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

Abstract

<p><strong>Abstract.</strong> Cassiterite (SnO<sub>2</sub>) is the most common ore phase of Sn. Typically containing 1–100 µg/g U and relatively low concentrations of common Pb the mineral has been increasingly targeted for U-Pb geochronology, principally via micro-beam 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, to date, there has been no published method permitting the complete, closed system decomposition of cassiterite under conditions where 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 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 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 CA-ID-TIMS zircon U-Pb ages and demonstrate the potential of this new method, for resolving high resolution timescales (< 0.1 %) of magmatic-hydrothermal systems. However, analyses 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 micro-beam precision levels, this issue is largely unresolvable and can result in significant inaccuracy in interpreted ages. However, this method can, for the first time, be used to properly characterise suitable reference materials for micro-beam cassiterite U-Pb analyses, thus improving the accuracy of the U-Pb cassiterite chronometer as a whole.</p>

From OXALID to GlobaLID: Introducing a modern and FAIR lead isotope database with an interactive application

“…Nanosecond LA‐ICP‐MS data of Pb isotopes (e.g., Neymark et al, 2021) has a low resolution compared to solution MC‐ICP‐MS, but femtosecond LA‐ICP‐MS promises better data. A novel KBr technique for complete dissolution of cassiterite (Tapster & Bright, 2020) will soon allow the production of Pb isotope data with a much higher resolution. These spatially resolved cassiterite analyses will be added to the database when common lead data are reported.…”

Discrete late Jurassic Sn mineralizing events in the Xianghualing Ore District, South China: Constraints from cassiterite and garnet U-Pb geochronology

“…Each cassiterite analysis began with a 20 s blank gas measurement followed by 40 s of analysis with the laser switched on. The downhole fractionation, instrument drift and mass bias correction factors for the Pb/U ratios of the cassiterite were calculated from two analyses of the primary AY-4 standard (Yuan et al, 2011), and analyses of secondary standards, i.e., the Cligga Head (Tapster and Bright, 2020) and Emmaville cassiterites (Prichard, 2013). These analyses were conducted at the beginning of each session and after 10 analyses of the cassiterite samples.…”

Sn-rich tourmaline from the Land’s End granite, SW England

“…20 Ma period, from ca. 294 to 274 Ma, based on U-Pb dating of xenotime, monazite and zircon (Chen et al 1993;Chesley et al 1993;Tapster and Bright 2020). The peraluminous LEG comprises mostly porphyritic, coarse-grained biotite granite (G3 in the classification of Simons et al 2016), with dikes and larger sheets of tourmaline granite (G4) intruding the G3 granite.…”