Non-traditional stable isotope fractionation during magmatic differentiation: In situ Fe-Mg-isotope analyses of phenocrysts by femtosecond laser ablation MC-ICP-MS

在这个项目中，我们旨在研究玄武岩(橄榄石、CPx)中斑晶的非传统稳定同位素(即Fe和Mg)的分馏，以提高我们对岩浆系统时间演化的认识。我们在第一个资助期获得的结果表明，通过激光烧蚀等离子体质谱进行原位Fe和Mg同位素分析，解决大洋中脊玄武岩中橄榄石的小尺度同位素变化(即同位素分带)是可行的。因此，我们能够追踪岩浆演化过程中矿物尺度上发生的扩散过程，并通过对这些过程进行建模来获得时间信息。在第二个资助期，我们希望将我们开发的技术应用于洋脊玄武岩中的单斜辉石斑晶，以解决以下问题：(1)cpx中Fe和Mg同位素的分带是否与橄榄石中观察到的分带相当，并与化学分带相关联？(2)我们能否使用这种分带来获得有关岩浆过程的时间信息？(3)cpx的信息如何与我们通过模拟橄榄石中的扩散分带获得的时间约束进行比较？我们的目标是研究沙茨基隆起大洋高原玄武岩中的橄榄石斑晶，以获得具有超大岩浆体积的岩浆系统的时间演化信息。最后，我们正在通过开发一个多组分扩散模型来充分描述铁和镁同位素在晶体中的耦合输运，从而对稳定同位素在晶体中的扩散分馏有一个更一般的理解。

In this project we aim to investigate the fractionation of non-traditional stable isotopes (i.e. Fe and Mg) of phenocrysts in basalts (olivine, cpx) to improve our knowledge on the temporal evolution of magmatic systems. Our results obtained during the first funding period show that resolving small-scale isotopic variations (i.e. isotopic zoning) in olivines from mid-ocean ridge basalts is feasible by in situ Fe- and Mg isotope analyses using laser ablation plasma mass spectrometry. We are thus able to trace diffusive processes that occur on the mineral scale during magma evolution and to obtain time information by modeling these processes.During the second funding period, we want to apply our developed technique to clinopyroxene phenocrysts in basalts from ocean ridges to address the following questions: (1) Is the zoning of Fe and Mg isotopes in cpx comparable to that observed in olivines and correlated with a chemical zoning? (2) Can we use this zoning to receive time information on magmatic processes? (3) How do the information from cpx compare to the time constraints we have received by modeling of the diffusion zoning in olivine?Additionally, we aim to investigate olivine phenocrysts in basalts from the Shatsky Rise oceanic plateau to gain information on the temporal evolution of magmatic systems with extraordinarily large magma volumes. Finally, we are working on a more general understanding of diffusive fractionation of stable isotopes in crystals by developing a multi-component diffusion model that adequately describes the coupled transport of Fe and Mg isotopes in a crystal.