Global MORBs reveal a warmer present-day ambient mantle and moderate secular cooling of Earth

Earth’s mantle temperature controls melting, crust formation and planetary cooling, but its present-day ambient value remains debated. Most geodynamic models assume ~1350 ± 50 °C, calibrated indirectly from experiments and the 6–7 km thickness of oceanic crust. Here we show, using primary-magma thermobarometry on >16,000 mid-ocean ridge basalts—seafloor lavas erupted where tectonic plates diverge—that only ~1% preserve primitive compositions that constrain mantle temperature. Under reduced redox conditions, these melts imply ~1421 °C, a value that would generate crust much thicker than seismic observations unless part of the melt is retained or crystallizes below the seafloor. More oxidized conditions lower estimates to ~1390 °C, reconciling basalt chemistry, crustal thickness and thermobarometers. Comparison with Palaeoproterozoic basalts suggests <150 °C of mantle cooling over 2 billion years. Present-day mantle is therefore modestly warmer than the canonical assumption and is better described by a dominant temperature range, with variability, than by a single value. Ganne et al. show that Earth’s mantle beneath modern mid-ocean ridges is hotter and more thermally heterogeneous than commonly assumed, refining our view of how the planet has cooled through time.