Του Γιώργου Λεκάκη
Κρύσταλλοι από ζιρκόνια
ξαναγράφουν την ιστορία της τεκτονικής των πλακών της Γης.
Η τεκτονική δραστηριότητα,
που σχηματίζει τα μυτερά κομμάτια του πλανήτη μας, ίσως έχει ξεκινήσει 700.000.000
χρόνια ενωρίτερα από ό,τι έλεγαν έως σήμερα άλλα στοιχεία.
Οι γεωφυσικοί εξέτασαν κρυστάλλους
ζιργκόν «τύπου S» από την Δυτική Αυστραλία – το μόνο υλικό που σώζεται από την «Εποχή
του Άδη» / Hadean Era, πριν από περισσότερα από 4.000.000.000 χρόνια.
Περιέχουν εγκλείσματα που θα
μπορούσαν να είναι απόδειξη ηπείρων και καταβύθισης – και συνεπώς ύπαρξης
τεκτονικών πλακών – την στιγμή που σχηματίστηκαν, λένε οι ερευνητές.
ΠΗΓΗ: J. Jiang «Sediment
subduction in Hadean revealed by machine learning», https://doi.org/10.1073/pnas.2405160121,
121 (30) PNAS, 8.7.2024. Και «Ancient
crystals point to a surprisingly early start for plate tectonics – Hardy
zircons suggest subduction of ocean crust began 4 billion years ago», 8.7.2024.
ΑΡΧΕΙΟΝ ΠΟΛΙΤΙΣΜΟΥ, 9.7.2024.
Zircons rewrite history of plate tectonics
The tectonic activity that forms our favourite planetary pointy bits, might have begun 700 million years earlier than other evidence suggests. Geophysicists looked at ‘S-type’ zircons from Western Australia, the only material that survives from the Hadean Era, more than 4 billion years ago. They contain inclusions that could be evidence of continents and subduction (and thus the existence of plate tectonics) at the time they were formed, say researchers.
Significance
The source of Earth’s oldest materials has been debated using
traditional approaches to trace elements and isotopes of zircon. Using a single
geochemical proxy, like P contents or oxygen isotopes, for source
fingerprinting can be ambiguous. Here, we employ machine learning in
multidimensional space, incorporating an array of zircon trace elements. This
approach is beneficial for low-P zircon, prevalent in the Hadean. We find
significant proportions of S-type granite in the Hadean Jack Hills zircon as
far back as 4.24 Ga. These proportions exhibit regular variations in
supercontinent-like cycles, mirroring patterns observed in global detrital
zircon throughout Earth’s history. This suggests that exposed continents,
weathering, and subduction-driven plate tectonics were active since the Hadean,
facilitating the habitability of early Earth.
Abstract
Due to the scarcity of rock samples, the Hadean Era predating 4 billion
years ago (Ga) poses challenges in understanding geological processes like
subaerial weathering and plate tectonics that are critical for the evolution of
life. The Jack Hills zircon from Western Australia, the primary Hadean samples
available, offer valuable insights into magma sources and tectonic genesis
through trace element signatures. However, a consensus on these signatures has
not been reached. To address this, we developed a machine learning classifier
capable of deciphering the geochemical fingerprints of zircon. This allowed us
to identify the oldest detrital zircon originating from sedimentary-derived
“S-type” granites. Our results indicate the presence of S-type granites as
early as 4.24 Ga, persisting throughout the Hadean into the Archean. Examining
global detrital zircon across Earth’s history reveals consistent
supercontinent-like cycles from the present back to the Hadean. These findings
suggest that a significant amount of Hadean continental crust was exposed,
weathered into sediments, and incorporated into the magma sources of Jack Hills
zircon. Only the early operation of both subaerial weathering and plate
subduction can account for the prevalence of S-type granites we observe.
Additionally, the periodic evolution of S-type granite proportions implies that
subduction-driven tectonic cycles were active during the Hadean, at least around
4.2 Ga. The evidence thus points toward an early Earth resembling the modern
Earth in terms of active tectonics and habitable surface conditions. This
suggests the potential for life to originate in environments like warm ponds
rather than extreme hydrothermal settings.
