Earth's mantle is known to harbor water in the form of hydrous and nominally anhydrous minerals. How much water the mantle holds and whether it has remained constant through time are open questions. Previous numerical studies of the deep‐water cycle have been limited to box models or 2‐D calculations. Here we present for the first time results from 3‐D mantle convection models. We address the evolution of the mantle's total water content by adapting a well benchmarked mantle convection code to track water, including its feedbacks on dynamics. While Earth's surface is presently covered by one ocean mass of water, our results suggest that the mantle holds approximately two ocean masses of water based on the best estimates from mineral physics. This value varies only weakly for a wide parameter space of additional complex dynamics such as viscosity laws, density controls, and phase change considerations. Our result of a mantle holding two ocean masses conforms with estimates from other branches of Earth science, suggesting that these models could be an excellent tool in understanding the spatial heterogeneity of the water found in the mantle.
Seismic tomography models have discerned the presence of two mantle structures close to the core-mantle interface characterized by low seismic velocities. These large low-shear velocity provinces (LLSVPs) sit beneath Africa and the Pacific plate and exhibit a shear velocity anomaly of δlnV s ≈ −2%. Spatially, they are both voluminous and broad (Cottaar & Lekić, 2016), with seismic tomography models showing high power in spherical harmonic degree 2 and to a lesser extent degree 3 structures (Koelemeijer et al., 2016;Ritsema et al., 2011) in the lower mantle. These structures exist within and above the D" layer and may extend >1,000 km into the mantle from the core-mantle boundary (CMB) (He & Wen, 2009;Lekic et al., 2012), with the African LLSVP being significantly taller than the Pacific LLSVP (Ni et al., 2002;Yuan & Li, 2022). Mantle plumes are claimed to cluster around their edges (Thorne et al., 2004) potentially entraining material to be sampled by melting beneath ocean islands. Despite the morphology of LLSVPs being well constrained, increasingly accurate measurements of their material properties and possible direct links to the surface via plumes, their origin and composition remain a matter of debate. This is because the very characteristics that define LLSVPs have a non-unique source; the velocity anomaly may originate thermally, chemically, or thermo-chemically. Consequently there are competing theories over the nature of LLSVPs, are they of a predominantly thermal origin (D. R. Davies et al., 2015;
For mid‐ocean ridge basalts and ocean island basalts, measurements of Pb isotope ratios show broad linear correlations with a certain degree of scatter. In 207Pb/204Pb—206Pb/204Pb space, the best fit line defines a pseudo‐isochron age (τPb) of ∼1.9 Gyr. Previous modeling suggests a relative change in the behaviors of U and Pb between 2.25 and 2.5 Ga, resulting in net recycling of HIMU (high U/Pb) material in the latter part of Earth's history, to explain the observed τPb. However, simulations in which fractionation is controlled by a single set of partition coefficients throughout the model runs fail to reproduce τPb and the observed scatter in Pb isotope ratios. We build on these models with 3D mantle convection simulations including parameterizations for melting, U recycling from the continents and preferential removal of Pb from subducted oceanic crust. We find that both U recycling after the great oxygenation event and Pb extraction after the onset of plate tectonics, are required in order to fit the observed gradient and scatter of both the 207Pb/204Pb—206Pb/204Pb and 208Pb/204Pb—206Pb/204Pb arrays. Unlike much previous work, our model does not require accumulations of subducted oceanic crust to persist at the core‐mantle boundary for long periods of time in order to match geochemical observations.
For mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs), measurements of Pb isotope ratios show broad linear correlations with a certain degree of scatter. In 207Pb/204Pb - 206Pb/204Pb space, the best fit line defines a pseudo-isochron age (τPb) of ~1.9 Gyr.Previous modelling suggests a relative change in the behaviours of U and Pb between 2.25-2.5 Ga, resulting in net recycling of HIMU (high U/Pb) material in the latter part of Earth's history, to explain the observed τPb. However, simulations in which fractionation is controlled by a single set of partition coefficients throughout the model runs fail to reproduce τPb and the observed scatter in Pb isotope ratios. We build on these models with 3D mantle convection simulations including parameterisations for melting, U recycling from the continents and preferential removal of Pb from subducted oceanic crust.We find that both U recycling after the great oxygenation event (GOE) and Pb extraction after the onset of plate tectonics, are required in order to fit the observed gradient and scatter of both the 207Pb/204Pb - 206Pb/204Pb and 208Pb/204Pb - 206Pb/204Pb arrays. Unlike much previous work, our model does not require accumulations of subducted oceanic crust to persist at the CMB for long periods of time in order to match geochemical observations.
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