The genesis of Earth’s incipient crust and its formative geological mechanisms remain largely inscrutable, primarily owing to the near-total absence of rock strata predating 4 billion years ago (the Hadean Eon) and the limited availability of rock samples from the subsequent 4 to 3.6 billion-year period (the Eoarchean Era). A persistent scientific enigma concerns the temporal commencement of mobile-lid tectonics. For investigations into the Hadean epoch, elemental evidence is predominantly derived from the mineral zircon, though Hadean zircons are exceedingly rare. In a recent scientific endeavor, researchers affiliated with Harvard University and other institutions meticulously examined Hadean to Eoarchean zircon crystals unearthed from the Barberton Greenstone Belt in South Africa. Their analysis indicated that zircons dating to over 3.8 billion years ago did not originate within a subduction zone environment, which is a characteristic feature of contemporary plate tectonics. Rather, the primordial crust appears to have resulted from the remelting of crustal material that itself was derived from a comparatively undepleted mantle; this early crust exhibited remarkable longevity when contrasted with mantle-derived crust of later epochs. Post-3.8 billion years ago, the geochemical signatures observed in the zircons begin to mirror those typically found in zircons associated with modern subduction zones. This observable transition, also evident in other zircon assemblages within the 3.8 to 3.6 billion-year timeframe, potentially signifies a profound alteration in crustal geochemistry and the global inauguration of mobile-lid tectonic activity during that geological period.
Hadean Earth. Image credit: Alec Brenner.
The uppermost layer of Earth, encompassing the crust and the lithospheric mantle beneath it, is fragmented into robust plates that drift slowly across more pliant, albeit mobile, underlying mantle rock.
The inexorable movement of these tectonic plates, which is the driving force behind seismic activity, volcanic eruptions, and orogenic processes, is propelled by thermal energy originating from our planet’s core.
Current estimations for when this dynamic geological regime intensified and the modern continental crust began to form vary considerably, ranging from over 4 billion years ago to as recently as 800 million years ago.
This considerable uncertainty stems from the paucity of geological evidence from Earth’s formative stages, a consequence of the very plate tectonic processes that continuously rework the planet’s surface.
Virtually no rock record survives from the Hadean Eon, the initial 500 million years of Earth’s existence.
“Before the 3.8 billion-year mark, Earth appears to have been less geologically active,” stated Dr. Nadja Drabon, a researcher involved with the Department of Earth and Planetary Sciences at Harvard University, as well as the Department of Geological Sciences at Stanford University.
“In contemporary times, a substantial volume of crust is continuously consumed at locations known as subduction zones, while simultaneously, new crust is generated.”
“Numerous prior zircon analyses indicated that once the primordial crust solidified, it maintained its integrity for exceptionally extended periods—approximately 600 million years in this specific context.”
“While some internal transformation occurred, we did not witness the formation of new granitic crust…. Then, around 3.8 billion years ago, a fundamental shift is observed.”
Zircons encapsulate chemical signatures that provide insights into Earth’s initial 500 million years. Certain zircons solidified from the planet’s magma over 4 billion years ago, a period when Earth, in geological terms, was still in its nascent stages of development. This makes them among the most ancient materials discovered on our planet.
The embedded information within these geological time capsules can be deciphered by subjecting them to laser ablation, a technique employed by the researchers in their meticulous analysis.
The investigators observed that approximately 3.8 billion years ago, concurrent with a significant cooling of the planet, a considerable amount of new crust began to form rapidly. Furthermore, the geochemical fingerprints within the zircons started to resemble those generated in subduction zones – the interfaces where converging tectonic plates interact, leading to one plate descending beneath the other and being reabsorbed into the mantle (effectively being incinerated).
“It remains unclear whether subduction zones were operational 3.8 billion years ago, but it is established that the newly forming crust was likely a product of some form of plate tectonic activity,” the researchers commented.
The scientific team collected 3,936 novel zircon samples during an expedition to South Africa in 2017. The age range of these crystals spanned from 4.1 billion to 3.3 billion years.
Their examination focused on three distinct geochemical characteristics of the zircons: their hafnium isotope composition, oxygen isotope ratios, and trace element concentrations. Each of these analytical approaches contributed a unique piece to the overarching geological puzzle.
For instance, the hafnium isotopic data offered clues regarding the genesis and evolutionary trajectory of Earth’s crust; the oxygen isotopes provided insights into the potential presence of ancient oceans; and the trace element profiles revealed information about the crust’s compositional makeup.
The gathered data suggested an acceleration in the rate of crustal formation commencing close to 4 billion years ago.
The authors also reviewed data from other scholarly investigations concerning ancient zircons collected from various global locations to ascertain if similar transitional patterns were observable. They indeed found such evidence when analyzing hafnium isotope data.
“All these datasets exhibit a discernible shift between 3.8 and 3.6 billion years ago,” remarked Dr. Drabon.
“Limited data was available for the other two geochemical indicators, and future research aims to concentrate on these aspects, including investigating the timing of ocean formation.”
“The scope of this undertaking is vast; there is an abundance of research avenues to explore.”
These groundbreaking findings were formally disseminated in the scientific journal AGU Advances.
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Nadja Drabon et al. Destabilization of Long-Lived Hadean Protocrust and the Onset of Pervasive Hydrous Melting at 3.8 Ga. AGU Advances, published online April 21, 2022; doi: 10.1029/2021AV000520
