Archean geodynamics and the thermal evolution of Earth

Abstract

Possible geodynamic regimes that may have prevailed in the Archean are investigated by back-tracking the thermal history of Earth from the present-day conditions. If the temporal evolution of plate-tectonic convection is modulated by strong depleted lithosphere created at mid-ocean ridges, more sluggish plate tectonics is predicted when the mantle was hotter, contrary to commonly believed, more rapid tectonics in the past. This notion of sluggish plate tectonics can simultaneously satisfy geochemical constraints on the abundance of heat-producing elements and petrological constraints on the degree of secular cooling, in the framework of simple whole-mantle convection. The geological record of supercontinents back to ~2.7 Ga is shown to be broadly consistent with the accelerating plate motion as predicted by the new model. Furthermore, the very fact of repeated continental aggregation indicates that thicker depleted lithosphere in the past needs to move more slowly to become negatively buoyant by thermal contraction and also needs to be strong enough to support resulting thermal boundary layer. The concept of many small plates covering Archean ocean basins is thus physically implausible. As a consequence of reduced heat flux in the past, mantle plumes are expected to have been weaker in the Archean. The chemical evolution of Earth's mantle may have been encumbered by sluggish plate tectonics and weak mantle plumes, maintaining its compositional heterogeneity at various spatial scales to the present day. Internal heat production probably played an important role in controlling plate dynamics in the early Archean, for which a different mode of mantle convection is suggested.

Reconciling magma‐ocean crystallization models with the present‐day structure of the Earth's mantle

“…Fe-rich cumulates should undergo efficient melting once they reach the base of the mantle, independent of any potential hybridization during sinking (see above). Melting in the hot thermal boundary layer near the CMB is expected in the hot Hadean eon (e.g., Korenaga, 2006) for any material with significant Fe-enrichment. Note that even today, moderately-enriched rocks such as basalts are thought to almost reach melting temperatures at the CMB (Andrault et al, 2014;Kato et al, 2016).…”

Palaeoproterozoic supercontinents and global evolution: correlations from core to atmosphere

“…'Conventional' models commonly calculate surface heat flux using scaling laws that assume a strong temperature dependency on viscosity such that hotter mantle convects more vigorously, thereby increasing surface heat flux. As clearly enunciated by Korenaga (2006), applying conventional heat flux scaling from current conditions back through geological time predict unrealistically hot mantle temperatures before 1 Ga and lead to the so called 'thermal catastrophe' (Davies 1980). Several numerical models have therefore addressed different ways of modifying the scaling laws to avoid these unrealistic mantle temperatures.…”

“…In the last few years, developments in the modelling of secular cooling of the mantle have utilized surface heat flux scaled to a 'plate tectonic' model involving mantle melting at mid-ocean ridges (Korenaga 2006) or intermittent plate tectonic models in which subduction flux, indicated by geochemical proxies (e.g. the Nb/Th ratios of Collerson & Kamber 1999 Fig.…”

Plate-tectonic evolution of the Earth: bottom-up and top-down mantle circulation

“…Then the whole mantle would need to cool at 225 K Gyr −1 and the entire Earth would need to cool at 180 K Gyr −1 (i.e., degrees of cooling per billion years; Sleep et al 2014). A low Urey number implies that mantle radioactivity was not an intense heat source during the Hadean (Korenaga 2006(Korenaga , 2008a(Korenaga , 2008bHerzberg et al 2010;Foley et al 2014). The global mantle heat flow in balance with radioactivity was comparable to or even less than the current mantle heat flow.…”

“…Radioactive heat available during 4.4-4.0 Ga was slightly more than current mantle heat flow. Consequently, Late Hadean and even Early Archean mantle circulation and plate-tectonic processes could have been relatively sluggish (Korenaga 2006;Foley et al 2014).…”