Overturn of ilmenite‐bearing cumulates in a rheologically weak lunar mantle

©2019. American Geophysical UnionThe crystallization of the lunar magma ocean (LMO) determines the initial structure of the solid Moon. Near the end of the LMO crystallization, ilmenite‐bearing cumulates (IBC) form beneath the plagioclase crust. Being denser than the underlying mantle, IBC are prone to overturn, a hypothesis that explains several aspects of the Moon's evolution. Yet the formation of stagnant lid due to the temperature dependence of viscosity can easily prevent IBC from sinking. To infer the rheological conditions allowing IBC to sink, we calculated the LMO crystallization sequence and performed high‐resolution numerical simulations of the overturn dynamics. We assumed a diffusion creep rheology and tested the effects of reference viscosity, activation energy, and compositional viscosity contrast between IBC and mantle. The overturn strongly depends on reference viscosity and activation energy and is facilitated by a low IBC viscosity. For a reference viscosity of 1021 Pa s, characteristic of a dry rheology, IBC overturn cannot take place. For a reference viscosity of 1020 Pa s, the overturn is possible if the activation energy is a factor of 2–3 lower than the values typically assumed for dry olivine. These low activation energies suggest a role for dislocation creep. For lower‐reference viscosities associated with the presence of water or trapped melt, more than 95% IBC can sink regardless of the activation energy. Scaling laws for Rayleigh‐Taylor instability confirmed these results but also showed the need of numerical simulations to accurately quantify the overturn dynamics. Whenever IBC sink, the overturn occurs via small‐scale diapirs

Effective viscosity with dislocation creep

The code for generating the effective viscosity coupling mantle diffusion-creep and mantle dislocation-creep. Rheological data derive from Hirth and Koldstedt (2003)

Figure 2: Effective viscosity with dislocation creep

This is a MATLAB script for generating the effective viscosity of a hybrid mantle rheology coupling mantle diffusion-creep and mantle dislocation-creep as a function of temperature. <div><br></div><div>The solid curve specifies the viscosity of mantle diffusion-creep, while the dashed curves specifies the effective viscosity of hybrid mantle rheology with different strain rates (10^-10 to 10^-14 s^-1).<br><div><div><br></div><div>How to use: please download the script and run it directly in MATLAB. The code will generate a figure with two sub-plots: the upper sub-plot represents the condition with a grain size of 5.6 mm, corresponding to a reference viscosity of 10^20 Pa s, whereas the lower sub-plot represents the condition with a grain size of 2.6 mm, corresponding to a reference viscosity of 10^19 Pa s.</div></div></div><div><br></div><div>The reference viscosity represents the viscosity probed at ~1600 K.</div

Figure 4: Crystallization of lunar magma ocean

The data of lunar magma ocean crystallization modeled by the software alphaMELTS. The attached .txt file corresponds to Figure 4.<div><br></div><div>The information of bulk composition derives from O'Neil (1990). For trace element see Taylor (1992).</div><div><div><div><br></div><div>In the data file, the first column represents radius (in km), the second column represents density, the third column represents temperature and the fourth column represents initial heat production rate.</div></div></div

Figure 3: Effective viscosity of ilmenite-olivine mixture

The MATLAB code for generating the viscosity of ilmenite-olivine mixture as a function of temperature. <div><br></div><div>The script will give two sub-plots: the upper plot specifies the condition with a grain size of 5.6 mm corresponding to a reference viscosity of 10^20 Pa s, whereas the lower plot specifies the condition with a grain size of 2.6 mm corresponding to a reference viscosity of 10^19 Pa s.</div><div><br></div><div>How to use: just download the script and run it directly in MATLAB.</div

Control and Signal Acquisition System of Broad-Spectrum Micro-Near-Infrared Spectrometer Based on Dual Single Detector

Based on the scanning grating mirror developed by us, this paper presents a method for precise control of the scanning grating mirror and high-speed spectrum data acquisition. In addition, a system circuit of the scanning grating mirror control and a spectrum signal acquisition system were designed and manufactured. The final results of the experiment show that the control system successfully allowed the precise control of the swing of the scanning grating mirror and the acquisition system successfully carried out the high-speed acquisition and transmission of the spectrum and angle data. The spectrum detection range of the NIR spectrometer was 80–2532 nm. The overall resolution of the spectrum was better than 12 nm

Replication Data for: Overturn of Ilmenite-Bearing Cumulates in a Rheologically Weak Lunar Mantle

The crystallization of the lunar magma ocean (LMO) determines the initial structure of the solid Moon. Near the end of the LMO crystallization, ilmenite-bearing cumulates (IBC) form beneath the plagioclase crust. Being denser than the underlying mantle, IBC are prone to overturn, a hypothesis that explains several aspects of the Moon's evolution. Yet the formation of stagnant lid due to the temperature dependence of viscosity can easily prevent IBC from sinking. To infer the rheological conditions allowing IBC to sink, we calculated the LMO crystallization sequence and performed high-resolution numerical simulations of the overturn dynamics. We assumed a diffusion creep rheology and tested the effects of reference viscosity, activation energy, and compositional viscosity contrast between IBC and mantle. The overturn strongly depends on reference viscosity and activation energy and is facilitated by a low IBC viscosity. For a reference viscosity of 1021 Pa s, characteristic of a dry rheology, IBC overturn cannot take place. For a reference viscosity of 1020 Pa s, the overturn is possible if the activation energy is a factor of 2–3 lower than the values typically assumed for dry olivine. These low activation energies suggest a role for dislocation creep. For lower-reference viscosities associated with the presence of water or trapped melt, more than 95% IBC can sink regardless of the activation energy. Scaling laws for Rayleigh-Taylor instability confirmed these results but also showed the need of numerical simulations to accurately quantify the overturn dynamics. Whenever IBC sink, the overturn occurs via small-scale diapirs