The Electrical Conductivity of Subducted Continental Crust

俯冲带是地壳和岩石圈循环进入地幔的区域。虽然洋壳在俯冲板块中占主导地位，但大陆地壳也可以被俯冲。据推测，俯冲大陆地壳可能解释了在深度大于300公里的某些俯冲带中观察到的高电导率。这种高传导性以前被归因于含水流体的释放。在这里，研究人员调查了另一种可能性：高传导性是由于锂贝壳甘露石的存在。这种矿物在深俯冲板块中普遍存在的极端压力下变得稳定。它可能占俯冲大陆地壳的很大一部分。具有相似结构的材料通常表现出超离子导电性。这种类型的导电性--其中载流子是固体中的离子和空位--不需要流体的存在。在这里，研究小组在地幔中普遍存在的极端条件下进行实验。他们使用的设备能够产生超过20万大气压的压力。它们量化了温度、压力和矿物组成对电导率的影响。他们逐渐揭开了锂铝榴石是否可以解释在俯冲带观察到的高电导率。这个项目的成果可能会在地球科学之外的材料科学中得到应用。该奖项支持一位来自科学界代表性不足的团体的首席研究员。它涉及高中生，特别是来自代表性不足群体的高中生。它还促进了对公众的教育推广。在这里，研究小组测量了(K，Na)AlSi3O8自由锰矿的电导率。他们研究了纯K端元以及K-Na固溶体的导电性。他们在多砧板高温装置中使用了阻抗谱技术。为了补充电导率测量，他们在类似的压力和温度条件下进行K自扩散和Na-K互扩散测量。这为从电导率测量中推断出的传导机制提供了进一步的见解。这些结果为计算广泛的地幔深度范围内Libermannite的电导率随压力、温度和成分的变化提供了基础。这项研究还改进了对大陆地壳电导率的估计。它确定了“干燥的”大陆地壳是否可以解释300公里以上深度的高电导率区域。该团队还研究了锂硼镁矿中电导率与晶体取向的关系。目的是评估俯冲的大陆地壳是否可能具有显著的电性各向异性。这些数据与之前公布的其他矿物数据一起用于计算俯冲带大陆地壳的电导率。它为通过大地电磁测量识别俯冲大陆地壳提供了更好的基础。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Subduction zones are regions where crust and lithosphere are recycled into the Earth’s mantle. While oceanic crust dominates in subducting plates, continental crust can also be subducted. It has been inferred that subducted continental crust may explain the high electrical conductivity observed in some subduction zones at depth greater than ~300 km. This high conductivity has previously been attributed to the release of hydrous fluids. Here, the researchers investigate an alternative possibility: that the high conductivity is due to the presence of liebermannite. This mineral becomes stable at the extreme pressure prevailing in deep subducted plates. It may account for a significant fraction of subducted continental crust. Materials with similar structures often display superionic conductivity. This type of conductivity - where charge carriers are ions and vacancies in the solid - does not require the presence of fluids. Here, the team carries out experiments at the extreme conditions prevailing in the mantle. They use an apparatus capable of generating pressures in excess of 200,000 atm. They quantify the effect of temperature, pressure, and mineral composition on electrical conductivity. They gradually unveil whether liebermannite can explain the high conductivities observed in subduction zones. Outcomes of this project may find applications beyond Earth Sciences in Materials Science. The award supports a principal investigator from a group underrepresented in science. It involves high-school students notably from underrepresented groups. It also promotes educational outreach toward the public.Here, the team measures the electrical conductivity of (K,Na)AlSi3O8 libermannite. They investigate the conductivity of the pure K-endmember, as well as that of K-Na solid solutions. They use the impedance spectroscopy technique in a multi-anvil high-pressure-temperature device. To complement the conductivity measurements, they perform K self-diffusion and Na-K interdiffusion measurements at similar conditions of pressure and temperature. This provides further insight on the conduction mechanism inferred from the conductivity measurements. The results provide the basis for calculating liebermannite conductivity as a function of pressure, temperature, and composition over a wide range of mantle depths. The study also improves estimates of the electrical conductivity of continental crust. It establishes whether “dry” continental crust may explain regions of high conductivity at depths greater than 300 km. The team also investigate the crystal orientation dependence of the electrical conductivity in liebermannite. The goal is to evaluate whether subducted continental crust may have significant electrical anisotropy. The data are used, along with previously published data on other minerals, to calculate the electrical conductivity of continental crust in subduction zone. It provides an improved basis for the identification of subducted continental crust by magnetotelluric measurements.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.