Single Zircon Evaporation Thermal Ionisation Mass Spectrometry: Method and Procedures†

Abstract

Zircon evaporation thermal ionisation mass spectrometry (TIMS) is used in geochronology to determine absolute ²⁰⁷Pb^*/²⁰⁶Pb^* ages and Th/U ratios of single zircon crystals. The process involves the breakdown of zircon (ZrSiO₄) to porous baddeleyite (ZrO₂) along a reaction front which progresses into the interior of the crystal. Evaporation of high quality zircons thus allows us to distinguish between crystal rim (overgrowth) and core, providing precise information about the time of magmatic crystal growth, partial dissolution, and/or metamorphic overgrowth. Derived Th/U ratios complement age data interpretation and provide valuable petrogenetic implications. A double Re-filament ion source is used. The zircon is encased in the evaporation filament and heated step-wise to 1200–1300 °C to strip off unsupported common and radiogenic Pb components. After cleaning, evaporation proceeds in temperature steps of ∼20 °C. The evaporate (SiO₂, Pb, REEs, and U from the zircon and Re from the evaporation filament) of each step is deposited for 45 min on the cold ionisation filament and subsequently analysed. Lead isotopic composition is determined using a dynamic secondary electron multiplier ion counter or static Faraday cup data acquisition schemes. Lead ratios are corrected for fractionation using correction factors derived from standard measurements of a 1 ng NBS SRM 982 sample. The precision on ²⁰⁷Pb/²⁰⁶Pb ratios is <1%. Only high temperature steps (>1300 °C) with ²⁰⁶Pb/²⁰⁴Pb > 5000 are used for age calculations. The ages reported (single temperature step, multi-temperature step means) are weighted means calculated from at least 20 measured ²⁰⁷Pb^*/²⁰⁶Pb^* ratios with 2 standard errors of the mean. Precision of ages is strongly dependent on age range and varies between 0.1 and 10%.