Experimental Investigation of the Link Between Water Loss and Oxygen Fugacity in Olivine-Hosted Melt Inclusions

智力优势。水在各种构造背景下岩浆的生成和演化过程中起着核心作用。确定岩浆喷发前水浓度的困难阻碍了量化其对岩浆过程的影响。水在硅酸盐熔体中的溶解度随着压力的降低而大幅下降，因此，在浅地壳中含有几个重量百分比溶解的水的岩浆在喷发后只剩下几千ppm的水。橄榄石托管熔融包裹体提供了一个直接的信息来源，因为主机晶体的强度保护的熔融包裹体从减压经历的夹带岩浆喷发前的H2O含量。从橄榄石托管的熔融包裹体中获得喷发前的H2O浓度的主要不确定性是通过宿主橄榄石扩散的H+（质子）的损失或增益的可能性。通过晶格扩散从橄榄石托管的熔体包裹体中损失或获得H2O需要每损失2个质子留下一个O2-，并且每获得2个质子清除一个O2-，以保持电荷中性。这分别导致包裹体中氧逸度的增加或减少。因此，有人提出，铁氧化还原反应与进出口的H+严重限制的量H2O，可以移动到或出橄榄石托管熔体包裹体。然而，最近的实验表明，铁的氧化还原反应不限制水的损失/增益的夹杂物的量或速率，这表明夹杂物内的氧逸度是由以前忽视的机制缓和。从概念上讲，熔体夹杂物中的H2O损失/增益可能与点缺陷介导的氧逸度再平衡有关。该模型表明，熔融包裹体中的氧逸度波动是有效的，有效地缓和与主机橄榄石的再平衡。提出了一个实验和理论研究，严格测试这个模型，并提供一个更深入的了解过程中的橄榄石托管熔融包裹体的H2O损失/增益。目标是通过一系列（1）对天然存在的富H2O包裹体进行脱水实验，和（2）使用同位素富集18 O和D（2 H）的H2O对低H2O包裹体进行水合实验，量化橄榄石熔融包裹体中H2O损失/增加的速率限制过程。从这些实验的结果，结合现有的分区和扩散的数据，从文献中，将被用来开发一个数值模型，以评估时间尺度的扩散再平衡的橄榄石托管熔融包裹体在一系列的地质情况。该模型将广泛提供给科学界，作为解释天然熔体包裹体的工具。从拟议的研究的实验和理论部分的综合结果将提供必要的见解，以更准确地评估橄榄石托管熔融包裹体的可靠性作为指标的喷发前的H2O含量的脱气熔岩。拟议的研究（1）推进发现和理解，同时促进教学，培训和学习，（2）扩大代表性不足的群体的参与，（3）通过研究生参与麻省理工学院/WHOI联合计划，本科生参与WHOI暑期和少数民族学生研究员计划，并将这项研究的结果纳入由PI教授的研究生课程，并开发一个数值模型，该模型将广泛提供给科学界，作为解释天然熔体包裹体的工具。

Intellectual Merit. Water plays a central role in the generation and evolution of magmas in every tectonic setting. Difficulties in determining pre-eruptive water concentrations in magmas have hindered quantification of its influence on magmatic processes. The solubility of H2O in silicate melts drops substantially with decreasing pressure, so that a magma containing several weight percent dissolved H2O in the shallow crust is left with only a few thousand parts-per-million H2O following eruption. Olivine-hosted melt inclusions provide a direct source of information on the pre-eruptive H2O contents of degassed magmas because the strength of the host crystal protects the melt inclusion from the decompression experienced by the entraining magma. The main uncertainty involved with deriving pre-eruptive H2O concentrations from olivine-hosted melt inclusions is the potential for loss or gain of H+ (protons) by diffusion through the host olivine. Loss or gain of H2O from olivine-hosted melt inclusions via lattice diffusion requires leaving behind an O2- for every 2 protons lost, and scavenging an O2- for every 2 protons gained in order to maintain charge neutrality. This produces an increase or decrease, respectively, of the fugacity of oxygen within the inclusion. It has, therefore, been proposed that Fe redox reactions associated with ingress or egress of H+ severely limit the amount of H2O that can move into or out of an olivine-hosted melt inclusion. However, recent experiments indicate that iron redox reactions do not limit either the amount or rate of water loss/gain by the inclusion, suggesting that oxygen fugacity within the inclusion is moderated by a previously overlooked mechanism. Conceptually, H2O loss/gain in melt inclusions may be linked to point defect-mediated oxygen fugacity re-equilibration. This model suggests that oxygen fugacity fluctuations in melt inclusions are efficiently and effectively moderated by re-equilibration with the host olivine. An experimental and theoretical study is proposed to rigorously test this model and to provide a more thorough understanding of the process of H2O loss/gain for olivine-hosted melt inclusions. Goals are to quantify the rate-limiting process for H2O loss/gain in olivine-hosted melt inclusions through a series of (1) dehydration experiments, carried out on naturally occurring H2O-rich inclusions, and (2) hydration experiments carried out on low-H2O inclusions using H2O that is isotopically enriched in both 18O and D (2H). Results from these experiments, combined with existing partitioning and diffusion data from the literature, will be used to develop a numerical model to evaluate timescales for diffusive re-equilibration of H2O in olivine-hosted melt inclusions over a range of geologic scenarios. The model will be made broadly available to the scientific community to be used as a tool for interpreting natural melt inclusions. The combined results from the experimental and theoretical portions of the proposed study will provide the insights necessary to more accurately assess the reliability of olivine-hosted melt inclusions as indicators of pre-eruptive H2O contents of degassed lavas.Broader Impacts. The proposed research (1) advances discovery and understanding while promoting teaching, training and learning, (2) broadens participation of underrepresented groups, and (3) enhances infrastructure for research and education through the involvement of graduate students in the MIT/WHOI Joint Program, undergraduate students participating in the WHOI Summer and Minority Student Fellow programs, and incorporation of results from this study into graduate courses taught by the PIs and by development of a numerical model that will be made broadly available to the scientific community to be used as a tool for interpreting natural melt inclusions.