# Sources of Extraterrestrial Rare Earth Elements: To the Moon and Beyond
## 4 Lunar Differentiation  
### The Lunar Magma Ocean  
Today, the Moon is a differentiated planetary object with a core, mantle, and crust (Figure 3a). The present day internal architecture of the Moon has been constrained to: an upper mantle from 60 to 400 km; a mantle transition zone from 400 to 800 km; a lower mantle from to (at least) 1100 km; a fluid outer core (350 km radius); and a solid inner core (160 km radius, Figure 3a: [63; 64; 65]). In the absence of plate tectonics, the lunar crust and mantle have remained physically separate for the past \(\sim\)4 billion years [64; 66]. In order to better evaluate potential lunar resources, the geochemical differentiation processes associated with lunar formation and evolution need to be understood [67]. Fundamental to recent interpretations of lunar evolution, is the Lunar Magma Ocean (LMO) model. Following formation of the Moon, _c._ 70-110 Myrs after the onset of Solar System formation through a giant impact between proto-Earth and an impacting body (Mars-sized Theia), molten material rapidly accumulated and began to solidify [33; 34; 35; 68]. Previous studies have investigated the extent to which the Moon was initially molten with models ranging from scenarios in which the initial LMO depth was as shallow as 400 km [41], molten to 1000 km [69], to a scenario where the whole Moon was initially molten [50]. Figure 3: **(a)** Schematic cross section of the lunar interior (modified from [64; 70]). Results from the GRAIL mission reveal an average crustal thickness of between 34 and 43 km, with impact basins exhibiting thicknesses near 0 km (the Moscoviense and Crisium basins [71]). The Moon’s largest and oldest basin is the \(\sim\)13 km deep South Pole Aitken Basin located on the lunar far side. From GRAIL, crustal thicknesses here are estimated at \(<\)5 km [71; 72]. Moonquakes have been detected at both shallow (50–220 km) and deep (800–1000 km) depths within the lunar mantle. Seismic velocities have identified a distinct discontinuity at \(\sim\)500 km, which has been associated with a phase change from spinel to garnet  
(300-500 km, e.g., [73]). One hypothesis has been that this ~500 km discontinuity represents the extent of the mare basalt mantle source region; (**b**) Schematic summary of the LMO model showing dense settling out of early formed olivine and pyroxene. These cumulates sink to the lunar interior and form the source regions to the younger mare basalts. Following ~75-80% LMO crystallization, Ca-rich plagioclase feldspar is a liquidus phase, and being less dense than the surrounding mantle, forms an anorthitic flotation crust. Following ~99% solidification, the ITEs are concentrated in the last remaining dregs of the LMO and form the urKREEP reservoir from which KREEP-like signatures are inferred to originate. The depth to which an LMO on a primordial Moon existed has been debated for decades (whole Moon vs. partial Moon melting). For a summary of previous models, see [70]. The presence of magma oceans (or magmaspheres) on early formed planetary bodies has been debated and widely discussed throughout the planetary science community for decades [39; 41; 51; 74]. The concept of a MO on an early formed Moon has established the context in which the geochemical and geochronological history of Earth's nearest neighbor in space is evaluated today. Simply, the widely-accepted sequence for crystallization of the LMO is the following: olivine \(\rightarrow\) orthopyroxene \(\pm\) olivine \(\rightarrow\) olivine \(\rightarrow\) clinopyroxene \(\pm\) plagioclase \(\rightarrow\) clinopyroxene + plagioclase \(\rightarrow\) clinopyroxene + plagioclase + ilmenite [75]. As a young Moon solidified, the first phases to fractionally crystallize out (olivine and pyroxene, \(\pm\) Fe-Ti oxides) were denser than the surrounding magma and sank towards the lunar interior (Figure 3b). Following ~75-80% solidification, low-density plagioclase feldspar became a liquidus phase and buoyantly rose to form an anorthitic feldspathic flotation crust (Figure 3b).