Abstract. Cassiterite (SnO2) is the most common ore phase of Sn. Typically
containing 1–100 µg g−1 of uranium and relatively low concentrations of common
Pb, cassiterite has been increasingly targeted for U–Pb geochronology,
principally via microbeam methods, to understand the timing and durations
of granite-related magmatic–hydrothermal systems throughout geological time.
However, due to the extreme resistance of cassiterite to most forms of acid
digestion, there has been no published method permitting the complete,
closed-system decomposition of cassiterite under conditions in which the basic
necessities of measurement by isotope dilution can be met, leading to a
paucity of reference and validation materials. To address this a new low
blank (< 1 pg Pb) method for the complete acid decomposition of
cassiterite utilising HBr in the presence of a mixed U–Pb tracer, U and Pb
purification, and thermal ionisation mass
spectrometry (TIMS) analyses has been developed. Decomposition rates have
been experimentally evaluated under a range of conditions. A careful balance
of time and temperature is required due to competing effects (e.g. HBr
oxidation), yet the decomposition of 500 µm diameter fragments of
cassiterite is readily achievable over periods comparable to zircon
decomposition. Its acid-resistant nature can be turned into an advantage by
leaching common Pb-bearing phases (e.g. sulfides, silicates) without
disturbing the U–Pb systematics of the cassiterite lattice. The archetypal
Sn–W greisen deposit of Cligga Head, SW England, is used to define accuracy
relative to chemical abrasion–isotope dilution–thermal ionisation mass
spectrometry (CA-ID-TIMS) zircon U–Pb ages and demonstrates the potential of
this new method for resolving high-resolution timescales (<0.1 %) of magmatic–hydrothermal systems. However, data also indicate that the
isotopic composition of initial common Pb varies significantly, both between
crystals and within a single crystal. This is attributed to significant
fluid–rock interactions and the highly F-rich acidic nature of the
hydrothermal system. At microbeam precision levels, this issue is largely
unresolvable and can result in significant inaccuracy in interpreted ages.
The ID-TIMS U–Pb method described herein can, for the first time, be used to
properly characterise suitable reference materials for microbeam
cassiterite U–Pb analyses, thus improving the accuracy of the U–Pb
cassiterite chronometer as a whole.
High-precision ID-TIMS cassiterite U–Pb systematics using a low-contamination hydrothermal decomposition: implications for LA-ICP-MS and ore deposit geochronology
