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Geochronology Advances in geochronological science
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Preprints
https://doi.org/10.5194/gchron-2019-22
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/gchron-2019-22
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  16 Jan 2020

16 Jan 2020

Review status
A revised version of this preprint was accepted for the journal GChron and is expected to appear here in due course.

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

Simon Richard Tapster1 and Joshua William George Bright1,2 Simon Richard Tapster and Joshua William George Bright
  • 1Geochronology and Tracers Facility, British Geological Survey, Keyworth, Nottinghamshire, NG12 5GG, UK
  • 2Department of Chemistry, University of Surrey, Guildford, GU2 7XH, UK

Abstract. Cassiterite (SnO2) 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.

Simon Richard Tapster and Joshua William George Bright

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Simon Richard Tapster and Joshua William George Bright

Simon Richard Tapster and Joshua William George Bright

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Latest update: 19 Oct 2020
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Short summary
Cassiterite is the primary ore mineral of tin and is associated with lots of other elements we need for green technology. It is deposited from fluids that are released from magmas. Because it is extremely acid resistant there has been difficulty in dissolving the mineral for analysis, we use a novel method to dissolve and extract radiogenic isotopes of the uranium to lead decay scheme from Cassiterite and analyse its age thus improving the ability to work out the formation processes.
Cassiterite is the primary ore mineral of tin and is associated with lots of other elements we...
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