After over half a century since the last Apollo mission in 1972, the successful launch of the Artemis Mission will bring humans back to the Moon. This exciting new mission carries goals for the latest scientific discovery and economic opportunities for lunar exploration, and it also inspires the next generation of space missions to Mars.
Figure 1 Lunar Magma Ocean model and Mantle Overturn. The slide of demonstration is from Y. Li’s talk given to Lunar Exploration Acceleration Program – Canadian Lunar Research Network monthly meeting in 2022 March. The figure combines the demonstration from Flemming et al. (2019), published work by McCubbin et al. (2015), and Elardo et al. (2011).
While the new scientific findings about the Moon from Artemis await, recent discoveries from other missions, e.g., Chang’E 5 in 2021 and lunar meteorites, also bring fresh perspectives to decode the petrogenesis mystery of the Moon. However, the inconsistency between the new findings from the Chang’E mission and lunar meteorites and the legacy Lunar Magma Ocean model developed by Apollo and Luna Mission exists, urging us to re-examine the current LMO model. Herein, the Artemis Mission provides an excellent opportunity to reinforce our understanding of the Moon. Before it arrives, it is worthwhile to summarize the current progress for lunar exploration and discuss the inconsistency between current findings and the legacy LMO model. This article will be going over the current petrogenesis theory of the Moon and its mineralogical, geochemical, and geochronological paradox with the lunar meteorites and new basalts found by the Chang’E mission.
The Moon is thought to form around 4.6 billion years ago by re-accreting the ejecta from a giant impact between a Mars-sized body with the proto-Earth. The giant impact resulted in a wholly or largely molten proto-Moon. The molten state and the subsequent crystallization are often termed as the lunar magma ocean (LMO) model. The model is largely developed by the geochemistry and mineralogy of Apollo and Luna samples from the Oceanus Procellarum Terrain (Canup and Asphaug, 2001, Tanton et al., 2002). The current paradigm of the LMO model is that the molten Moon differentiated into a solid metallic core first, followed by an olivine-pyroxene-rich silicate mantle. In the end, an ancient ferroan anorthositic highland crust was crystallized from the magma ocean. The light anorthositic highland crust is ancient and floated on the top of the denser residue that was enriched in incompatible elements (e.g., titanium, Ti and potassium, K, rare earth elements (REE), and phosphorous P, yielding the KREEP composition). In the later stage of the Moon’s evolution, the gravitational instability between heavy ilmenite-rich KREEP cumulates and light mantle lithologies caused the mantle overturn and re-melt the Moon’s interior. This magmatic activity was thought to be global and formed the magnesian rock and alkali-feldspar rocks (Mg-suite) discovered by Apollo and Luna missions in Oceanus Procellarum Terrain (Tanton et al., 2002, Gross et al., 2020). Because this magmatism is thought to have involvement of KREEP mantle residues, the Mg-suites are believed to have a “KREEP” signature as well. This secondary global magmatism is believed to cease around the same age as the KREEP formation (~4 Ga). By remote sensing and the cratering analysis, it suggests that the Moon is likely to have younger magma activities; however, we lack the actual samples to support such young magma activities, nor can we explain the potential heat source for the regional or prolonged young magma generation.
Last year, Chang’E 5 discovered a basalt unit on the Moon within Oceanus Procellarum Terrain that had an age as young as 1.9 – 2.1Ga (Che et al., 2021, Li et al., 2021). Moreover, this basalt unit does not have similar KREEP signatures as the basalts discovered by Apollo and Luna missions, indicating that the Chang’E basalts are crystallized independently from the KREEP formation (Che et al., 2021, Li et al., 2021). This finding is contradictory to the current lunar magma ocean model that, so far, there is no explanation for such abnormally young lunar basalts in terms of its potential heating source and its protolith. It was speculated that the high abundance of radiogenic elements might provide thermal heat by radioactive decay. However, such a hypothesis was not valid for the Chang’E basalt because the preliminary geochemistry analysis showed that the Chang’E basalt unit did not contain much higher radiogenic elements than other discovered units. The radiogenic element abundance in Chang’E basalt cannot sustain the magmatic activity to melt the protolith rock and reproduce the basalt. The other hypothesis is that it was formed by the impact melting process that shock wave released enormous energy, transferring the kinetic energy to heat. So far, this hypothesis requires more investigation in relict grains in Chang’E basalts to determine if they experienced such impact history, resulting in the shock-melting metamorphism.
Figure 2. Scanned Image and R-G-B False Color Map for Lunar Meteorite NWA 11515 Figure is from Li et al., (2022, LPSC Proceeding) and Li et al., (2022 MAPS in submission). Fig. 2A is the scanned image showing different morphology and color for different clasts in the sample. Fig. 2B is the R-G-B false color map for Ca-Fe-Si using micro-X-ray fluorescence scan. Rough mineralogy can be estimated by the color scheme: purple color indicates felsic mineral, probably anorthite; green color indicates mafic minerals, probably olivine and pyroxene. More detail can refer to Li et al., (2022 LPSC Proceeding) and Li et al., (2022 MAPS, in submission).
Besides the Chang’E basalt, mineralogical and geochemical findings from recently discovered lunar meteorites also posed concerns for us to revise the lunar magma ocean model. In the recent work by Li et al. (2022, MAPS submitted), they reported a hybrid lunar meteorite Northwest Africa 11515 that has mixed lithology, potentially containing the lithology from the far side of the Moon or the lithic clasts had not been recognized by the current LMO model. In detail, they reported the composition of various spinel group oxides, changing from chromite to ulvöspinel and to spinel. The Al-rich spinel is compositionally similar to spinel species reported in Allan Hill A81005 (Li et al., 2022 MAPS submitted, Gross et al., 2014). In general, chromite is one of the most common spinel group oxides reported in lunar rocks, especially in lunar mare basalts. Al-rich spinel is rare but has been identified in various meteorites, for example, in Apollo 14 basalts and in meteorites like NWA 11515 and ALHA81005 (Prissel et al. 2014; Steele 1972; Taylor et al. 2004; Li et al., 2022 MAPS submitted, Gross et al., 2014). The exact formation of Al-spinel is unclear, but one possible hypothesis is that Al-spinel is formed by picritic magma assimilation of the ancient anorthositic crust and the anorthite in the crust provided the sufficient Al source (Li et al., 2022 MAPS submitted, Gross et al., 2014). Recent Moon Mineralogy Mapper (or M3) identified a region on the Moon where it has a similar composition to the spinel being identified in the lunar breccia (Li et al., 2022 MAPS submitted, Gross et al., 2014). This region may be the potential source region for these Al-spinel grains. However, there are questions that still need to be answered in terms of the timing of the assimilation process, the protolith of the picritic magma, and the potential relationship of this formation to other lunar units. More investigations on lunar meteorites and the returned samples are needed in order to address these questions.
The other notable finding in these meteorites is that they lack of KREEP signatures as well. In ALHA81005, Gross et al. (2014) reported its mineralogy and geochemistry. The meteorite has Mg-rich clasts forming troctolitic clasts; however, the sample does not contain any KREEP signatures. It has been argued a “simple-mixing” scenario of ferroan anorthites with Mg-suite rocks to form such lithology; however, if that’d be the case, the sample should show some level of geochemical affinity to KREEP formation. Here, the high magnesian clasts in ALHA81005 have been argued to form independently from KREEP. Probably the mantle directly melted the deep crustal materials. However, more investigation is needed.
Similar to ALHA 81005, NWA 11515 studied by Li et al. (2022, submitted to MAPS) is also suspected of having no KREEP signatures. They reported high magnesian clasts, for example, olivine with forsterite content up to 80-82 mole percent; nevertheless, they did not observe any sodic or alkali-feldspar that the plagioclase composition remains unchanged at all with anorthite composed of 96-mole percent. Multiple shock stages have been identified in different clast types, which can be responsible for hybrid lithic clasts observed in this sample. The other notable texture reported in NWA 11515 is the orthopyroxene-augite exsolution clasts. Orthopyroxene is crystallized at low temperatures with a slow cooling speed. Pigeonite is stable at a higher temperature, which is preserved during the fast cooling process. Herein, the orthopyroxene exsolution, instead of pigeonite, indicated the clast was crystalized with a slow cool speed and formed in the deep crust. The shock level they estimated from the orthopyroxene clast is up to S5, which is reasonable for a deep crustal origin for the clast that was excavated during the impact cratering event.
With the current investigation on lunar meteorites and findings from Chang’E basalt, a conclusion can be drawn that the KREEP formation is not necessary for the later lunar magmatism. It is likely that the volcanic activities were (or are) still active for another 2 billion years after KREEP-involved magmatism ceased. However, it is unclear the driven heat source for such local or regional magma activity to happen. The mineralogical and geochemical inconsistency in the new lunar samples requires us to revise the legacy lunar magma ocean model. Shock and mineralogical study in NWA 11515 provide circumstantial evidence for the impact-driven deep-crustal origin magmatism, which provides an alternative explanation of the formation of high magnesian clasts in KREEP-depleted rocks. Nevertheless, more evidence is needed to conclude such a hypothesis. With the recently launched Artemis Mission, we are getting closer to make more breakthrough discoveries about our neighbor, the Moon. To understand the Moon is to understand more about the Earth and the solar system. The mission will further help us to probe other terrestrial planets and asteroid bodies in the solar system.
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