Influence of continental growth on mid‐ocean ridge depth

Abstract

The interconnectedness of life, water, and plate tectonics is strikingly apparent along mid‐ocean ridges (MOR) where communities of organisms flourish off the disequilibrium of chemical potentials created by circulation of hydrothermal fluids driven by Earth's heat. Moreover, submarine hydrothermal environments may be critical for the emergence of life on Earth. Oceans were likely present in the Hadean but questions remain about early Earth's global tectonics, including when seafloor spreading began and whether mid‐oceanic ridges were deep enough for maximum hydrothermal activities. For example, plate tectonics influences global sea level by driving secular variations in the volume of ocean basins due to continental growth. Similarly, variations in the distribution of seafloor age and associated subsidence, due to assembly and dispersal of supercontinents, explain the largest sea level variation over the past 140 Myr. Using synthetic plate configurations derived from numerical models of mantle convection appropriate for early Earth, we show that MOR have remained submerged and their depths potentially constant over geologic time. Thus, conditions in the early Earth existed for hydrothermal vents at similar depths as today, providing environments conducive for the development of life and allowing for processes such as hydrothermal alteration of oceanic crust to influence the mantle's geochemical evolution.

Gravitational Potential Energy per Unit Area as a Constraint on Archean Sea Level

“…Consistent with inferences drawn by de Wit and Hynes (), Galer (), and Sim et al () from assumptions somewhat different from those made here, the results shown in Figure suggest that only in the most extreme of plausible conditions might Archean spreading centers have been emergent. Specifically, if heat loss were Q = 3.0 Q 0 , spreading centers would have been submerged.…”

“…(3) Spreading centers were emergent, and subaerial weathering preferentially extracted 18 O from the basic crust, so that water entering the ocean would have been depleted in 18 O (e.g., Jaffrés et al, 2007;Kasting et al, 2006). Kamber (2010) Consistent with inferences drawn by de Wit and Hynes (1995), Galer (1991), and Sim et al (2016) from assumptions somewhat different from those made here, the results shown in Figure 12 suggest that only in the most extreme of plausible conditions might Archean spreading centers have been emergent. Specifically, if heat loss were Q = 3.0Q 0 , spreading centers would have been submerged.…”

“…These values of δ ρ c are similar to others based on petrological arguments (e.g., Arndt et al, ; Jordan, ; Schutt & Lesher, ) and on the geoid (Doin et al, ; Turcotte & McAdoo, ) but roughly half those proposed by O'Neill et al () and Poudjom Djomani et al (). Calculated differences in temperatures between the base of the lithosphere and the continental Moho range from 850 to 950 °C. If the asthenosphere were hotter than that at present and with a temperature of ∼1600–1700 °C, temperatures at the Moho would have been ∼700–900 °C in Archean time. Spreading centers almost surely were not emergent in Archean time, as de Wit and Hynes (), Galer (), and Sim et al () inferred, especially if the volume of seawater in Archean time exceeded that today (Figure ). Emergent Archean continents are permitted by the following conditions (Figure ): (a) the volume of Archean seawater was not much greater than that today, (b) little continental crust had formed, 20%, as might have been the case in early Archean time (Figure d); (c) heat loss from the Earth was closer to twice that today, 2 Q 0 , than the more expected 3 Q 0 at 3.5 Ga, so that the mean depth of the ocean floor was not as shallow as it would be with Q = 3 Q 0 (Figure b); and (d) Archean spreading centers lay at least 1 km, and more likely 2 km, below low continents (Figure c). For deep spreading centers, 2 km below surfaces of low continents, oceanic crust would have been thinner than ∼25 km (Figures ).…”

“…3. Spreading centers almost surely were not emergent in Archean time, as de Wit and Hynes (1995), Galer (1991), and Sim et al (2016) inferred, especially if the volume of seawater in Archean time exceeded that today ( Figure 12). 4.…”

An Anticipation Experiment for Plate Tectonics

“…To perform this test, we input ellipse, circle, and Voronoi data to a discriminator trained on synthetic geodynamic data. We also pass a new set of synthetic geodynamic data, which was built from former 3-D spherical calculation of convection, with different parameters: We used the model of Mallard et al (2016) without continents, and the models used in Sim et al (2016) with different continent sizes, and the model presented in Coltice et al (2017) with a more realistic rheology.…”