Li isotope fractionation in magmatic systems: Constraints from in situ analyses on magmatic minerals by femtosecond-laser ablation-MC-ICP-MS

本项目旨在研究火山岩硅酸盐矿物（即橄榄石和单斜辉石）中稳定锂（Li）同位素的分馏，以获得岩浆过程的时间信息。由于以前的研究表明（i）岩浆中稳定同位素分馏主要是由扩散过程引起的动力学效应驱动的，以及（ii）Li是矿物（和熔体）中扩散非常快的微量元素，因此可以预期矿物尺度上的Li同位素变化，即同位素分带。因此，在原位方法（如激光烧蚀等离子体质谱法）必须使用，允许进行高精度的同位素分析与高空间resolution.The本项目的第一个目标包括建立一个分析例行单斜辉石和橄榄石的锂同位素原位测量。由于这些硅酸盐矿物中的锂浓度非常低（通常为几ppm），因此此类分析具有挑战性。然而，初步测试表明，汉诺威现有的飞秒激光烧蚀-多收集器-等离子体质谱（Femtosecond-Laser Abilation-Multi-Collector-ICP-MS）能够以高精度和高空间分辨率对晶体中的微量元素进行原位同位素测量。本项目的第二部分将集中于火山岩中化学环带单斜辉石和橄榄石晶体中Li同位素环带的研究，以解决以下问题：（a）化学（和Fe-Mg同位素）分带是否通常与Li同位素分带相关联？(b)锂同位素分带的形成机制是什么？这种分带记录了哪些（岩浆）过程？(c)我们可以通过使用观察到的矿物内分区扩散建模获得这些过程的时间信息（持续时间）吗？(d)岩浆体系中Li扩散的空间尺度是什么--在手标本尺度之内还是之外？

This project aims to investigate the fractionation of stable lithium (Li) isotopes in silicate minerals (i.e. olivine and clinopyroxene) from volcanic rocks in order to gain time information on magmatic processes. Since previous studies indicate (i) that stable isotope fractionation in magmas is largely driven by kinetic effects due to diffusive processes and (ii) that Li is a very fast diffusing trace element in minerals (and melts), one can expect large Li isotope variations, i.e. isotopic zoning, on the mineral scale. Therefore, in situ methods (e.g. laser ablation plasma mass spectrometry) have to be used which allow to performing high-precision isotope analyses with high spatial resolution.The first objective of this project comprises establishing an analytical routine for in situ Li isotope measurements on clinopyroxene and olivine. As the Li concentration in these silicate minerals is very low (usually a few ppm), such analyses are challenging. However, preliminary tests indicate that the analytical setup (femtosecond-laser ablation-multi collector-ICP-MS) available in Hannover is capable of in situ isotope measurements of trace elements in crystals with high precision and high spatial resolution.The second part of this project will focus on the investigation of Li isotopic zoning in chemically zoned clinopyroxene and olivine crystals in volcanic rocks in order to address the following questions:(a) Is chemical (and Fe-Mg isotopic) zoning commonly coupled with Li isotopic zoning?(b) Which mechanisms generate Li isotopic zoning and which (magmatic) processes are recorded by such zoning?(c) Can we obtain time information (durations) on these processes by using the observed intra-mineral zoning for diffusion modeling?(d) What are the spatial scales of Li diffusion in magmatic systems -- within or beyond hand-specimen scale?