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

硅酸盐熔体中锂和硼同位素的扩散分馏：研究上升岩浆脱气的工具

基本信息

批准号：
349804343
负责人：
Professor Dr. Max Wilke
金额：
--
依托单位：
Institut für Geowissenschaften
依托单位国家：
德国
项目类别：
Research Grants
财政年份：
2017
资助国家：
德国
起止时间：
2016-12-31 至 2020-12-31
项目状态：
已结题

来源：
https://gepris.dfg.de/gepris/projekt/349804343?language=en
关键词：
Diffusive fractionation lithium boron isotopes

项目摘要

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.

我们建议通过实验校准岩浆脱气过程中锂同位素（6Li vs. 7Li, 10B vs. 11B）的分馏，作为减压速率的函数。这种校准将成为一种新的喷发速度计的基础，定量地限制火山管道中的岩浆上升速率。岩浆上升速率是控制火山喷发类型（喷涌型与爆发性）和火山危险性的主要参数之一。但是先前关于气泡数密度（BND）与上升速度的相关研究表明，均匀气泡的成核和生长强烈依赖于减压路径（即单步减压与多步减压和连续减压），特别是在低减压速率下，气泡的特征可以被气泡生长和合并的相互作用（如奥斯特瓦尔德成熟）所覆盖。此外，连续减压熔体的实验BND值仅限于减压速率大于0.02 MPa/s，并且均匀气泡成核的实验BND值在几个数量级上显著分散。因此，仅在bnd上进行直接关联是困难的，这表明需要更好地量化岩浆上涌期间脱气时间尺度的方法。所提出的熔体和气泡之间的锂和硼同位素分馏校准将是补充气泡结构分析的有力工具。锂是易挥发的，在岩浆中扩散迅速，而硼具有类似的特征，但与锂相比扩散较慢。Li和B的稳定同位素以完全不同的速度扩散，因此在典型的火山喷发的小时到天的时间尺度上发生分馏。因此，火山产物中6Li与7Li、10B与11B同位素的测量具有很大的潜力，可以作为一种速度计来估计与一系列喷发风格相关的岩浆上升速率。该测速仪的有效研制需要对6Li/7Li和10B/11B同位素分馏进行定量标定。拟议的工作将进行两组精心设计的实验，以便在内加热压力容器（IHPV）中进行校准：(1)锂和硼在无Li和/或B熔体和平衡Li和/或B富集熔体之间的扩散偶联实验，以量化Li和B同位素扩散系数；(2)减压诱导脱气实验，使用专门设计并配置气体释放阀的IHPV模拟连续上升，以量化熔体和溶解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（对应于美国新墨西哥州Valles Caldera的Los Posos黑曜岩），随后将扩展到更基本的熔体。