Doctor of Philosophy
Secco, Richard A.
The source of the fluid stirring mechanism that powers the dynamo of terrestrial-type bodies during their active magnetic field era is debated. Prior to the formation of a solid inner core, thermal convection may cause enough mechanical stirring of the core fluid to generate a magnetic field through dynamo action. After inner core formation, compositional convection in the liquid outer core becomes the main source of fluid stirring mechanism to power a dynamo. Constraints on the likelihood and duration of these convection mechanisms may be obtained by the experimental determination of the thermal properties of core materials. These cores consist of complex Fe-alloys and the effects of impurities have not yet been established under high pressures and temperatures. The first objective of this thesis was to investigate the behavior of electrical resistivity along the melting boundary of metals by measurements in large volume presses (1000 ton, 3000 ton) using the 4-wire method. Unprecedented measurements on Au up to liquid temperatures from 2-5 GPa show a decrease in resistivity along the melting boundary, conflicting with the prediction of invariant behavior. In contrast, the first measurements on Fe-8.5wt%Si revealed a constant behavior of resistivity along the melting boundary from 10-24 GPa. The second objective of this thesis was to investigate the effect of impurities on the resistivity via measurements on Fe-xSi (x is 2, 8.5, 17 wt%) and Fe-10wt%Ni-10wt%Si from 2-24 GPa and up to liquid temperatures. The similarity in Fe-8.5wt%Si and Fe-10wt%Ni-10wt%Si measurements indicate a negligible effect of Ni. Finally, the estimated heat flow at the top of an Fe-10wt%Ni-10wt%Si Earth core is estimated to be 14 TW. The results of an Fe-Si lunar core date the end of the high magnetic field dynamo to be in the range of 3.32-3.80 Gyr. A similar analysis of an Fe-8.5wt%Si Mercurian core suggests a thermally driven dynamo up to 4.28-4.42 Gyr. However, an Fe-10wt%Ni-10wt%Si Mercurian core indicates a thermally driven dynamo up to 4.29-4.48 Gyr. Lastly, measurements of Fe-10wt%Ni-10wt%Si suggest the lack of dynamo in Venus can be explained by a solid inner core and at least partially liquid outer core.
Summary for Lay Audience
It is difficult to directly determine the thermal processes inside terrestrial-type bodies due to the inaccessibility of these regions. These bodies are usually comprised of a crust, a mantle, and a core, and the core is typically divided into a liquid outer and solid inner core. The presence of an inner core suggests that compositional convection occurs in the outer core. This convection mechanism, and the stirring of core fluid that it causes, may be enough to generate a magnetic field through dynamo action. In contrast, thermal convection is required prior to the formation of an inner core. High pressure and high temperature experimental determination of the thermal properties of core materials can help constrain the likelihood and duration of these convection mechanisms. A recent theory proposed that simple metals have a constant electrical resistivity along their pressure- and temperature-dependent melting boundary. A constant resistivity behavior represents a solution to the technical difficulties of reproducing the extreme conditions of terrestrial-type cores in a laboratory. Electrical resistivity is used to calculate thermal conductivity and adiabatic heat flow, both necessary to identify the age of the inner core and presence of convection. The first objective of this thesis is to investigate the possibility of constant resistivity behavior on the melting boundary. Electrical resistivity measurements of Au from 2-5 GPa and into the liquid state show a decrease in electrical resistivity along the melting boundary. However, measurements on Fe-8.5wt%Si and Fe-10wt%Ni-10wt%Si up to 24 GPa and in the liquid state show a constant behavior starting at 10 GPa and 7 GPa, respectively. The constant behavior at the pressures studied may be applicable at higher pressures in core materials. The second objective of this thesis is to evaluate the effects of impurities on the resistivity of simple metals. Measurements on Fe-2wt%Si, Fe-8.5wt%Si, and Fe-17wt%Si from 2-5 GPa and up to liquid temperatures show a decrease in the impurity effect as temperature is increased. The results of these measurements are used to draw conclusions on the age of the core and magnetic field of Earth, the moon, Mercury, and Venus.
Berrada, Meryem, "Heat flow in terrestrial-type bodies from high P,T electrical resistivity measurements of Au, Fe-Si and Fe-Ni-Si solid and liquid alloys" (2022). Electronic Thesis and Dissertation Repository. 8612.
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