Synthesis of Lead-Doped Monazite Standards for Electron Microprobe Chemical Dating

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Cherniak0230019Monazite is a tremendously versatile and durable recorder of rock events. It resists high-temperature diffusive equilibration, yet is reactive at low temperatures in the presence of hydrothermal fluids or significant deviatoric stress fields. Monazite responds to major phase reactions, and tags specific major or accessory phase reservoirs participating in the reaction through compositional zoning of elements such as Y, Th, U, Ca, Pb, and Si. It also records temperature information via (Y+HREE) distributions, and as monazite may be highly thorogenic, it can preserve the ages of some or all whole-rock events it experiences. However, natural monazites are compositionally quite complex, leading to inherent difficulty in both conventional analysis and electron microprobe (EMP) chemical age determination. Moreover, calibration for EMP chemical age determinations usually involves a secondary standardization to monazite with a measured isotopic age. This is problematic because chemical age may be distinct from one or more isotopic ages, and calibration should be set to a concentration, not an age, as elemental concentrations are measured during analysis. We propose the following work: 1) Pb-doped REE phosphate will be synthesized and fully characterized via EMP and XRD as a chemical age standard. We demonstrate that minor amounts (to wt. % level) of lead are stable in monazite, and incorporation of known amounts of lead, along with Th- and U-monazite components, will allow calibration to a standard of known composition rather than inferred age; 2) Pb-doped unary REE phosphates will be synthesized to aid EMP age analysis protocols, by isolating lead with a single rare earth element so as to examine Pb interferences as a function of detector gas plus detector and collimator settings; 3) Isotopic monazite standards will be synthesized by doping (REE, Th, U, Ca) phosphate with Pb of known isotopic composition. Standards will be freely distributed to interested analysts, fostering mutual collaboration and sharing of methods and results.