Speed dating with ion specificity: in situ Rb-Sr ages using novel mass-spectrometry

一个理想的地质年代学技术应该能够从一个简单的分析中得出一个日期。虽然锆石的原位测年已经接近这一目标，但它仅限于这种副相，通常需要从大量样品中分离出来。更有吸引力的是能够从岩相学切片中确定样品的年代，最好是主要矿物相。对于使用87 Rb-86 Sr系统的一些高Rb-Sr相存在这种可能性，但这目前需要耗时的微钻、分离化学和随后的分析。87 Rb-87 Sr对和其他通常用于地质年代测定的β衰变系统的问题在于，母元素不可避免地会与子元素形成等压干扰，因此具有高母元素/子元素比的相位，有可能给出最精确的年代，具有最多的干扰同位素比。即使是最大的几何质谱仪也无法解析这些元素推论。一种新的方法是使用碰撞池来实现化学特异性，而不是质量分辨率。碰撞池中的离子-气体反应可以将同量异位素干扰降低许多数量级。这样的计划存在的Rb-Sr系统，我们希望探索这种方法与我们独特的碰撞池，等离子体多收集器质谱仪（Proteus）的潜力。学生将致力于改进反应方案以从Sr中去除Rb（与我们的CASE合作伙伴Thermo Fisher Scientific合作），并利用这些结果开发Proteus协议以测量激光烧蚀材料的同位素比。由此产生的方法将被应用于超深钻石中包裹体的年代测定，其年龄是长期争论的一个来源。激光烧蚀是获取这些有价值的深部地幔样品的有效手段，将激光烧蚀与同位素分析相结合的前景是一个有价值的目标。

An ideal geochronological technique should be able to produce a date from a single, easy analysis. Although in situ dating of zircons has come close to this goal, it is limited to this accessory phase, which typically needs to be separated from larger volumes of sample. Even more appealing would be the ability to date samples from a petrographic section, preferably on major mineral phases. This possibility exists for some high Rb-Sr phases using the 87Rb- 86Sr system, but this currently requires time consuming micro-drilling, separation chemistry and subsequent analysis. The problem with the 87Rb- 87Sr pair, and other beta decay systems commonly used in geological dating, is that the parent inevitably forms an isobaric interference with the daughter, such that phases with high parent daughter ratios, which have the potential to give the most precise dates, have the most interfered isotope ratios. These elemental inferences cannot be resolved by even the largest geometry mass spectrometers. A novel approach is to use a collision cell to achieve chemical specificity rather than mass resolution. Ion-gas reactions in the collision cell can reduce isobaric interferences by many orders of magnitude. Such a scheme exists for the Rb-Sr system and we wish to explore the potential of this approach with our unique collision cell, plasma multi-collector mass-spectrometer (Proteus). The student will work in both refining the reaction schemes to remove Rb from Sr (in collaboration with our CASE partner, Thermo Fisher Scientific), and harnessing these results to develop protocols on Proteus to measure isotope ratios from laser ablated material. The resulting methodology will be applied to the dating of inclusions in ultra-deep diamonds, the ages of which are a source of long standing debate. Laser ablation is an efficient means to sample these valuable samples of the deep mantle and the prospect of coupling this with isotopic analysis is a valuable goal.