Tamblyn,Renée1, Hasterok, Derrick1, Hand, Martin1, Gard,Matthew1
1Department of Earth Sciences, the University of Adelaide, South Australia, Australia
The thermal state of the solid Earth determines the interactions between the mantle and the crust. The only way to probe the thermal conditions of the ancient Earth is from the mineralogical and geochemical record of thermally-driven processes, i.e. metamorphism and magmatism. The generally accepted model for the thermal budget of the Earth balances heat accumulated from accretion and the decay of heat producing elements, and indicates an overall cooling trend from ca. 3 Ga to present, encompassing the emergence of modern plate tectonics. The geological record however indicates this simple cooling model may not hold true. Thermally sensitive metamorphic mineral assemblages, such as eclogites, emerge in the rock record transiently from 2.2–1.8 Ga, and disappear again until ca. 0.8 Ga. Coincident with this transient emergence of eclogite, the global record of arc granite chemistry also shows significant step changes, most notably decreased Sr and Eu and increased Y and rare earth element concentrations, from 2.0–1.8 Ga, both of which point to a global increase in thermal gradients that intersected granite genesis. We suggest these changes occurred as the secular cooling of the mantle and crust was reversed by a net increase in the spatial extent of continental crust between 2–1.8 Ga, resulting in thermal insulation of the mantle. The following 1.2 billion years on Earth was dominated by a warm, insulated mantle and crust, maintained by stable continental volumes, which eventually cooled to allow the second emergence and widespread preservation of eclogites from ca. 0.8 Ga until present. While novel, this idea combines unrelated global petrological and geochemical datasets to explore the sensitivity of switches in the thermal evolution of the solid Earth.
Renee is finishing her PhD at the University of Adelaide. Her early PhD focussed on the pressure-temperature-time evolution of high-pressure rocks formed in the subduction channel. More recently, she has looked into the emergence of these subduction-related rocks on Earth.