Browsing by Author "Richards, Mark"
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Item Buffering of mantle conditions through water cycling and thermal feedbacks maintains magmatism over geologic time(Springer Nature, 2022) Seales, Johnny; Lenardic, Adrian; Richards, MarkThe Earth has remained magmatically and volcanically active over its full geologic history despite continued planetary cooling and a lack of thermal equilibrium in the mantle. Here we investigate this conundrum using data-constrained numerical models of deep-water cycling and thermal history. We find that the homologous temperature - the ratio of upper mantle to melting temperatures - initially declined but has been buffered at a nearly constant value since 2.5-2.0 billion years ago. Melt buffering is a result of the dependence of melting temperature and mantle viscosity on both mantle temperature and water content. We show that thermal and water cycling feedbacks lead to a self-regulated mantle evolution, characterised by a near-constant mantle viscosity. This occurs even though the mantle remains far from thermal equilibrium. The added feedback from water-dependent melting allows magmatism to be co-buffered over geological time. Thus, we propose that coupled thermal and water cycling feedbacks have maintained melting on Earth and associated volcanic/magmatic activity.Item Influence of the asthenosphere on earth dynamics and evolution(Springer Nature, 2023) Cathles, Lawrence; Fjeldskar, Willy; Lenardic, Adrian; Romanowicz, Barbara; Seales, Johnny; Richards, MarkThe existence of a thin, weak asthenospheric layer beneath Earth’s lithospheric plates is consistent with existing geological and geophysical constraints, including Pleistocene glacio-isostatic adjustment, modeling of gravity anomalies, studies of seismic anisotropy, and post-seismic rebound. Mantle convection models suggest that a pronounced weak zone beneath the upper thermal boundary layer (lithosphere) may be essential to the plate tectonic style of convection found on Earth. The asthenosphere is likely related to partial melting and the presence of water in the sub-lithospheric mantle, further implying that the long-term evolution of the Earth may be controlled by thermal regulation and volatile recycling that maintain a geotherm that approaches the wet mantle solidus at asthenospheric depths.