Insufficient Energy From MgO Exsolution to Power Early Geodynamo

Abstract

The origin of Earth's ancient magnetic field is an outstanding problem. It has recently been proposed that exsolution of MgO from the core may provide sufficient energy to drive an early geodynamo. Here we present new experiments on Mg partitioning between iron‐rich liquids and silicate/oxide melts. Our results indicate that Mg partitioning depends strongly on the oxygen content in the iron‐rich liquid, in contrast to previous findings that it depends only on temperature. Consequently, MgO exsolution during core cooling is drastically reduced and insufficient to drive an early geodynamo alone. Using the new experimental data, our thermal model predicts inner core nucleation at ~850 Ma and a nearly constant paleointensity.

Experimental Constraints on an MgO Exsolution‐Driven Geodynamo

“…Our results confirm the previous findings that K Mg strongly depends on X o, oxygen concentration in the Fe alloy (Du et al, ), evident from Figure b as discussed above. Du et al () did not constrain the temperature dependence of K Mg due to limited data set. Our new results provide important constraints: moderate temperature and pressure control on K Mg , namely, small b and c , for 1/ T and P / T terms, respectively.…”

“…So sulfur does not directly influence Mg partitioning, in agreement with our regression result, where parameter g is statistically insignificant. In our study, Mg content is below detection limit in S‐poor Fe alloy, primarily due to its low oxygen contents, in agreement with previous findings (Badro et al, ; Du et al, ).…”

“…The measured K Mg values are plotted as red circles in Figure , along with previous experimental results in blue symbols (Badro et al, ; Badro et al, ; Chidester et al, ; Du et al, ; Jackson et al, ; Suer et al, ). K Mg values span ~3 orders of magnitude, nearly covering the ranges of previous studies.…”

“…Similar to the approach by Du et al (), K Mg may be expressed as a function of temperature ( T , kelvin), pressure ( P , GPa), and the composition of Fe alloy ( X i , the mole fraction of elements i (O, Si, C, S)), as follows: where a – g are fitting parameters. We conducted a stepwise, nonweighted least squares regression to our “combined” data set, including this study and previous studies (Badro et al, ; Badro et al, ; Chidester et al, ; Du et al, ; Jackson et al, ; Suer et al, ). We find a = −3.0 ± 0.26, b = −2314 ± 662, c = 26 ± 10, d = −10 ± 0.9, e = 1.6 ± 0.3, with R 2 = 0.77.…”

Ab Initio Molecular Dynamics Investigation of Molten Fe–Si–O in Earth's Core

“…But those same experiments (e.g., Badro et al, 2016Badro et al, , 2018Blanchard et al, 2017;Bouhifd & Jephcoat, 2011;Chidester et al, 2017;Du et al, 2017;Fischer et al, 2015;Huang & Badro, 2018;Jackson et al, 2018;Ricolleau et al, 2011;Siebert et al, 2012Siebert et al, , 2013Tsuno et al, 2013) have not observed any SiO 2 exsolution and even support that the present-day core could be undersaturated in Si and O (Brodholt & Badro, 2017). But those same experiments (e.g., Badro et al, 2016Badro et al, , 2018Blanchard et al, 2017;Bouhifd & Jephcoat, 2011;Chidester et al, 2017;Du et al, 2017;Fischer et al, 2015;Huang & Badro, 2018;Jackson et al, 2018;Ricolleau et al, 2011;Siebert et al, 2012Siebert et al, , 2013Tsuno et al, 2013) have not observed any SiO 2 exsolution and even support that the present-day core could be undersaturated in Si and O (Brodholt & Badro, 2017).…”