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1. Reevaluating the evidence for a Hadean-Eoarchean dynamo

   https://doi.org/10.1126/sciadv.aav9634  

   Borlina, Cauê S.; Weiss, Benjamin P.; Lima, Eduardo A.; Tang, Fengzai; Taylor, Richard J.; Einsle, Joshua F.; Harrison, Richard J.; Fu, Roger R.; Bell, Elizabeth A.; Alexander, Ellen W.; et al (April 2020, Science Advances)

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   The time of origin of the geodynamo has important implications for the thermal evolution of the planetary interior and the habitability of early Earth. It has been proposed that detrital zircon grains from Jack Hills, Western Australia, provide evidence for an active geodynamo as early as 4.2 billion years (Ga) ago. However, our combined paleomagnetic, geochemical, and mineralogical studies on Jack Hills zircons indicate that most have poor magnetic recording properties and secondary magnetization carriers that postdate the formation of the zircons. Therefore, the existence of the geodynamo before 3.5 Ga ago remains unknown. 

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2. Destabilization of Long‐Lived Hadean Protocrust and the Onset of Pervasive Hydrous Melting at 3.8 Ga

   https://doi.org/10.1029/2021AV000520  

   Drabon, Nadja; Byerly, Benjamin_L; Byerly, Gary_R; Wooden, Joseph_L; Wiedenbeck, Michael; Valley, John_W; Kitajima, Kouki; Bauer, Ann_M; Lowe, Donald_R (April 2022, AGU Advances)

   Abstract The nature of Earth's earliest crust and crustal processes remain unresolved questions in Precambrian geology. While some hypotheses suggest that plate tectonics began in the Hadean, others suggest that the Hadean was characterized by long‐lived protocrust and an absence of significant plate tectonic processes. Recently proposed trace‐element proxies for the tectono‐magmatic settings in which zircons formed are a relatively novel tool to understand crustal processes in the past. Here, we present high‐spatial resolution zircon trace and rare earth element geochemical data along with Hf and O isotope data of a new location with Hadean materials, 4.1–3.3 Ga detrital zircons from the 3.31 Ga Green Sandstone Bed, Barberton Greenstone Belt. Together, the hafnium isotope and trace element geochemistry of the detrital zircons record a major transition in crustal processes. Zircons older than 3.8 Ga show evidence for isolated, long‐lived protocrust derived by reworking of relatively undepleted mantle sources with limited remelting of surface‐altered material. After 3.8 Ga, Hf isotopic evidence for this protocrust is muted while relatively juvenile source components for the zircon's parental magmas and flux‐like melting signatures become more prominent. This shift mirrors changes in Hf isotopes and trace element geochemistry in other Archean terranes between ∼3.8 and 3.6 Ga and supports the notion that the global onset of pervasive crustal instability and recycling—A possible sign for mobile‐lid tectonics—Occurred in that time period. 

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3. Direct age constraints on the magnetism of Jack Hills zircon

   https://doi.org/10.1126/sciadv.add1511  

   Taylor, Richard J. M.; Reddy, Steven M.; Saxey, David W.; Rickard, William D. A.; Tang, Fengzai; Borlina, Cauê S.; Fu, Roger R.; Weiss, Benjamin P.; Bagot, Paul; Williams, Helen M.; et al (January 2023, Science Advances)

   A potential record of Earth’s magnetic field going back 4.2 billion years (Ga) ago is carried by magnetite inclusions in zircon grains from the Jack Hills. This magnetite may be secondary in nature, however, meaning that the magnetic record is much younger than the zircon crystallization age. Here, we use atom probe tomography to show that Pb-bearing nanoclusters in magnetite-bearing Jack Hills zircons formed during two discrete events at 3.4 and <2 Ga. The older population of clusters contains no detectable Fe, whereas roughly half of the younger population of clusters is Fe bearing. This result shows that the Fe required to form secondary magnetite entered the zircon sometime after 3.4 Ga and that remobilization of Pb and Fe during an annealing event occurred more than 1 Ga after deposition of the Jack Hills sediment at 3 Ga. The ability to date Fe mobility linked to secondary magnetite formation provides new possibilities to improve our knowledge of the Archean geodynamo. 

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4. The Signature of Metasomatized Subcontinental Lithospheric Mantle in the Basaltic Magmatism of the Payenia Volcanic Province, Argentina

   https://doi.org/10.1029/2021GC010071  

   Chilson‐Parks, Benjamin H.; Calabozo, Fernando M.; Saal, Alberto E.; Wang, Zhengrong; Mallick, Soumen; Petrinovic, Ivan A.; Frey, Frederick A. (January 2022, Geochemistry, Geophysics, Geosystems)

   Abstract The Payenia region of Argentina (34.5–38°S) is a large Pliocene‐Quaternary volcanic province of basaltic compositions in the Andean Cordillera foothills representing the northernmost extent of back‐arc volcanism in the Andean Southern Volcanic Zone (SVZ). Although the chemical diversity of the Payenia basalts has been characterized previously, the processes and sources responsible for such variation remain controversial. Here, we report new whole‐rock major and trace element concentrations, Sr‐, Nd‐, Hf‐, and Pb‐isotope ratios and high‐precision olivine oxygen‐isotope ratios in a suite of 35 alkaline basalts from Payenia. These lavas have major and trace elements that define a compositional range from arc‐influenced to intraplate signature. Variable crustal contamination and/or recent slab‐derived inputs inadequately account for elemental and isotopic systematics and spatial compositional variations of Payenia lavas. We present a simple forward model indicating that early metasomatism and subsequent melting of the metasomatized subcontinental lithospheric mantle (SCLM) has significantly contributed to the Payenia lava compositional range. Isotopic ingrowth calculations of radiogenic Sr, Nd, Hf, and Pb suggest that the SCLM metasomatism occurred at 50–150 Ma, consistent with the timing of the breakup of Gondwana and the development of the proto‐Pacific Andean arc. Variations in δ¹⁸O_olivinevalues from modeled melts indicate that the metasomatism and melting within the SCLM can fractionate oxygen isotopes even when the metasomatizing melt has MORB‐like δ¹⁸O values, providing a different explanation for the low‐δ¹⁸O signatures observed in continental arc settings. 

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5. Oxygen isotope (δ18O, Δ′17O) insights into continental mantle evolution since the Archean

   https://doi.org/10.1038/s41467-022-31586-9  

   Bindeman, Ilya N.; Ionov, Dmitri A.; Tollan, Peter M. E.; Golovin, Alexander V. (July 2022, Nature Communications)

   Abstract Oxygen isotopic ratios are largely homogenous in the bulk of Earth’s mantle but are strongly fractionated near the Earth’s surface, thus these are robust indicators of recycling of surface materials to the mantle. Here we document a subtle but significant ~0.2‰ temporal decrease in δ¹⁸O in the shallowest continental lithospheric mantle since the Archean, no change in Δ′¹⁷O is observed. Younger samples document a decrease and greater heterogeneity of δ¹⁸O due to the development and progression of plate tectonics and subduction. We posit that δ¹⁸O in the oldest Archean samples provides the best δ¹⁸O estimate for the Earth of 5.37‰ for olivine and 5.57‰ for bulk peridotite, values that are comparable to lunar rocks as the moon did not have plate tectonics. Given the large volume of the continental lithospheric mantle, even small decreases in its δ¹⁸O may explain the increasing δ¹⁸O of the continental crust since oxygen is progressively redistributed by fluids between these reservoirs via high-δ¹⁸O sediment accretion and low-δ¹⁸O mantle in subduction zones. 

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