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. Cassiterite (SnO2) is the most common ore phase of Sn. Typicallycontaining 1–100 µg g−1 of uranium and relatively low concentrations of commonPb, cassiterite has been increasingly targeted for U–Pb geochronology,principally via microbeam methods, to understand the timing and durationsof granite-related magmatic–hydrothermal systems throughout geological time.However, due to the extreme resistance of cassiterite to most forms of aciddigestion, there has been no published method permitting the complete,closed-system decomposition of cassiterite under conditions in which the basicnecessities of measurement by isotope dilution can be met, leading to apaucity of reference and validation materials. To address this a new lowblank (&lt; 1 pg Pb) method for the complete acid decomposition ofcassiterite utilising HBr in the presence of a mixed U–Pb tracer, U and Pbpurification, and thermal ionisation massspectrometry (TIMS) analyses has been developed. Decomposition rates havebeen experimentally evaluated under a range of conditions. A careful balanceof time and temperature is required due to competing effects (e.g. HBroxidation), yet the decomposition of 500 µm diameter fragments ofcassiterite is readily achievable over periods comparable to zircondecomposition. Its acid-resistant nature can be turned into an advantage byleaching common Pb-bearing phases (e.g. sulfides, silicates) withoutdisturbing the U–Pb systematics of the cassiterite lattice. The archetypalSn–W greisen deposit of Cligga Head, SW England, is used to define accuracyrelative to chemical abrasion–isotope dilution–thermal ionisation massspectrometry (CA-ID-TIMS) zircon U–Pb ages and demonstrates the potential ofthis new method for resolving high-resolution timescales (&lt;0.1 %) of magmatic–hydrothermal systems. However, data also indicate that theisotopic composition of initial common Pb varies significantly, both betweencrystals and within a single crystal. This is attributed to significantfluid–rock interactions and the highly F-rich acidic nature of thehydrothermal system. At microbeam precision levels, this issue is largelyunresolvable and can result in significant inaccuracy in interpreted ages.The ID-TIMS U–Pb method described herein can, for the first time, be used toproperly characterise suitable reference materials for microbeamcassiterite U–Pb analyses, thus improving the accuracy of the U–Pbcassiterite chronometer as a whole.

geochronology

High-precision ID-TIMS cassiterite U–Pb systematics using a low-contamination hydrothermal decomposition: implications for LA-ICP-MS and ore deposit geochronology