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.

我们建议在岩浆脱气过程中实验校准锂同位素（6Li vs. 7li，10b vs. 11b）的分级，这是减压速率的函数。该校准将基于新的喷发速度表的基础，在火山导管中定量约束岩浆上升速率。MAGMA上升速率是控制火山喷发样式（爆炸性与爆炸性）的主要参数之一，从而相关的火山危害。 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成熟）。此外，连续解压缩熔体的实验BND值仅限于大于0.02 MPa/s的减压速率，并且在均质气泡成核的几个数量级上显着散射。因此，很难仅在BND上直接相关性，并证明需要更好地量化岩浆上升过程中脱气的时间标度的方法。熔融和气泡之间的锂和硼同位素分馏的拟议校准将是补充气泡纹理分析的强大工具。LITHIUM是挥发性的，并且在岩浆中迅速扩散，而硼的特征相似，但与锂相比，扩散较慢。 LI和B的稳定同位素以足够不同的速度扩散，因此在典型的火山喷发的小时时间尺度中，分馏出现。因此，在火山产品中的6LI与7LI和10B同位素的测量值很大程度上可以用作估计与一系列喷发样式相关的岩浆上升速率的速度表。该速度计的有效开发需要定量校准6LI/7LI和10B/11B同位素分馏。拟议的工作将进行两组仔细设计的实验，以在内部加热的压力容器（IHPV）中执行此校准：（1）锂和硼扩散式实验实验和/或无B-FR-and B-FR-FR-FR-FR-FR-FR-FR-FRILE和/或平衡的LI-和/或BENEDENEDEN-BENEDEDEDEDEDEDERED MELT，以量化Li and b isosepepers-deg iSosporde-degriim degfimim degfimim degfimim degfiped-degs-deg ressive-nimimim degfiped（2）（2）（2）（2）为了使用特殊设计和配置的IHPV量化熔体和溶解气相之间的锂和硼同位素分馏，以模拟连续上升。两组实验都将主要在流纹岩熔体上进行（对应于研究良好的Los Posos Obsidian，Valles Caldera，New Mexico，USA），后来将扩展到更基本的熔体。