Electrical Resistivity of Fe and Fe‐3 wt%P at 5GPa With Implications for the Moon's Core Conductivity and Dynamo
Electrical Resistivity of Fe and Fe‐3 wt%P at 5GPa With Implications for the Moon's Core Conductivity and Dynamo
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DOI：
10.1029/2021je007116
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发表时间：
2022
期刊：
影响因子：
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通讯作者：
Yingwei Fei
中科院分区：
文献类型：
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作者：
Yuan Yin;Lin Wang;Shuangmeng Zhai;Yingwei Fei
在月球的早期历史中曾经存在过一个高磁场，这表明月球的核心曾经拥有一台热驱动的发电机。铁芯材料的导热系数是发电机的一个重要参数。月球核心的成分被认为是铁或含有一些轻元素(如S、P、Si和C)的铁合金化，但其输运性质仍不确定。我们测量了铁和Fe-3wt%P合金在5 Gpa和高温下的电阻率。除了准四点法外，还采用了四探针范德波法测量纯铁的电阻率。在铁中加入∼3wt%的磷，由于杂质的影响，在5 Gpa和1000-1500K时的电阻率略有增加。Fe-3wt%P合金的电阻率在熔化初期增大。根据Wiedemann-Franz定律，无轻元素核的月核-地幔边界的热导率为28.6-34.2Wm−1K−1，含磷核的热导率为31.5Wm−1K−1(∼3wt%P)。因此，月核中的少量磷对其导热系数略有影响。据估计，穿过月球CMB的传导热流在4.5到5.7GW之间，绝热热流在3.3到4.2GW/m2之间，这取决于核心的成分(Fe或Fe-3wt%P)。结合我们的结果和以前的月核演化模型，我们认为一个热驱动的发电机一直持续到3.63-3.88年前。
A high‐magnetic field once existed in the early history of the Moon, suggesting the core once possessed a thermally driven dynamo. The thermal conductivity of core materials is a significant parameter of the dynamo. The lunar core composition is thought to be iron or iron‐alloyed with some light elements (e.g., S, P, Si, and C), but its transport properties remain uncertain. We measured the electrical resistivity of iron and Fe‐3 wt%P alloys at 5 GPa and high temperatures. Apart from the quasi four‐point technique, the four‐probe van der Pauw technique was also employed to measure the resistivity of pure iron. Adding ∼3 wt% phosphorus to iron slightly increases the resistivity at 5 GPa and 1000–1500 K due to the impurity effect. The resistivity of Fe‐3 wt%P alloys increases at the onset of melting. Via the Wiedemann‐Franz law, the thermal conductivity at the lunar core‐mantle boundary (CMB) is estimated to be 28.6–34.2 Wm−1K−1for a light‐element free core and 31.5 ± 1.9 Wm−1K−1for a phosphorus‐bearing (∼3 wt% P) core. Therefore, small amounts of phosphorus in the lunar core slightly impact its thermal conductivity. The estimated conductive heat flow across the lunar CMB varies from 4.5 to 5.7 GW, and the adiabatic heat flux varies from 3.3 to 4.2 mW/m2, depending on the core's composition (Fe or Fe‐3 wt%P). Integrating our results with previous lunar core evolution models, we suggest that a thermally driven dynamo persisted until 3.63–3.88 Ga ago.