Exceptionally Low Thermal Conduction of Basaltic Glasses and Implications for the Thermo‐Chemical Evolution of the Earth's Primitive Magma Ocean

The thermal properties of the Earth's primordial magma are the key factors that constrained crystallization and other thermo‐chemical processes in Earth's primitive magma ocean and therefore controlled the Earth's long‐term evolution. Thermal conductivity of the primordial magma is conventionally assumed to be a constant of about 4 W m−1 K−1 under the high pressure‐temperature conditions of the primitive magma ocean. Here we measured the lattice thermal conductivity of a variety of basaltic and silicate glasses at high pressures and a wide range of temperatures. Our results suggest that the primordial magma, if it is indeed represented by basaltic melts, had a thermal conductivity of ∼1.0–1.9 W m−1 K−1, much lower than previously thought. Such low thermal conduction reduced heat loss and thus prolonged the cooling time of the early magma ocean, promoting convection in the solidifying mantle and preventing a global overturn. Moreover, if the seismic ultralow velocity zones presently observed in the lowermost mantle are made of basaltic melts, originating either from remnants of the primitive magma ocean or pieces of subducted crust, the material in these zones must have an ultralow thermal conductivity, which would reduce cooling and thus influence the thermo‐chemical evolution of the present day core‐mantle boundary. Plain Language Summary It is believed that a global magma ocean existed in the early Earth's interior. The thermo‐chemical evolution of the primitive magma ocean holds the key to understand the Earth's subsequent evolution and even its present day structure. Thus, knowledge of the fundamental physical properties of the primitive magma ocean, in particular, the thermal properties, would significantly advance our comprehension of our planet's history. Here, we precisely determined the thermal conductivity of a series of basaltic and silicate glasses at the high pressure‐temperature conditions assumed to be characteristic of the primitive magma ocean. Our results suggest that the primitive magma ocean, if it indeed consisted of basaltic melts, had a thermal conductivity much lower than previously assumed, which substantially slowed down cooling and allowed convection of the magma ocean before it entirely solidified. Furthermore, if the ultralow seismic velocity zones observed at present are remnants of the primitive magma ocean, their exceptionally low thermal conductivity should hinder heat transfer and thus strongly affect the thermo‐chemical s