Formation and evolution of a lunar core from ilmenite-rich magma ocean cumulates

In the absence of comprehensive seismic data coverage, the size, composition and physical state of the lunar core are still debated. It has been suggested that a dense ilmenite-rich layer, which originally crystallised near the top of the lunar magma ocean, may have sunk to the centre of the Moon to form either an outer core, surrounding a small metallic inner core, or a complete core if the lunar metallic iron content is insignificant. Here we study the formation, gravitational stability and thermal evolution of both an ilmenite-rich outer core and a full ilmenite-rich core, using a two-dimensional cylindrical thermo-chemical convection model. Gravity acceleration in the lunar mantle decreases quickly with depth. Since the gravity acceleration directly influences the buoyancy of materials, the low gravity acceleration near the centre was explicitly taken into account. Core formation and evolution were investigated by assessing the effects of varying two parameters, the Mg# (density) and the internal heat production of the ilmenite-rich layer. Varying these parameters changes the compositional and thermal buoyancy of the dense layer. Models show that a stable ilmenite-rich (outer) core may indeed have formed in the lunar interior and that its density depends on the internal heating in and the Mg# of the ilmenite-rich layer. Furthermore, the sharpness of the core-mantle boundary is shown to depend on the internal heat production in the ilmenite-rich material. Surprisingly, a higher internal heat production results in a sharper core-mantle boundary and a higher ilmenite content in the core. This is caused by lower viscosities as a result of higher temperatures. Although maximum core temperatures vary between different models by 700-1000 K around 2 Gyr after the start of the models, present-day estimates vary by only about 350-500 K. Further narrowing of the range of internal heating values is essential for a better determination of both the present day lunar core temperature and its physical state. © 2009 Elsevier B.V. All rights reserved.