Constraints on the Depths and Temperatures of Basaltic Magma Generation on Earth and Other Terrestrial Planets

Basaltic magmatism is a common feature of dynamically active terrestrial planets. Because the compositions of basalts reflect temperatures and pressures of magma generation, these basalts are windows into a planet's thermal state. Here, we present thermobarometric results based on Si-barometry and Mg-thermometry to estimate the pressures and temperatures of basaltic magma generation on Earth and other terrestrial planets. Most melting on Earth occurs in two very different environments, mid-ocean ridges and subduction zones, yet the temperatures and pressures of melting are similar between the two. Melting in these two environments is mostly driven by decompression associated with solid state convection. Only in the case of cold subduction zones is the influence of hydrous flux melting important in terms of total amount of melt produced. Melting on Venus occurs at higher temperatures and higher pressures than on Earth, perhaps because Venus is characterized by a thick and stagnant upper thermal boundary layer that retards convective heat loss. Earth, in contrast, has plate tectonics and hence a mobile lid, but thick continents on Earth may represent local analogs of stagnant lids as intraplate magmas on Earth tend to be hot, even when there is no obvious expression of a thermal plume. Mantle potential temperatures of active planetary bodies, including the Moon and Mars when they were active, all appear to be slightly above the melting point of the respective planetary mantles. The similar temperatures of active planets may be due to buffering by the phase change associated with melting and the negative feedbacks associated with rapid transport of the latent heat of melting to a planet's surface by volcanic eruptions.