Emergence of blueschists on Earth linked to secular changes in oceanic crust composition

The oldest blueschists - metamorphic rocks formed during subduction - are of Neoproterozoic age¹, and 0.7-0.8 billion years old. Yet, subduction of oceanic crust to mantle depths is thought to have occurred since the Hadean, over 4 billion years ago². Blueschists typically form under cold geothermal gradients of less than 400 °C GPa^-1, so their absence in the ancient rock record is typically attributed to hotter pre-Neoproterozoic mantle prohibiting such low-temperature metamorphism; however, modern analogues of Archaean subduction suggest that blueschist-facies metamorphic conditions are attainable at the slab surface³. Here we show that the absence of blueschists in the ancient geological record can be attributed to the changing composition of oceanic crust throughout Earth history, which is a consequence of secular cooling of the mantle since the Archaean⁴. Oceanic crust formed on the hot, early Earth would have been rich in magnesium oxide (MgO). We use phase equilibria calculations to show that blueschists do not form in high-MgO rocks under subduction-related geothermal gradients. Instead, the subduction of MgO-rich oceanic crust would have created greenschist-like rocks-metamorphic rocks formed today at low temperatures and pressures. These ancient metamorphic products can hold about 20% more water than younger metamorphosed oceanic crust, implying that the global hydrologic cycle was more efficient in the deep geological past than today.