Geothermal gradient change during subduction recorded by ultrahigh-pressure eclogite

Abstract The subduction of crustal rocks to, and exhumation from, depths greater than 90 km in the stability field of the mineral coesite is known as ultrahigh-pressure metamorphism, widely regarded as evidence for cold subduction. This assumption stems from peak metamorphic conditions of ultrahigh-pressure rocks, which indicate low subduction gradients of less than 330 °C GPa −1 , but direct evidence for such a cold prograde evolution is lacking. Here we report the discovery of a young ultrahigh-pressure eclogite in eastern Papua New Guinea. We find (sub)micrometre-sized inclusions of coesite in intermediate growth zones of garnet crystals and zircon inclusions revealing a U–Pb age of approximately 4.4 million years. Petrographic observations and thermo-barometric methods show that the prograde subduction path was initially warm with an apparent average gradient of approximately 470 °C GPa −1 until a depth of about 45 km, before switching to a cold thermal regime with ultrafast subduction rates of greater than 60 mm yr −1 (vertical component). This change in geothermal gradients can be explained by shear heating and/or by formation during the non-steady-state phase of a young subduction zone. We propose that high geothermal gradients at shallow levels of deep subduction systems might be common, challenging the use of peak metamorphic conditions as indicator for subduction zone thermal structures.