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

Cherniak0230019独居石是一种非常多功能和耐用的摇滚事件记录器。它抵抗高温扩散平衡，但在低温下在热液或显著偏离应力场存在的情况下具有反应性。独居石对主相反应作出反应，并通过Y、Th、U、Ca、Pb和Si等元素的组成分区标记参与反应的特定主相或副相储集层。它还通过(Y+HREE)分布记录温度信息，由于独居石可能具有高度的气原性，它可以保存其经历的一些或所有全岩事件的年龄。然而，天然独居石的成分非常复杂，这给常规分析和电子探针(EMP)化学年龄测定带来了固有的困难。此外，EMP化学年龄测定的校准通常涉及对独居石进行二次标准化，以测量同位素年龄。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，校准应设置为浓度，而不是年龄，因为元素浓度是在分析期间测量的。1)合成了掺铅稀土磷酸盐，并以EMP和X射线衍射仪为化学年龄标准进行了表征。我们证明了微量的铅在独居石中是稳定的，加入已知数量的铅，以及Th和U独居石成分，将允许校准到已知组成的标准，而不是推断年龄；2)将合成掺铅的一元稀土磷酸盐，以帮助EMP年龄分析协议，方法是用单一的稀土元素分离铅，以检查铅的干扰随着检测器气体加检测器和准直器设置的变化而变化；3)通过掺杂(REE，Th，U，Ca)磷酸盐和已知同位素组成的铅，将合成同位素独居石标准。标准将免费分发给感兴趣的分析员，促进方法和结果的相互协作和共享。

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.