Diffusive fractionation of lithium and boron isotopes in silicate melts: a tool to study degassing in ascending magmas

我们建议在实验上校准在岩浆脱气过程中锂同位素的分馏(6Li比7Li，10B比11B)，作为减压速率的函数。这种定标将成为新的喷发速度计的基础，定量地限制火山导管中的岩浆上升速度。岩浆上升速度是控制火山喷发方式(喷发与爆炸)以及由此而产生的火山灾害的主要参数之一。但以往关于气泡数密度(BND)与上升速度相关的研究表明，均匀气泡的成核和长大强烈依赖于减压路径(即单步和多步以及连续减压)，特别是在低减压速率下，气泡的生长和合并(如Ostwald熟化)的相互作用可以叠加气泡的特征。此外，连续减压熔体的实验BND值被限制在大于0.02 Mpa/S的减压速率范围内，并且对于均匀的气泡成核，BND值显著地分散在几个数量级上。因此，很难仅仅根据BNDS进行直接的关联，这表明需要更好地量化岩浆上涌过程中脱气的时间尺度的方法。拟议的锂和硼同位素在熔体和气泡之间分馏的校准将是补充气泡结构分析的有力工具。锂是挥发性的，在岩浆中扩散迅速，而硼具有类似的特征，但扩散速度比锂慢。Li和B的稳定同位素扩散速度非常不同，因此分馏作用在火山喷发的典型时间尺度上以小时到天的时间尺度发展。因此，火山产物中6Li对7Li和10B对11B同位素的测量具有很大的潜力，可以作为速度计来估计与一系列喷发类型有关的岩浆上升速率。这种速度计的有效研制需要对6Li/7Li和10B/11B同位素分馏进行定量标定。拟议的工作将实施两套精心设计的实验，以在内部加热压力容器(IHPV)上进行这一校准：(1)锂和硼扩散--无Li和/或B与平衡的Li和/或B富熔体之间的耦合实验，以量化Li和B同位素扩散系数；(2)减压诱导脱气实验，以便使用专门设计并配置气体释放阀来模拟连续上升的IHPV，以量化熔体和出溶气相之间的锂和硼同位素分馏。这两组实验将主要在流纹岩熔体上进行(对应于研究得很好的美国新墨西哥州Valles Caldera的Los Posos Bobsidian)，稍后将扩展到更基性的熔体。

We propose to experimentally calibrate fractionation of lithium isotopes (6Li vs. 7Li, 10B vs. 11B) during magma degassing as a function of the decompression rate. This calibration will b the basis of a new eruption speedometer, quantitatively constraining magma ascent rates in volcanic conduits.Magma ascent rate is one of the main parameters controlling volcanic eruption style (effusive vs. explosive) and thus associated volcanic hazards. But previous studies on correlating bubble number densities (BND) to ascent velocities have shown that the homogeneous bubble nucleation and growth strongly depends on the decompression path (i.e. single-step vs. multi-step and vs. continuous decompression) and that especially at low decompression rates the bubble signatures can be overprinted by the interaction of bubble growth and coalescence (e.g. Ostwald ripening). Furthermore, the experimental BND values of continuously decompressed melts are limited to decompression rates larger than 0.02 MPa/s and scatter significantly over several orders of magnitude for homogeneous bubble nucleation. Thus, it is difficult to make straightforward correlations solely on BNDs, and demonstrates the need for methods that better quantify the timescales of degassing during magma upwelling. The proposed calibration of lithium and boron isotope fractionation between melt and gas bubbles will be a powerful tool to complement bubble texture analysis.Lithium is volatile and diffuses quickly in magmas, while boron has similar characteristics but diffuses slower compared to lithium. The stable isotopes of Li and B diffuse at sufficiently different speeds so that fractionation develops in the hours-to-days timescales typical of volcanic eruptions. Therefore, measurements of 6Li vs. 7Li and 10B vs. 11B isotopes in volcanic products have great potential to be used as a speedometer to estimate magma ascent rates associated with a range of eruption styles. Effective development of this speedometer requires quantitative calibration of 6Li/7Li and 10B/11B isotopic fractionation. The proposed work will conduct two sets of experiments carefully designed to perform this calibration in internally heated pressure vessels (IHPV): (1) lithium and boron diffusion-couple experiments between a Li- and/or B-free and an equilibrated Li- and/or B-enriched melt, in order to quantify Li and B isotope diffusivity and (2) decompression-induced degassing experiments, in order to quantify lithium and boron isotope fractionation between melts and exsolved gas phases using an IHPV specially designed and configured with a gas release valve to simulate continuous ascent. Both sets of experiments will be performed mainly on rhyolite melts (corresponding to well-studied Los Posos obsidian, Valles Caldera, New Mexico, USA) and will be later extended to more basic melts.