The Fate of Water within Earth-like Planets and Implications for the Onset of Plate Tectonics

Introduction: Water is thought to be vital for the development of plate tectonics because it lowers viscosities in the asthenosphere and enables subduction. However, the following issue persists: if water is necessary for plate tectonics, but subduction itself hydrates the upper mantle, how is the upper mantle initially hydrated? Here we present models demonstrating that processes associated with magma ocean solidification and overturn may segregate sufficient quantities of water within the Earth’s upper mantle to induce partial melting, produce a damp asthenosphere, and thereby facilitate a rapid onset for plate tectonics. Model framework: Geochemical studies suggest that the Earth likely accreted from chondritic planetesimals (see review in [1]) that may have contained up to 20 wt. % water as evidenced from meteorites [2]. It is likely that volatiles were partially retained through planet formation and therefore that the majority of Earth’s water was acquired during accretion. In addition to delivering water, the giant impacts of late accretion created magma lakes and oceans [3], which degassed during solidification to produce a heavy atmosphere [4]. However, some water would have remained in the mantle, trapped within crystallographic defects in nominally anhydrous minerals. Magma oceans on Earth-sized planets are thought to solidify from the bottom-up because the solidus and adiabat would intersect at depth [3]. Because the magnesium ion is smaller than that of iron, magnesium is preferentially incorporated into mantle silicate minerals during the initial stages of magma ocean solidification. The remaining magma ocean liquid is increasingly enriched with dense iron and incompatible elements, yielding an unstable mantle profile with density radially increasing as solidification progresses from the bottom up. As a result, the solid mantle overturns until it reaches a gravitationally stable configuration [3]. As solidification proceeds from the bottom-up, water is preferentially incorporated up to saturation into late-crystallizing, dense cumulates that are gravitationally unstable. During overturn, these relatively waterrich cumulates sink into the lower mantle (Fig. 1). Because lower mantle minerals such as perovskite and magnesiowüstite have water saturation limits 10-100 times lower than upper mantle phases such as olivine and pyroxene, sinking cumulates must undergo dewatering as they enter the lower mantle. Figure 1: Cumulate mantle density before and after overturn for a 2000-km deep terrestrial magma ocean with an initial water content of 0.25 wt. % at solidus temperatures and a reference pressure of 1 atm. The green highlighted area denotes the lower mantle boundary (i.e., the top of the perovskite stability zone) where dewatering occurs.