SGER: Preliminary Experiments on the Grain Boundary Diffusivity of Water

氢含量可能是地幔中唯一最重要的化学参数，因此量化其分布和传输速率是非常重要的。地幔矿物中H的体积扩散测量表明，与几乎所有其他元素不同，H的扩散平衡时间非常快。在中等温度（~300-1000°C）下，晶界扩散的添加成分有可能使平衡时间加快一个数量级。然而，迄今为止还没有关于水通过上地幔的有效扩散速率的资料。这些都是迫切需要的数据，因为地球科学家已经在基于水扩散率的假设进行计算了。在这项小额探索性研究资助中，研究者建议进行初步实验，以确定水通过橄榄石聚集体的晶界扩散系数的大小。橄榄石的选择是至关重要的，因为这是最丰富的上地幔矿物。氘将被用作氢的替代品，以避免样品制备过程中的污染，并提高分析背景/检测限。对这些实验可行性的怀疑足以保证对实验设计的具体方面进行初步测试，包括在活塞缸中合成的橄榄石聚集体的质量，水热退火过程中聚集体的机械完整性，以及在拟议实验背景下通过SIMS（次级离子质谱）探测氘的检测极限。

Hydrogen content may be the single most important chemical parameter within the mantle, and hence it is very important to quantify its distribution and rates of transport. Volume diffusion measurements for H in mantle minerals indicate extremely fast diffusive equilibration times, unlike that for nearly all other elements. The added component of grain boundary diffusion has the potential to speed equilibration times by an order of magnitude at moderate temperatures (~300-1000°C). However, to date there is no information on effective diffusion rates of water through the upper mantle. These are urgently needed data, as geoscientists are already making calculations based on assumptions about bulk water diffusivity.In this Small Grant for Exploratory Research, the investigator proposes to do the preliminary experiments to determine the magnitude of grain boundary diffusivity of water through olivine aggregates. The choice of olivine is critical as this is the most abundant upper mantle mineral. Deuterium will be used as a proxy for hydrogen in order to avoid contamination by sample preparation and to improve the analytical background/detection limit. Doubt about the feasibility of these experiments is significant enough to warrant an initial test of the specific aspects of the experimental design, including the quality of olivine aggregates synthesized in the piston-cylinder, the mechanical integrity of aggregates during hydrothermal anneals, and the detection limit of deuterium by SIMS (Secondary Ion Mass Spectrometry) probing in the context of the proposed experiments.