K- and Na-rich davemaoite inclusion in diamond is not inherited from deeply subducted oceanic crusts

Abstract

Deep potassium (K) and sodium (Na) cycles can affect the physiochemical properties and geodynamic processes of the Earth's interior, to some extent. Recently reported natural K- and Na-rich davemaoite inclusion in superdeep diamond (Tschauner et al., 2021), which is still controversial (Walter et al. 2022a; Tschauner et al., 2022), motivated us to reinvestigate the recycling and partitioning behaviours of K and Na in the subducting oceanic crust under deep-mantle P-T conditions. In this study, we experimentally determined the partitioning of Na and K, as well as other major elements, in a natural phengite-rich eclogite system under 5–30 GPa and 850–1550 °C, relevant to P-T conditions from the deep upper mantle to the uppermost lower mantle. We found that the produced davemaoite with the perovskite structure contained minimal amounts of other elements, including K and Na. Therefore, we conclude that K- and Na-rich davemaoite inclusion in the superdeep diamond cannot be inherited from deeply subducted oceanic crusts. In addition, our experimental results further support previous arguments overestimating the K and Na, and other elemental contents in natural davemaoite, which may further indicate that the reported diamond “inclusion” with abnormally high heteroatoms (Tschauner et al., 2021) is not a davemaoite from the lower mantle (Walter et al. 2022a). Finally, we combined the high-pressure phase relationship in the upper continental crust, oceanic basalt, and pyrolite systems from previous studies, and constructed a geodynamical model for the deep K and Na cycles along with the subduction of the oceanic slabs into the uppermost lower mantle.