Did the Neoproterozoic Revolution Extend to the Deep Mantle?

Following the Earth's formation about 4.6 Ga, its initial basaltic outer layer underwent differentiation, generating the oldest continental crust. However, how and when this differentiation took place and what processes led to the formation of early silicic continental crust are controversial topics, which are rooted in a bigger question of when modern-style plate tectonics started operating in Earth's history. In general, there is a well-accepted consensus that the Earth transitioned from single lid (squishy, stagnant, and/or sluggish lid?) to plate tectonics at some point, largely as a result of the establishment of the first subduction zone(s). Members of the geoscience community are divided in answering the question of the timing of the onset of plate tectonics, as reflected in the four perspective papers by different international research teams here. Using different proxies and datasets, the proposed models range from the Late Archean to Paleoproterozoic, and Neoproterozoic for the beginning of a plate tectonic convective regime. Another interpretation states that we do not know when plate tectonics might have started based on the extant observations and data. Although the beginning of plate tectonics is still a subject of debate, most researchers agree on the definition of plate tectonics as a globally networked system of laterally moving rigid plates riding on the aesthenosphere and mantle convection cells, which facilitated recycling of the top boundary layer through time. Testing the proposed models and concepts in these perspective papers with new data, observations and geodynamic modelling efforts will be critical to better understanding the Earth's thermal history and evolution since 4.4 Ga.

A synopsis of recent conceptual models on supercontinent tectonics in relation to mantle dynamics, life evolution and surface environment

Continental growth has been episodic, reflecting the episodic nature of mantle dynamics as well as surface dynamics of the Earth, the net result of which is exhibited by the present mantle with two huge reservoirs of TTG rocks, one on the surface continents and the other on the D″ layer on the Core-Mantle Boundary (CMB). During the early half of the Earth history, the felsic continental crust on the surface which formed in an intra-oceanic environment has mostly been subducted into the deep mantle, except in the rare case of parallel arc collision. The growth history of continental crust shows that with its simultaneous formation, a considerable amount must have also been subducted. Such ongoing subduction processes can be seen in the western Pacific region, through tectonic erosion, arc subduction, and sediment-trapped subduction.

Mantle melting, which leads to the formation of oceanic and continental crust, together with crust recycling through plate tectonics, are the primary processes that drive the chemical differentiation of the silicate Earth. The present-day mantle, as sampled by oceanic basalts, shows large chemical and isotopic variability bounded by a few end-member compositions. Among these, the HIMU end-member (having a high U/Pb ratio, μ) has been generally considered to represent subducted/recycled basaltic oceanic crust. However, this concept has been challenged by recent studies of the mantle source of HIMU magmas. For example, analyses of olivine phenocrysts in HIMU lavas indicate derivation from the partial melting of peridotite, rather than from the pyroxenitic remnants of recycled oceanic basalt. Here we report data that elucidate the source of these lavas: high-precision trace-element analyses of olivine phenocrysts point to peridotite that has been metasomatized by carbonatite fluids. Moreover, similarities in the trace-element patterns of carbonatitic melt inclusions in diamonds and HIMU lavas indicate that the metasomatism occurred in the subcontinental lithospheric mantle, fused to the base of the continental crust and isolated from mantle convection. Taking into account evidence from sulfur isotope data for Archean to early Proterozoic surface material in the deep HIMU mantle source, a multi-stage evolution is revealed for the HIMU end-member, spanning more than half of Earth's history. Before entrainment in the convecting mantle, storage in a boundary layer, upwelling as a mantle plume and partial melting to become ocean island basalt, the HIMU source formed as Archean-early Proterozoic subduction-related carbonatite-metasomatized subcontinental lithospheric mantle.

Subduction erosion: Rates, mechanisms, and its role in arc magmatism and the evolution of the continental crust and mantle

<p>The present-day Earth exhibits subduction-driven plate tectonics, which is a surface expression of processes happening in the deep interior. For the early Earth, following the magma ocean solidification stage, a variety of tectonic regimes have been proposed albeit without any consensus: heat-pipe tectonics, plutonic-squishy lid, stagnant lid. Furthermore, the rheological changes required to make the (supposedly gradual) transition to modern style plate tectonics on Earth remain hotly debated. Also, different estimates of mantle potential temperature (Herzberg et al., 2010; Aulbach and Arndt, 2019) for the Archean have been proposed.</p><p>Recently, it has been proposed that sediments accumulated at continental margins as a result of surface erosion processes could have acted as a lubricant to stabilise subduction and aid with the initiation of plate tectonics after the emergence of continents around 3 Ga (Sobolev and Brown, 2019). Before that time, the flux of sediments to the ocean was very limited. It was further suggested that subduction zones were already present at that time but were likely initiated only above hot mantle plumes. This tectonic regime of regional plume-induced retreating subduction zones was very different from the modern type of plate tectonics, but nevertheless might have been efficient in production of early continental crust and recycling of water and pre-existing crust into the deep mantle.</p><p>In this work, we test this hypothesis of surface-erosion controlled plate tectonics preceded by plume-induced retreating subduction tectonic regime in global convection models by introducing magmatic weakening of lithosphere above hot mantle plumes. We also adapt the effective friction coefficient in brittle deformation regime to mimic the lubricating effect of sediments. Furthermore, these models employ a more realistic upper mantle rheology and are capable of self-consistently generating oceanic and continental crust while considering both intrusive (plutonic) and eruptive (volcanic) magmatism (Jain et al., 2019). We also investigate the influence of lower mantle potential temperatures on crust production and compare our models with geological data.</p><p>When compared to models with just diffusion creep, the models with composite rheology (diffusion creep and dislocation creep proxy) result in more efficient mantle cooling, higher production of continental crust, and higher recycling of basaltic-eclogitic crust through delamination and dripping processes. These models also show higher mobilities (Tackley, 2000), which have been previously shown for diffusion creep models only with low surface yield stress values (Lourenço et al., 2020). Preliminary results from models initialised with lower mantle potential temperatures show an affect on the initial growth of TTG rocks over time. However, no considerable differences in terms of total crust production or mantle cooling are observed.</p>

Continental crustal volume, thickness and area, and their geodynamic implications

Supercontinents, mantle dynamics and plate tectonics: A perspective based on conceptual vs. numerical models

The most important process affecting both major and trace-element concentrations in the mantle and crust is melting producing silicate liquids which then migrate. Another process whose effects are becoming more apparent is the transport of elements by CO 2 - and H 2 O-rich fluids. Due to the relatively small amounts of fluids involved they have but little effect on the major-element abundances but may severely affect minor- and trace-element abundances in their source and the material through which they travel. The Archaean crust was a density filter which reduced the possibility of komatiite or high FeO melts with relative densities greater than about 3.0 from reaching the surface. Those melts retained in the lower crust or at the crust-mantle boundary would have enhanced the possibility of melting in the lower crust. The high FeO melts may have included the Archaean equivalents of alkali basalt whose derivatives may form an important component in the Archaean crust. The occurrence of ultramafic to basic to alkaline magmas in some Archaean greenstone belts is an assemblage most typical of modern ocean-island suites in continental environments. The rock types in the assemblage were modified by conditions of higher heat production during the Archaean and thus greater extents of melting and melting at greater depths. If modern ocean-island suites are associated with mantle plumes, which even now may be an important way to transport heat upward from the deeper mantle, it is suggested that during the Archaean mantle plumes were an important factor in the evolution of the continental crust. It appears that the Archaean continental crust was of comparable thickness to that of the present based on geobarometeric data. If the freeboard concept applied then, this would suggest that plate tectonics was also an active process during the Archaean. If so, it is probably no more realistic to assume that all Archaean greenstone belts had a similar tectonic setting than to assume that all modern occurrences of basic rocks have a common tectonic setting.

Continental collision zones are primary sites for net continental crust growth — A testable hypothesis

Oceanic hotspots with extreme enriched mantle radiogenic isotopic signatures—including low 143Nd/144Nd indicative of subducted continental crust—are linked to plume conduits sampling the southern hemispheric mantle. However, the mechanisms responsible for concentrating subducted continental crust in the austral mantle are unknown. We show that subduction of sediments and subduction eroded material, and lower continental crust delamination, cannot generate this spatially coherent austral geochemical domain. However, continental collisions—associated with the assembly of Gondwana‐Pangea—were positioned predominantly in the southern hemisphere during the late Neoproterozoic appearance of widespread continental ultra‐high‐pressure metamorphic terranes, which marked the onset of deep subduction of upper continental crust. We propose that deep subduction of upper continental crust at ancient rifted‐passive margins during ca. 650‐300 Ma austral supercontinent assembly resulted in enhanced upper continental crust delivery into the southern hemisphere mantle. Similarly enriched mantle domains are absent in the boreal mantle plume source, for two reasons. First, continental crust subducted after 300 Ma—when the continents drifted into the northern hemisphere—has had insufficient time to return to the surface in plumes sampling the northern hemisphere mantle. Second, before the first known appearance of continental ultra‐high‐pressure rocks at 650 Ma, deep subduction of upper continental crust was uncommon, limiting its subduction into the northern (and southern) hemisphere mantle earlier in Earth history. Our model implies a recent formation of the austral enriched mantle domain, explains the geochemical dichotomy between austral and boreal plume sources, and may explain why there are twice as many austral hotspots as boreal hotspots.

Precambrian tectonics and the physicochemical evolution of the continental crust. II. Lithosphere delamination and ensialic orogeny

Very long-term periodicity of episodic zircon production and Earth system evolution

Hadean geodynamics and the nature of early continental crust

Rhenium abundances and systematics in oceanic basalts