Estimates of the Temperature and Melting Conditions of the Carpathian‐Pannonian Upper Mantle From Volcanism and Seismology.

What drives the formation of basaltic melts beneath intraplate volcanoes not associated with extensive thermal anomalies or lithospheric extension? Detailed constraints on the melting conditions and source region are imperative to resolve this question. Here we model the geochemistry of alkali basalts and mantle nodules brought up by young (12–0.1 Ma) intraplate volcanoes distributed across the Carpathian‐Pannonian region and combine the results with geophysical observations. Rare earth element inversion and forward calculation of elemental concentrations show that the basalts require the mantle to have undergone less than 1% melting in the garnet‐spinel transition zone, at depths of about 63–72 km. The calculated melt distributions correspond to a mantle potential temperature of ∼1257°C, equivalent to a real temperature of 1290°C at 65 km beneath the Pannonian Basin. The composition, modal mineralogy, and clinopyroxene geochemistry of some of the entrained mantle nodules closely resemble the basalt source, though the latter equilibrated at greater depths. The gravity anomalies and topography of the Basin reveal no large‐scale features that can account for the post‐extensional volcanism. Instead, the lithospheric thickness and geotherm show that melting occurs because the base of the lithosphere, at ∼50‐km depth, is close to or at the solidus temperature over a large part of the Basin. Hence, only a small amount of upwelling is required to produce minor volumes (up to a few cubic kilometers) of melt. We conclude that the Pannonian volcanism originates from upwelling in the asthenosphere just below thinned lithosphere, which is likely to be driven by thermal buoyancy. Plain Language Summary: On Earth's continents, there are surprising instances of basaltic volcanism that are not related to plate stretching or hotspots. To understand just such activity in the Carpathian‐Pannonian region, in eastern central Europe, during the last 12 million years, we studied the chemical composition of the erupted lavas. Using a computer code that takes as input the concentrations of elements in basaltic rocks, we calculated the fraction of melt and the depth at which it formed within Earth's mantle. We find that the mantle has undergone less than 1% melting in the garnet‐spinel transition zone—a region at about 63‐ to 72‐km depth where these minerals both occur. Using a model of mantle melting, we find that the temperature of the mantle at 65 km below the Pannonian Basin is about 1290°C. The plate's thickness and the temperature variations with depth tell us that the magma source region is at a temperature close to that at which melting begins (the solidus). This means that if mantle material is moved upwards just slightly, it will produce small volumes of melt that can rise to the surface. The most likely cause of such upward movement is convection in the mantle below the rigid plate. Key Points: Carpathian‐Pannonian intraplate basalts are formed by less than 1% melting in the garnet‐spinel transition zone, at depths of about 63–72 kmThe calculated melt distributions correspond to a mantle potential temperature of 1257°C beneath the Pannonian BasinMelting occurs because the base of the lithosphere (at about 1300°C) is close to the solidus temperature over a large part of the Basin [ABSTRACT FROM AUTHOR]