Deep-water cycling and the Magmatic History of the Earth

Abstract

Earth is a magmatically active planet. Magmatism connects Earth's interior to its atmosphere, hydrosphere, and biosphere through cycling of volatiles, greenhouse gasses, and nutrients. Earth's magmatic history is intertwined with its thermal and tectonic evolution. How magmatism has evolved and been maintained in the face of planetary cooling remains an open question. We address this question using data-constrained deep-water cycling and thermal history models. We track magmatic potential using a homologous temperature: the ratio of upper mantle to melting temperatures. After an initial decline, homologous temperature is buffered at a nearly constant value from roughly 2.5-2.0 Ga to the present day. Melt buffering reflects two factors: 1) The dependence of melting temperature on water content, and 2) The dependence of mantle viscosity on temperature and water content. The latter allows solid Earth evolution to self-regulate via feedbacks that keep mantle viscosity at a near constant value. Self-regulation occurs even though the mantle remains far from thermal equilibrium, consistent with heat flow data. The added feedback from water-dependent melting allows magmatism to be co-buffered over geological time. This indicates that coupled thermal and water cycling feedbacks have maintained melting on Earth and associated volcanic/magmatic activity. Magmatic self-regulation affects not only the lifetime of geological activity on Earth but also, to the degree that planetary life connects to volcanic activity, the maintenance of conditions favorable for life.