The operation of a hydrological cycle (i.e., exchange of water between the land, oceans, and atmosphere) has significant implications for the emergence of life. The oldest confirmed single-celled organisms at ~3.48 billion years ago (Ga) (Pilbara Craton, Western Australia) are thought to have formed in the presence of meteoric (fresh) water on emerged (subaerial) land in a hot spring environment. However, when widespread interaction between fresh water and emerged continental crust first began is poorly constrained. In this study, we use >1000 oxygen isotope analyses of Jack Hills detrital zircon to track fluid-rock interactions from the Hadean to the Paleoarchean (~4.4–3.1 Ga). We identify extreme isotopically light O (i.e., δ18O < 4.0 ‰) values older than 3.5 Ga. The data define two periods of magmatism with extreme isotopically-light O as low as 2.0 ‰ and –0.1 ‰ at around 4.0 and 3.4 Ga, respectively. Using Monte Carlo simulations, we demonstrate that such values can only be generated by the interaction of crustal magmatic systems with meteoric water. Our data constrains the earliest emergence of continental crust on Earth, the presence of fresh water, and the start of the hydrological cycle that likely provided the environmental niches required for a life less than 600 million years after Earth’s accretion.
Abstract The nature and evolution of Earth’s crust during the Hadean and Eoarchean is largely unknown owing to a paucity of material preserved from this period. However, clues may be found in the chemical composition of refractory minerals that initially grew in primordial material but were subsequently incorporated into younger rocks and sediment during lithospheric reworking. Here we report Hf isotopic data in 3.9 to 1.8 billion year old detrital zircon from modern stream sediment samples from West Greenland, which document successive reworking of felsic Hadean-to-Eoarchean crust during subsequent periods of magmatism. Combined with global zircon Hf data, we show a planetary shift towards, on average, more juvenile Hf values 3.2 to 3.0 billion years ago. This crustal rejuvenation was coincident with peak mantle potential temperatures that imply greater degrees of mantle melting and injection of hot mafic-ultramafic magmas into older Hadean-to-Eoarchean felsic crust at this time. Given the repeated recognition of felsic Hadean-to-Eoarchean diluted signatures, ancient crust appears to have acted as buoyant life-rafts with enhanced preservation-potential that facilitated later rapid crustal growth during the Meso-and-Neoarchean.
The Mesoarchean Akia Terrane in West Greenland contains a detailed magmatic and metamorphic mineral growth record from 3.2 Ga to at least c. 2.5 Ga. This time span makes this region an important case study in the quest to track secular changes in geodynamic style which may ultimately inform on the development of plate tectonics as a globally linked system of lateral rigid plate motions. The common accessory mineral titanite has recently become recognised as a powerful high temperature geochronometer whose chemistry may chart the thermal conditions of its growth. Furthermore, titanite offers the potential to record the time-temperature history of mafic lithologies, which may lack zircon. Although titanite suffers from higher levels of common Pb than many other UPb chronometers, we show how measurement of 207Pb/206Pb in texturally coeval biotite may assist in the characterization of the appropriate common Pb composition in titanite. Titanite extracted from two samples of mafic gneisses from the Akia Terrane both yield UPb ages of c. 2.54 Ga. Although coeval, their chemistry implies growth under two distinctly different processes. In one case, the titanite has elevated total REE, high Th/U and grew from an in-situ partial melt, consistent with an identical date to granite dyke zircon. In contrast, the second titanite sample contains greater common Pb, lower total REE, lower Th/U, and grew from dominantly hydrothermal fluids. Zr-in-titanite thermometry for partial melt-derived titanite, with activities constrained by phase equilibrium modelling, indicates maxima of c. 690 °C. Elsewhere in the Akia Terrane, coeval metamorphism linked to growth of hydrothermal titanite is estimated at temperatures of c. 670 °C. These new results when coupled with existing findings indicate punctuated, repeated metamorphic events in the Akia Terrane, in which high temperature conditions (re)occurred at least three times between 3.0 and 2.5 Ga, but crucially changed in style across a c. 3.0 Ga change point. We interpret this change in metamorphism as reflecting a fundamental shift in geodynamic style in West Greenland at 3.0 Ga, consistent with other estimates for the onset of widespread plate tectonic-type processes.
The evolution of faults and paleodrainage patterns on the southwestern Australian passive margin during and after the breakup of Gondwana in the Early Cretaceous remains poorly understood. This contribution investigates the fault and paleodrainage evolution in the southern Perth Basin with the use of the ‘Bunbury Basalt’, the only lava flows known to be synchronous with continental breakup. New aeromagnetic data have been integrated with well intersections and outcrop constraints to establish the first 3D model of the Bunbury Basalt. The model reveals that flows are up to 100 m thick and are predominantly confined to two north–south-trending paleovalleys and their tributaries situated in the Bunbury Trough in the southern Perth Basin. The Donnybrook Paleovalley flow ponded in a paleovalley proximal to the Darling Fault and is truncated by the two later flows within the Bunbury Paleovalley, which is positioned centrally in the Bunbury Trough. Offsets of the Bunbury Basalt have been used to identify new northeast- and northwest-trending faults in the southern Perth Basin, and broad folding is interpreted as a consequence of drag into the Darling and Busselton faults. The model has been used to determine post-basalt net displacements for the Darling and Busselton faults of 370 and 210 m, respectively, and <175 m for the northeast and northwest-trending faults. The source vents for the Bunbury Basalt were probably located at extensional jogs at intersections between the Darling Fault and subordinate oblique faults. These results challenge the views on longstanding quiescence of the post-breakup western Australian passive margin.
Several studies focused on the ultramafic bodies of the Archaean continental crust in southern Greenland in order to gain information on early Earth petrogenetic, metamorphic and metasomatic processes. This research provides the first petrological dataset of the Miaggoq Ultramafic Complex (~1 km2) in the Akia terrane, with a minimum age of 2997 ± 15 Ma. It comprises ultramafic (dunite, peridotite) and mafic (orthopyroxenite, norite) rocks along with chromitites and provides a window into Mesoarchaean mantle compositions. Field observations, such as chromitite bands, mineral layering, and orthopyroxenite oikocrysts in peridotites coupled with chemical analysis displaying high abundance of chromites in the dunitic rocks and high forsterite contents (Mg# 91 to 92.5) of the olivines, all point to a layered cumulate origin for the Miaggoq body. Pseudosection calculations along with geothermometry estimations reveal peak metamorphic conditions of 850–1100 °C at pressures of 0.7–1.25 GPa under anhydrous conditions followed by a possible metamorphic overprint at 650–800°C and 0.7 GPa with relatively dry melting (0.025–0.125 wt.% H2O). MELTS fractional crystallization coupled with cumulate modelling approximated the compositional trends with conditions on ~3 kbar with 1 wt.% H2O. This research concludes that the Miaggoq body represents a layered cumulate complex that was derived by large degrees of partial melting of the mantle with possible assimilation (synonymous with contamination) of basalts in the crust. Overall, this study provides complementary data for the topic of Mesoarchaean cumulate bodies of the Akia Terrane and their petrological processes.
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