secular equilibrium with regards to 230Th and 238U during alteration around veins. In these cases the 232Th–208Pb system would not provide a realistic proxy for initial 230Th disequilibrium in cassiterite.

It must also be considered that the hydrothermal fluids, and therefore the initial cassiterite, may not have been in secular equilibrium for 238U–234U. A relative excess of 234U or a 234U / 238U activity ratio > 1 at the time of cassiterite precipitation will result in an excess of 206Pb / 238U, an older measured date relative to the true age. These effects therefore counteract effects of 206Pb deficiency due to 230Th disequilibrium. Mixing with meteoric fluids appears to be an important process in cassiterite formation (Fekete et al., 2016), and as highlighted by the compilation of Roberts et al. (2020) the 234U / 238U activity of crustal fluids can be in excess of 1 (shallow groundwater median value of 2.25; hydrothermal fluid median of 1.41).

Estimates of the effects of initial disequilibrium (Fig. 9) indicate that corrections for the exclusion (or absence) of initial 230Th, potentially inferred by the characteristic low 232Th / U in cassiterite, produces a maximum possible date (∼ 285.23 Ma) that is still within uncertainty of the maximum age permitted by the granite host. Effects of excess initial 234U indicate that an activity ratio of 1.5 produces dates ∼ 200 kyr younger (284.92 Ma) than that of secular equilibrium, yet it also lies within the analytical uncertainties of the host granite. In the more extreme scenario of a 234U / 238U activity ratio of 2.5, this is reduced to 284.55 Ma and would be significantly younger.

Not accounting for the uncertainty that arises from the potential effects of disequilibrium, the lower intercept of the four-point isochron C1.1 yields a precision of ± 250 kyr or 0.088 % (2σ, analytical uncertainties only) and agreement with the possible window for hydrothermal activity defined by the CA-ID-TIMS zircon dates of the intrusions (∼ 0.7 %). Moreover, it also demonstrates a clear temporal association with the granitic stock that hosts the Cligga Head deposit (the cassiterite date is 0.021±0.268 Myr younger than the zircon date for the granitic stock) (Figs. 9, 10). High-precision constraints on the durations of relatively simple ore-forming magmatic–hydrothermal systems spatially associated with single intrusions indicate timeframes of tens to hundreds of thousands of years (Li et al., 2017; Tapster et al., 2016). It is therefore feasible that the emplacement of the host granite stock, cooling to the 400–350 ∘C temperature interval of cassiterite precipitation (Smith et al., 1996), and the transfer of ore-forming volatiles from depth all occurred within the timeframes defined by the < 0.1 % uncertainty of the zircon and cassiterite data presented here. However, considering the potential uncertainty due to initial U–Th disequilibrium effects over geologically reasonable assumptions, the cassiterite uncertainty could be expanded by ca. -200 kyr and would be asymmetric (Fig. 9). At present the poor understanding of isotopic disequilibrium effects due to elemental partitioning into ore fluids and cassiterite represents a limitation for the accurate interpretation of absolute ages of cassiterite beyond the <∼ 500 kyr timescale.

In this study we have generated ID-TIMS U–Pb data on cassiterite that has undergone full decomposition in a single stage. These include samples that have previously been used for LA-ICP-MS studies using an approach to U / Pb normalisation using an ad hoc cassiterite reference material with an inferred U / Pb age. The comparison of the ID-TIMS and LA-ICP-MS U–Pb data provides a direct means to assess the accuracy of the microbeam dates (Moscati and Neymark, 2019; Neymark et al., 2018), which show agreement at the ca. 1 % level and are therefore comparable to the quoted levels of accuracy and similar to the accuracy for U–Pb (zircon) studies (Horstwood et al., 2016). Additional characterisation over a wider range of cassiterite materials by the ID-TIMS methods described here will establish a focal point for the refinement of cassiterite measurement uncertainties by LA-ICP-MS.

The ID-TIMS U–Pb data from the Cligga Head cassiterite strongly suggest that the initial Pb isotopic composition varies and that a simple binary mixture between a radiogenic and initial Pb may not be expected within and between crystals. A high proportion of LA-ICP-MS U–Pb cassiterite datasets record over-dispersion (e.g. ∼2/3 of samples, typically analysed from single crystals; reported by Neymark et al., 2018), suggesting that these are also not strict binary mixtures. We postulate that some of the over-dispersion can be attributed to variable initial Pb isotopic compositions within cassiterite that are inherited from the hydrothermal systems. This complexity in the U–Pb systematics represents a potential limitation on the accuracy of the resulting age interpretation of cassiterite. This limitation is interdependent with the single data-point precision of the U–Pb analyses and the ability to resolve different populations of the Pbc mixing endmember that will bias the regression of a dataset away from the accurate lower intercept.

We explore how the U–Pb data can be leveraged with variable Pbc by correcting the Cligga Head dataset using a widely employed strategy for minerals that accommodates Pb into their lattice upon formation, which involves correcting the component of initial Pbc using a model Pbc value at ∼ 285 Ma (Stacey and Kramers, 1975). Figure 10 demonstrates that the resulting Pbc-corrected 206Pb / 238U dates range from being in good agreement with the host granite age to being “too old” by ca. 4 Myr (∼ 1.5 %). Without the maximum constraint from the age of the granite, these “model” U–Pb cassiterite dates alone could be misinterpreted as a protracted episode of mineralisation. Comparable shifts, both in magnitude and direction, can be observed (Fig. 10) within one of the two samples from the same deposit that were analysed by LA-ICP-MS U–Pb (Moscati and Neymark, 2019).

Figure 11 illustrates how the ability to resolve this effect and the interplay with lower precision on single data points can impact the accuracy of the regression and age interpretation. The isochron we construct is based upon all Cligga Head cassiterite ID-TIMS U–Pb analyses but with uncertainties expanded to ±1 % (2σ) (Fig. 11). It shows no dispersion (MSWD = 1.2, n=12); however, the resulting age is biased too old (lower intercept at 286.80±0.95 Ma) for the known maximum possible age that is defined by the granite.

The approach of taking weighted means of common Pb-corrected 206Pb / 238U data is strongly advised against, as it diminishes the evaluation of dispersion within the data and ignores the uncertainty of 207Pb / 206Pb measurements (see Horstwood et al., 2015, for community discussion of this issue in LA-ICP-MS U–Pb). However, to demonstrate that this approach does not resolve the issues of variable initial Pbc composition, the weighted-mean date of Pbc-corrected 206Pb / 238U dates for the same dataset with uncertainties expanded to 1 % is 286.23±0.83 Ma (MSWD = 1.3; n=12; therefore, despite its seemingly precise date, it also lies outside the uncertainty of the maximum possible age defined by the granite.

Any bias resulting from Pbc variation would be in addition to greater scatter on individual data points resulting from lower-precision measurements. This reinforces the limitations on the accuracy of the interpreted ages when derived from high-n but lower-precision datasets due to heterogeneity being masked by the single data-point uncertainty. In addition to the limits of systematic uncertainties defined by validation RMs, this limitation should be taken into account within future microbeam work with cassiterite, alongside any mineral phase that contains initial Pbc.

We note that the reference values of any cassiterite material used as an RM should not be corrected for Pbc or any initial isotopic disequilibrium effects; rather, they should reflect the isotopic values of the material ablated (Horstwood et al., 2016), as with the data presented in Figs. 4, 5, 7 and 8. HF leaching of cassiterite targets inclusions and is unlikely to affect lattice-bound Pb and modify the isotope ratios as chemical abrasion does for zircon RMs.