地幔样品稀有气体和活性气体联测

Obtaining abundances and isotopic ratios of both noble gases and active volatiles (e.g., H2O, CO2, and CH4) from the same mantle sample (or the same gas-release duration) is scientifically important. However, it is difficult to obtain them simultaneously in technical respect. Usually, they are measured on a noble gas mass spectrometer and a stable isotopic mass spectrometer respectively for different sample aliquots, through which quantitative relationship between noble gases and active gases cannot be achieved. This proposal is to build a cavity ring-down spectroscopy (CRDS) facility onto a noble gas measurement line and design corresponding analytical methods in order to measure both noble gas isotopes and active volatile isotopes on the same sample aliquot. CRDS is a highly sensitive optical spectroscopic technique that enables measurement of absolute optical extinction by samples that scatter and absorb light, and in turn to determine mole fractions and isotopic ratios of gaseous samples. Because of its portability, simple process for measuring and high precision comparable to isotope ratio mass spectrometer (IRMS), the CRDS technique is widely used for measuring abundances and isotope ratios of trace gases both in the air and fluid inclusions of geological samples. This proposal firstly combines a CRDS and a noble gas mass spectrometer together and provides a new analytical technique to measure isotopes of both noble gases and active volatiles.

同时获得地幔样品的稀有气体和活性气体（如H2O、CO2、CH4等）的含量、同位素信息具有重要的科学意义，但从分析技术的角度很难实现。以往的技术手段多采用两份样品在稀有气体质谱和稳定同位素质谱上分别测量，但无法获得稀有气体与活性气体的准确比值。本项目拟在已有的稀有气体同位素测试系统上添加一套光腔衰荡光谱技术（Cavity ring-down spectroscopy, CRDS）设备，并建立相应的实验方法，实现二者的在线联测。CRDS是灵敏度超高的直接吸收光谱技术，通过测量激光经过气体样品吸收后的光强衰荡时间来获得样品的分压进而测算气体的浓度和同位素组成信息。CRDS因便携、测量过程简单且具有和同位素质谱技术相当的精度而广泛应用于空气中痕量气体以及地质样品流体包裹体中气体的浓度、同位素测试。本项目首次实现稀有气体质谱和CRDS技术联用，为同时测量稀有气体和活性气体提供新思路和新技术。

以CRDS（Cavity Ring-Down Spectroscopy，光腔衰荡光谱）为代表的光谱技术在大气痕量气体含量和同位素测试方面的重要性和优势逐步凸显，但这项技术对于分析地质样品流体包裹体中微量气体组分方面尚不成熟，尤其是在近真空环境下的应用几乎是空白。本项目拟设计一套全新的CRDS测试系统，用于少量固体地幔岩石样品中微量气体组分（如CO2、CH4等）的含量和同位素测试，同时能够与已有的稀有气体同位素分析测试系统对接，实现二者同时线上测试。项目总体进展比原计划滞后，目前已基本完成CRDS的设计与调试工作，具体包括完成了衰荡光腔的三次技术迭代，解决了传统光谱检测仪器在极高真空下的应用难题，实现了真空下CO2、CH4及δ13C-CH4的检测以及最终样机的成功研发。