Effects of water on the evolution of the Early Moon and deep Earth investigated by experiments
Authors: Lin Yanhao 
Published: March 2024

The Moon is thought to have been covered initially by a deep magma ocean, its gradual solidification leading to the formation of the aluminium-rich plagioclase-bearing highland crust. We performed the first high-pressure, high-temperature experimental study of the mineralogical and geochemical evolution accompanying the full solidification of a lunar magma ocean (LMO) and provide new constraints on the presence of water in the early lunar interior. In a dry Moon, plagioclase appears after 68 per cent solidification and yields a crust with a thickness of ~68km1, well above the lunar crustal thickness suggested by recent GRAIL mission gravitational data (34&#8211;43 km). Water-bearing experiments show a delay in the start and lowering of the volume of plagioclase formed during LMO crystallization, as observed previously for terrestrial magma. Using crustal thickness as a hygrometer we conclude that at least ~800 ppm water was present in the Moon at the time of LMO crystallization, indicating the Earth-Moon system was water-rich from the start2,3.
Water does not only have remarkable effects on early LMO evolution, but also on the properties of Earth&#8217;s mantle rocks, however, how and how much transporting water into the deep Earth is still debated due to high mantle temperatures. Subduction of oceanic lithosphere transports surface H2O into the mantle. Recent studies proved that stishovite and post-stishovites as high-pressure phases of SiO2 have the potential to carry weight percent levels of water into the Earth&#8217;s interior along the geotherm of the subducting oceanic crust4. Regression of our experimental data indicates an H2O storage capacity in stishovite of ~3.5 wt% in the transition zone and shallow lower mantle, decreasing to about 0.8 wt% at the base of the mantle5. As slabs subducted to the deepmost mantle, dehydration of these dense hydrous silica phases (DHS) can potentially change physicochemical properties of the Earth&#8217;s mantle by reducing melting point, forming new high-pressure phases and enhancing the oxygen fugacity heterogeneity of lower mantle6.
References:
[1] Lin Y., Tronche E. J., Steenstra E. S., and van Westrenen W. Experimental constraints on the solidification of a nominally dry lunar magma ocean. Earth Planet. Sci. Lett. 471, 104&#8211;116 (2017).
[2] Lin Y., Tronche E. J., Steenstra E. S., and van Westrenen W. Evidence for an early wet Moon from experimental crystallization of the lunar magma ocean. Nat. Geosci. 10, 14&#8211;18 (2017).
[3] Lin Y., Hui H., Xia X., Shang S., and van Westrenen W. Experimental constraints on the solidification of a hydrous lunar magma ocean. Meteorit. Planet. Sci. 55, 207&#8211;230 (2020).
[4] Lin Y., Hu Q., Meng Y., Walter M. and Mao H.-K. Evidence for the stability of ultrahydrous stishovite in Earth&#8217;s lower mantle. Proc. Natl. Acad. Sci. U. S. A. 117, 184&#8211;189 (2020).
[5] Lin Y et al. Hydrous SiO2 in subducted oceanic crust and H2O transport to the core-mantle boundary. Earth Planet. Sci. Lett. 594, 117708 (2022).
[6] Lin Y. and Mao H. K. Dense hydrous silica carrying water to the deep Earth and promotion of oxygen fugacity heterogeneity. Matter Radiat. Extremes 7, 068101 (2022).