Constraints on the depths and temperatures of basaltic magma generation on Earth and other terrestrial planets using new thermobarometers for mafic magmas

Basaltic magmatism is a common feature of dynamically active terrestrial planets. The compositions of basalts reflect the temperatures and pressures of magma generation, providing windows into a planet's thermal state. Here, we present new thermobarometers based on magma Si and Mg contents to estimate the pressures and temperatures of basaltic magma generation on Earth and other terrestrial planets. Melting on Earth is intimately tied to plate tectonics and occurs mostly at plate boundaries: mid-ocean ridges and subduction zones. Beneath ridges, melting is driven by adiabatic decompression of passively upwelling mantle at 1300-1400 degrees C. Similar temperatures of melting are found for some arcs, suggesting that decompression melting is also important in arcs and that enhanced melting by hydrous fluxing is superimposed on this background. However, in arcs where melting temperatures are low (1200 degrees C), hydrous fluxing is required. Temperatures hotter than ridges (>1400 degrees C) are primarily found away from plate boundaries: beneath thick continental lithosphere and oceanic hotspots like Hawaii. Oceanic hotspots are thought to derive from deep thermal upwellings (plumes), but some hot anomalies beneath continents are not associated with deep-seated plumes and hence must have different origins, such as thermal insulation or radioactive heating of metasomatized zones. Melting on Venus, as constrained from spectral data of its surface, occurs at higher temperatures (1500 degrees C) and pressures than on Earth, perhaps because Venus is characterized by a thick and stagnant upper thermal boundary layer that retards convective heat loss. In this regard, Venus' upper thermal boundary layer may be analogous to thick continents on Earth. Mars appears to have cooled off to <1300 degrees C within its first billion years, but considerable controversy exists over the interpretation of young (<500 My) basaltic meteorites that record temperatures of 1550 degrees C. As for the first billion years of Earth's history, its upper mantle was hotter than 1700 degrees C, hence melting commenced at pressures greater than 7 GPa, where melts could have been denser than residual solids, resulting in downward fertilization of the Earth's mantle. (C) 2008 Elsevier B.V. All rights reserved.