Timescales of magma transfer in intraplate volcanic systems from diffusion modelling of lithium isotopes in zircon and olivine

用岩浆转移和上升速率的时间尺度衡量的地壳中岩浆的动态行为是一个关键的未知问题，限制了更可靠地监测火山喷发和相关灾害的发展。由于动态岩浆系统中的各种化学不平衡，应用固体或熔体中的扩散定律可以指导提取对许多岩浆作用速率的一级定量约束。在这一建议中，锂在锆石和橄榄石中的快速扩散被认为是表征板内火山岩田从源头到喷口的岩浆管道系统的有力组合。然而，这些定量方法的可靠性需要彻底了解锂在这两种矿物中的分布和动力学行为。正在进行的地幔锆石中锂同位素研究的初步结果有力地支持了这样一种假设，即对锆石中锂扩散的模拟具有限制岩浆从储存到喷发的时间的特殊潜力。结果可以通过模拟橄榄石中的锂离子扩散来进一步验证，这种模式已经被成功地用于研究短期岩浆过程。将研究地壳包体中的锆石和玄武岩主岩中的橄榄石中的Li丰度和同位素比率，以确定热激活扩散时间尺度，该时间尺度可转化为岩浆转移和上升速率，以便合理假设包体夹带的深度。从锆石的拉曼光谱推断的退火期和包体在岩浆中的平均沉降速度进一步验证了锂在锆石和橄榄石中扩散的速率。本研究的地壳包体和寄主玄武岩样品来自三个具有明显地壳特征的板内火山岩田，它们是中央地块(法国)、库拉(土耳其)和汉诺巴(中国)。这项研究有望对锂在自然体系中的锆石和橄榄石中的扩散有更深入的了解。锂扩散计时与锆石拉曼光谱相结合来限制退火持续时间，有望为推断地壳层面的岩浆迁移和岩浆上升速率的时间尺度提供有意义的数据，从而进一步发展锂同位素作为一种新兴的地球速度计和高温火成过程的岩石学示踪剂的应用。

The dynamic behaviour of magma in the crust as measured by timescales of magma transfer and ascent rates is a key unknown limiting development of more reliable monitoring of volcanic eruptions and related hazards. As a result of various chemical disequilibria in dynamic magmatic systems, applying the laws of diffusion in solids or melts can guide the extraction of quantitative, first-order constraints on the rates of many magmatic processes. In this proposal, the fast diffusion of Li in zircon and olivine is proposed as a powerful combination for characterizing the magmatic plumbing systems from source to vent in intraplate volcanic fields. The reliability of these quantitative methods, however, requires a thorough understanding of the distribution and kinetic behaviour of Li in both minerals. Preliminary results of an ongoing study of Li isotopes in mantle-derived zircon lend strong support to the assumption that modelling of Li-in-zircon diffusion has an exceptional potential to constrain the duration of magma from storage to eruption. Results can be further cross-checked by modelling Li-in-olivine diffusion which has been already successfully utilized to investigate short-term magmatic processes. Li abundances and isotopic ratios in zircon from crustal xenoliths and olivine from their basaltic host rocks will be investigated to determine thermally activated diffusion timescales which can be translated into magma transfer and ascent rates for reasonable assumptions regarding the depth of xenolith entrainment. Annealing durations inferred from Raman spectroscopy of zircon and the average settling rate of xenoliths in magma allow further validation of rates extracted from Li diffusion in zircon and olivine. Samples of crustal xenoliths and host basalts for the proposed study stem from three intraplate volcanic fields with distinct crustal characteristics, which are the Massif Central (France), Kula (Turkey), and Hannuoba (China). It is expected that the study provides an in-depth understanding of Li-diffusion in zircon and olivine in natural systems. Li-diffusion chronometry combined with Raman spectroscopy of zircon to constrain annealing durations is expected to provide meaningful data for inferring timescales of magma transfer and magma ascent rates at crustal levels, thereupon further developing the application of Li-isotopes as an emerging geospeedometer and petrological tracer for high temperature igneous processes.