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1. Nb/Ta systematics in arc magma differentiation and the role of arclogites in continent formation

   https://doi.org/10.1038/s41467-018-08198-3  

   Tang, M. (January 2019, Nature communications)

   null (Ed.)

   The surfaces of rocky planets are mostly covered by basaltic crust, but Earth is unique in that it also has extensive regions of felsic crust, manifested in the form of continents. Exactly how felsic crust forms when basaltic magmas are the dominant products of melting the mantles of rocky planets is unclear. A fundamental part of the debate is centered on the low Nb/Ta of Earth’s continental crust (11–13) compared to basalts (15–16). Here, we show that during arc magma differentiation, the extent of Nb/Ta fractionation varies with crustal thickness with the lowest Nb/Ta seen in continental arc magmas. Deep arc cumulates (arclogites) are found to have high Nb/Ta (average ~19) due to the presence of high Nb/Ta magmatic rutiles. We show that the crustal thickness control of Nb/Ta can be explained by rutile saturation being favored at higher pressures. Deep-seated magmatic differentiation, such as in continental arcs and other magmatic orogens, is thus necessary for making continents. 

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2. Was There Land on the Early Earth?

   https://doi.org/10.3390/life11111142  

   Korenaga, Jun (November 2021, Life)

   The presence of exposed land on the early Earth is a prerequisite for a certain type of prebiotic chemical evolution in which the oscillating activity of water, driven by short-term, day–night, and seasonal cycles, facilitates the synthesis of proto-biopolymers. Exposed land is, however, not guaranteed to exist on the early Earth, which is likely to have been drastically different from the modern Earth. This mini-review attempts to provide an up-to-date account on the possibility of exposed land on the early Earth by integrating recent geological and geophysical findings. Owing to the competing effects of the growing ocean and continents in the Hadean, a substantial expanse of the Earth’s surface (∼20% or more) could have been covered by exposed continents in the mid-Hadean. In contrast, exposed land may have been limited to isolated ocean islands in the late Hadean and early Archean. The importance of exposed land during the origins of life remains an open question. 

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3. Argon constraints on the early growth of felsic continental crust

   https://doi.org/10.1126/sciadv.aaz6234  

   Guo, Meng; Korenaga, Jun (May 2020, Science Advances)

   null (Ed.)

   The continental crust is a major geochemical reservoir, the evolution of which has shaped the surface environment of Earth. In this study, we present a new model of coupled crust-mantle-atmosphere evolution to constrain the growth of continental crust with atmospheric 40 Ar/ 36 Ar. Our model is the first to combine argon degassing with the thermal evolution of Earth in a self-consistent manner and to incorporate the effect of crustal recycling and reworking using the distributions of crustal formation and surface ages. Our results suggest that the history of argon degassing favors rapid crustal growth during the early Earth. The mass of continental crust, highly enriched in potassium, is estimated to have already reached >80% of the present-day level during the early Archean. The presence of such potassium-rich, likely felsic, crust has important implications for tectonics, surface environment, and the regime of mantle convection in the early Earth. 

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4. Rapid emergence of subaerial landmasses and onset of a modern hydrologic cycle 2.5 billion years ago

   https://doi.org/10.1038/s41586-018-0131-1  

   Bindeman, I N; Zakharov, D O; Palandri, J; Greber, N D; Dauphas, N; Retallack, G J; Hofmann, A; Lackey, J S; Bekker, A (May 2018, Nature)

   The history of the growth of continental crust is uncertain, and several different models that involve a gradual, decelerating, or stepwise process have been proposed1,2,3,4. Even more uncertain is the timing and the secular trend of the emergence of most landmasses above the sea (subaerial landmasses), with estimates ranging from about one billion to three billion years ago5,6,7. The area of emerged crust influences global climate feedbacks and the supply of nutrients to the oceans8, and therefore connects Earth’s crustal evolution to surface environmental conditions9,10,11. Here we use the triple-oxygen-isotope composition of shales from all continents, spanning 3.7 billion years, to provide constraints on the emergence of continents over time. Our measurements show a stepwise total decrease of 0.08 per mille in the average triple-oxygen-isotope value of shales across the Archaean–Proterozoic boundary. We suggest that our data are best explained by a shift in the nature of water–rock interactions, from near-coastal in the Archaean era to predominantly continental in the Proterozoic, accompanied by a decrease in average surface temperatures. We propose that this shift may have coincided with the onset of a modern hydrological cycle owing to the rapid emergence of continental crust with near-modern average elevation and aerial extent roughly 2.5 billion years ago. 

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5. Making Continental Crust on Water-bearing Terrestrial Planets

   Borisova, Anastassia; Bernadet, Justine; Guitreau, Martin; Safonov, Oleg; Asimow, Paul D; Nedelec, Anne; Bohrson, Wendy A; Kosova, Svetlana; de_Parseval, Philippe (December 2024, American Geophysical Union)

   Debate regarding early Earth differentiation focuses on the fate, nature, origin, volume, and processes responsible for protocrust(s) and felsic crust formation. One specific aspect of this debate is how Hadean zircons and their felsic parental magmas fit with an expected ultramafic environment. Based on our new experiments, thermodynamic modeling, and elemental partitioning, we infer that felsic liquids could have been generated by shallow (< 20 km) interaction between primordial hydrated peridotite (serpentinite) and basaltic magmas. Felsic melts (SiO2 ≥ 55 wt%) can be generated at a maximum melt fraction of 0.4 when starting serpentinite:basalt mass ratio is high (i.e., higher than 1.5:1). Here we show that felsic melts obtained in our experimental runs can account for the Hf isotope evolutionary array displayed by Hadean detrital zircons worldwide. We propose that open system interactions between serpentinite and basaltic melts at the end of the magma ocean stage after magma degassing and water ocean precipitation allowed the formation of extensive early Hadean felsic crust (4.4 - 4.5 Gy ago). Our calculations indicate that this felsic crust accounts for up to 50% of present-day continental crust mass. The abundant production of primordial felsic crust throughout the Hadean could be due to the impact-induced melting owing to frequent impacts. A similar process could have also occurred on Mars, and other rocky planets, provided that water was abundant at shallow and surficial levels, which would account for the existence of a thick felsic crust. The serpentinised protocrust had a dual role in the primitive planetary environment: to provide the first and most abundant felsic crust and to facilitate the emergence of life in the shallow hydrothermal environments of water-bearing terrestrial planets. 

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