LAUSR

Abstract Among the Apollo returned samples were basaltic rocks belonging to the KREEP suite, which are thought to have been erupted ~4.1–3.9 ​Ga via dike propagation from the residual magmatic layer residing below the lunar crust as the last part of the solidifying lunar magma ocean. The emplacement dynamics of KREEP magma reaching the lunar surface are generally poorly understood, in part because no rheological data are available to model the conditions of KREEP ascent in detail, even ~50 years after KREEP basalt discovery. This study provides the first rheological data for KREEP magmas, as a function of temperature and crystal content, from the liquidus temperature (~1205 ​°C) to ~16% crystals by volume at ~1177 ​°C. The lava’s rheological behavior becomes non-Newtonian at just a few degrees of cooling below the liquidus and is well represented by a shear-thinning power-law equation. We applied these data to a buoyancy-driven dike ascent model, suggesting that if KREEP magma ascended through dikes comparable in size to those that fed the lunar mare, it most likely erupted below its liquidus in the range of 1180 ​°C to 1160 ​°C, with effective viscosities in the range 103–107 ​Pas and traversing the lunar crust on timescales of minutes to hours.