Water Diffusion in Olivine: Experiments and Application to Phenocrysts

对矿物橄榄石及其熔体包裹体中水分的测量，为了解诸如板块构造、行星熔融和火山爆发等基本地球过程提供了约束条件。然而，对这些测量结果的解释存在很大争议，因为水可能在从地球内部运输到收集地点的过程中与周围环境迅速交换。这种交换的程度取决于水在橄榄石中的扩散速度。水在橄榄石中的扩散可以通过几种不同的机制发生，在相同的温度下，其中一些机制比其他机制快几个数量级，导致应用哪种扩散速率存在相当大的不确定性。该项目旨在解决这种模糊性，同时支持一位早期职业女性科学家；本科生也将参与调查。研究结果将与公众和研究界分享。该项目将包括有针对性的脱水实验以及自然样品的测量，以阐明不同扩散机制作为氧逸度，温度和时间的函数的作用。取向橄榄石单晶首先在目标温度下完全脱水和平衡，然后在高压下完全水化，最后放入1atm气体混合炉中逐级加热，直到完全脱水。在每一步之后，红外光谱（FTIR）测量将沿着样品的每个维度剖面进行，以量化内部水浓度和扩散程度。这种方法利用了一种新的数值技术，通过未切割的晶体块来解释红外光谱测量结果，首次允许在多个方向上对同一样品进行氢扩散数据的时间序列。这些实验室实验将由富埃戈火山各种大小的橄榄石斑晶的FTIR测量来补充。这项研究将首次尝试识别和应用样品中特定地点的扩散系数，这些样品具有脱气和上升历史的良好特征。

Measurements of water in the mineral olivine and its melt inclusions provide constraints on fundamental Earth processes such as the occurrence of plate tectonics, planetary melting, and volcanic explosivity. However, interpretations of these measurements are highly debated because of the possibility that water may exchange rapidly with the surrounding environment during transport from the earth's interior to the site of collection. The extent of this exchange depends on how fast water diffuses in olivine. Water diffusion in olivine can occur by several different mechanisms, some of which are orders of magnitude faster than others at the same temperature, resulting in considerable uncertainty about which diffusion rate to apply. This project aims to resolve that ambiguity while supporting an early-career female scientist; undergraduate students will also contribute to the investigation. The research results will be shared with the general public and the research community.This project will include targeted dehydration experiments as well as measurements of natural samples to elucidate the role of different diffusion mechanisms as a function of oxygen fugacity, temperature, and time. Oriented single crystals of olivine will be first completely dehydrated and equilibrated at the temperature of interest, then fully hydrated at high pressure, and finally placed in a 1-atm gas-mixing furnace and heated in steps until fully dehydrated. After each step, infrared spectroscopy (FTIR) measurements will be taken along profiles in each dimension of the sample to quantify the internal water concentration and extent of diffusion. This approach takes advantage of a new numerical technique for interpreting infrared spectroscopic measurements through uncut crystal blocks, allowing for the first time a time-series of hydrogen diffusion data on the same sample in multiple orientations. These laboratory experiments will be complemented by FTIR measurements of various sized olivine phenocrysts from Fuego Volcano. This study will be the first attempt to identify and apply site-specific diffusivities in samples that are well-characterized for their degassing and ascent history.