An Experimental Attempt to Determine History of 3He, 4He, and Isotopes of Ne and Ar in the Mantle

该项目将根据对钻石和相关地幔样品的研究，确定地幔中氦、Ne和Ar同位素的历史。在进行这项研究时，一些人认为某些钻石含有原始氦同位素成分的记录。钻石似乎不太可能携带地球起源时残留的氦成分记录；最有可能的是，具有高3He/4He比率的钻石在其滞留在冲积层期间通过宇宙射线相互作用，或通过产生放射性的中子与锂的相互作用，如果钻石中的浓度超过1ppm，就获得了3He。在冲积钻石中发现了宇宙成因的3He组分，观察到钻石中具有非常奇怪的释放模式的极大的氦扩散系数，以及其他观察迫切需要扩大氦的研究，包括Ne(和Ar)同位素。Ne的同位素组成可能确实与旧钻石的形成有关。此外，还计划使用21Ne和10Be作为示踪剂来确定冲积钻石的宇宙射线暴露历史。该实验室是为3He测量而建立的；低空白系统是在之前的NSF拨款下建立的。由于同位素实验室很快将在美国国家科学基金会的资助下获得一台三收集器N_2-Ne质谱仪，因此现在可以以所需的精度进行Ne同位素测量。

This project will determine the history of He, Ne and Ar isotopes in the mantle, based on studies of diamonds and associated mantle samples. When this study was undertaken, it was believed by some that certain diamonds bore records of pristine helium isotopic composition. It is how seems unlikely that diamonds could carry vestigial records of He composition at genesis of the Earth; most likely diamonds having high 3He/4He ratios acquired their3He during their residence in the alluvium by cosmic ray interactions, or by interactions of radiogenic neutrons with lithium if present in the diamond at concentrations exceeding 1 ppm. Discovery of the cosmogenic 3He component in alluvial diamonds, observations of very large helium diffusivity in diamonds, with a very strange release pattern, and other observations urgently warrant extension of helium studies, including Ne (and Ar) isotopes. The isotopic composition of Ne may indeed have a bearing on the formation of old diamonds. Also it is planned to determine cosmic ray exposure histories of alluvial diamonds using 21Ne and 10Be as tracers. The lab is set up for 3He measurements; low blank systems were built under the previous NSF grant. Neon isotopic measurements can now be carried out with the required precision since the Isotope Laboratory will soon be acquiring a triple collector N2-Ne mass spectrometer under a grant from NSF.