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

Cherniak 0230019独居石是一个非常多才多艺和持久的岩石事件记录。它抵抗高温扩散平衡，但在低温下在热液流体或显著的偏应力场的存在下是反应性的。独居石响应主要相反应，并通过Y、Th、U、Ca、Pb和Si等元素的成分分带标记参与反应的特定主相或副相储层。它还通过（Y+HREE）分布记录了温度信息，由于独居石可能具有高度的钍成因，它可以保存它所经历的部分或全部岩石事件的年龄。然而，天然独居石的成分相当复杂，导致传统分析和电子探针（EMP）化学年龄测定的固有困难。此外，EMP化学年龄测定的校准通常涉及用测得的同位素年龄对独居石进行二次标准化。这是有问题的，因为化学年龄可能不同于一个或多个同位素年龄，并且校准应该设置为浓度，而不是年龄，因为元素浓度是在分析期间测量的。主要工作如下：1）合成了Pb掺杂的稀土磷酸盐，并通过EMP和XRD对样品进行了表征。我们证明，少量（wt. %水平）的铅在独居石中是稳定的，并且将已知量的铅与Th-独居石和U-独居石组分一起沿着，将允许校准到已知组成的标准而不是推断的年龄; 2）将合成Pb掺杂的一元稀土磷酸盐以辅助EMP年龄分析方案，通过用单一稀土元素隔离铅，以便检查作为检测器气体加上检测器和准直器设置的函数的Pb干扰; 3）用已知同位素组成的Pb掺杂（REE、Th、U、Ca）磷酸盐合成同位素独居石标准品。标准将免费分发给感兴趣的分析人员，以促进相互协作和分享方法和结果。

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.