Project title:“Application of the novel geochronological and isotope-geochemical methods to the early Earth studies: testing models of the continental crust formation and evolution, and initiation of the plate tectonics”
Acronym: Early Earth
Project manager:
Dr hab. Leonid Shumlyanskyy
Institute of Geological Sciences of the Polish Academy of Sciences Research Center in Krakow, Senacka 1, 31-002 Krakow
e-mail: ndshumly@cyf-kr.edu.pl
tel. +48 12 37 05 223
Description of the research base
Funded under the agreement No. UMO-2021/43/P/ST10/02283 between the National Science Center of Science based in Krakow and the Institute of Geological Sciences of the Polish Academy of Sciences for the implementation and financing of the research project, it received funding as part of the "POLONEZ BIS 1” competition.
“This research is part of the project No. 2021/43/P/ST10/02283 co-funded by the National Science Centre and the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 945339”.
Mankind is preparing to take a step towards other planets. Several countries pursue plans for establishing permanent inhabited bases on Moon and Mars. These bases will have to develop eventually the self-sustained production of all the necessary products, including mineral resources. In contrast to Earth, which continues to evolve, other planets got nearly “frozen” at those stages of evolution that Earth has passed 3-4 billion years ago. Due to this, investigation of the early stages of Earth evolution has a direct implication for the other rocky planets. It will allow us to better understand what kind of mineral resources can be found on other planets, in which planetary environment, etc. Deciphering the regularities of the early Earth evolution is also important for the understanding of the origin of life and its early survival on our planet. Hence, knowledge about the early stages of Earth evolution is needed to better understand the geological processes occurring on our planet, predict what kind of minerals resources (and where) can we expect on other planets, and make a progress in the understanding of the origin and evolution of life. The investigation of the early evolution of Earth is very complicated due to several circumstances. First of all, the oldest rocks have experienced several episodes of metamorphic reworking, that have almost completely erased their primary features that can be used for the understanding of the early Earth. Moreover, most of the oldest rocks have been destroyed during the long history of Earth. The oldest material, available for the direct study is represented by mineral zircon, which due to its remarkable chemical and physical durability can preserve information about geological processes that acted on our planed during the first days of its existence. The oldest terrestrial zircon was found in Western Australia and dated at around 4.4 billion years. However, zircons older than 4.0 billion years are extremely rare. The rock record starts at ca. 3.9-3.8 billion years, and such rocks were found on all continents. In the proposed project we plan to investigate zircons (and mineral inclusions in zircons) separated from the oldest rocks in the Ukrainian Shield (these are between 3.8 and 2.8 billion years old), Slave Craton in Canada (these are the oldest rocks in the world dated at ca. 4.0 billion years), and Yilgarn and Pilbara cratons in Australia. The main focus will be made on the poorly studied Archean rocks of the Ukrainian Shield. Australian samples are represented in our collection by zircon from the Jack Hills conglomerates (the famous occurrence of the zircons older than 4.0 billion years), Archean granitoids and the oldest known basic rocks (anorthosites) from the Narryer terrane (ca. 3.7 billion years old). In total, we plan to investigate around 50 samples. During the project implementation, we will address the following research questions: (1) How did the first stable continental crust develop? (2) How fast did the process of formation of the stable continental crust proceed? (3) When did plate tectonics start? (4) What was the fundamental difference between the Hadean and Archean mafic crust that did not allow the Hadean crust to persist? We plan to address these questions by achieving the following research goals: 1. Construction of the hafnium isotope composition map of the Ukrainian Shield, which will allow estimation of the volume of continental crust produced at different times. Using this map, we will be able to contribute to the understanding of the rates of continental crust formation. 2. We will also construct the εHf – age plot and compare it to already existing plots for other Archean domains worldwide. This plot will allow us to trace mantle evolution through time. 3. Detailed petrochronological study of a few key samples collected in the Ukrainian Shield and other areas of interest. The combination of various isotope methods will allow us to constrain the geological history of the studied rocks, including those relatively low-temperature events that are not revealed in the mineral composition of the rocks. 4. We will apply the recently developed methods of isotope-geochemical studies of mineral inclusions in zircon that allow obtaining reliable information regarding Sr isotope composition in apatite or Pb isotope composition in feldspar. 5. We plan also to investigate the usefulness of the emerging methods of isotope studies, namely Zr isotopes in zircon and Cl and O isotopes in apatite. 6. Finally, we aim at collecting a large volume of trace element data in zircon that can provide valuable data regarding the chemical composition of the melt from which these zircons have crystallized, the degree of melt fractionation oxidation state, etc. We expect that the successful implementation of the proposed project will have a great impact on our knowledge regarding the evolution of early Earth. We plan to contribute to the understanding of the evolution of different isotope systems through time. By doing this, we will be able to provide additional information about the processes and rate of continental crust formation. We aim also at providing additional information regarding the time of the plate tectonics initiation. This information may be also used for the understanding of the evolution of other rocky planets and better prediction of mineral resources distribution. In a more distant sense, our results will contribute to a better understanding of the origin of life, formation of the hydrosphere, evolution of atmosphere composition etc.
Jack Hills conglomerates (Western Australia) were the oldest terrestrial zircons (up to 4400 million years old) found |
Ca. 38000 million years old enderbites in the Bouh area of the Ukrainian Shield |
AKTUALNOŚCI NA TEMAT REALIZACJI PROJEKTU
Key achievements of the project
The following research tasks were achieved:
The continental crust's formation rate is traditionally explored using variations in the isotope composition of the crustal rocks. Insomuch as crustal rocks differ in chemical composition from the mantle rocks, with time this difference becomes visible in radiogenic isotope composition because different reservoirs (i.e., mantle and continental crust) differ in terms of parental and daughter element ratios (e.g., Rb/Sr, Sm/Nd, Lu/Hf). As the volume of the continental crust increases with time, the difference between the crust and mantle increases, resulting in ever-increasing differences in isotope composition between these two reservoirs. At the same time, this means that it is hard to estimate the continental growth rate and the continental crust's volume in the distant past as the chemical difference between the different reservoirs was still small, and the time was insufficient to develop detectable isotope differences.
Numerous crustal growth models were developed by different authors. Some of the models were based on speculative assumptions, like a model by Armstrong, 1981, who believed that the entire continental crust was created in the early Archean and had not changed significantly since then. Other models suggested continuous continental growth with slightly different rates at different times caused by various reasons such as the beginning of the plate tectonics or peaked formation/preservation of the continental crust caused by the continental assembly. Although the distribution of zircon, monazite and rutile ages, which correspond to the main crustal-forming episodes, is apparently very irregular (e.g., Cerri et al., 2024), the most commonly accepted model of the depleted mantle evolution, which foresees constant rate of the continental crust formation through the entire history of Earth, seems to be supported by the observations (e.g., Sundell and Macdonald, 2022).
Our extensive Hf isotope data collected for the Ukrainian Shield (Fig. 1) also support the model of the uniform crustal growth rate through the Earth’s history. Although our data clearly indicate the episodic growth of the continental crust in the Ukrainian Shield, hafnium isotope composition confidently aligns with the global model of continuous crustal growth.
Figure 1. εHf vs. age diagram for rocks from all domains of the Ukrainian Shield and surrounding areas. The total amount of spots in this plot is ca. 3500, some 60 % of them were obtained during the project implementation.
The question about the timing of the onset of plate tectonics is one of the most hotly debated in the Earth sciences. Numerous recent papers (e.g., Dhuime et al., 2012; Næraa et al., 2012; Palin et al., 2020; Hastie et al., 2023; Ouyang et al., 2024, and many others) discussed a question of the transition from the stagnant-lid regime to the system of mutually linked and interacting tectonic plates, known as plate tectonics. However, there is still no common agreement regarding what can be considered the initiation of plate tectonics. Subduction, one of the key features of plate tectonics, may occur locally and does not indicate the formation of the global self-sustained system of interacting tectonic plates. Hence, the main question is what kind of secular variations in isotope characteristics or chemical composition will reflect the initiation of plate tectonics. Besides plate tectonics, there are numerous mechanisms for returning crustal material to the mantle, which may lead to the same chemical consequences as subduction.
In our work, we adopted the approach used by Dhuime et al. (2012, 2015) who used systematic variations of the Rb/Sr ratio in whole-rock samples and Hf and O isotopes in zircons of different ages to reveal a marked decrease in the rate of crustal growth at ~3 Ga that may be linked to the onset of subduction-driven plate tectonics. Our zircon O and Hf isotope data indicate a significant change in the isotope composition of the crustal rocks that may be attributed to a fundamental change in the geodynamic regime and transition to the plate tectonic processes. Specifically, there is a significant shift in the Hf isotope composition after ca. 3.0 Ga (Fig. 1). Rocks formed earlier than 3.0 Ga have predominantly juvenile Hf isotope composition, indicating their short crustal residence time and link to the mantle-plume-related processes. The younger rocks record shifts towards negative εHf values, which reflect the predominance of the crustal reworking processes, possibly linked to the collision- or subduction-related involvement of the older crust. Another indication of the change in the tectonic regime is the appearance of the rocks with elevated δ18O values, which signifies an increased proportion of the (supra)crustal material in the source of felsic rocks. In the Ukrainian Shield, such rocks appear for the first time at ca. 3.0 Ga (Fig. 2). It is important to note that the first true K-Na granites (in contrast to Na-dominated tonalite-trondhjemite-granodiorite association) appear in the Ukrainian Shield between 2.9-2.8 Ga.
Considering our newly obtained data for the Ukrainian Shield, we conclude that the transit from the stagnant-lid tectonic mode to the predominantly plate tectonic mode took place at ca. 3.0 Ga. Our data are in good agreement with the results reported for other Early Precambrian domains worldwide (Dhuime et al., 2012; Næraa et al., 2012).
Figure 2. δ18O vs. age diagram for rocks from all domains of the Ukrainian Shield. All data was obtained during the project implementation. There is a significant rise in δ18O values after ca. 3000 Ma, indicating transit from the stagnant-lid tectonic mode to the predominantly plate tectonic mode.
The oldest stage of the Earth’s evolution, which persisted from the moment of its formation until 4.0 Ga, is known as Hadean. This is the most poorly studied stage in the Earth’s history because almost no material evidence has survived since then. The rock record extends to ca. 4.03 Ga when the rock known as Acasta gneiss in Northern Canada was formed. Information about even older geological events can be obtained from indirect evidence such as old zircons, or younger zircons with very unevolved Hf isotope compositions, or rocks with anomalous 142Nd/144Nd isotope compositions. Although zircons with ages over 4.0 Ga are extremely rare, they were found on almost all continents. There is only a single location on Earth where Hadean zircons can be found in abundance, i.e., Jack Hills Ridge in Western Australia, where Archean conglomerates contain ca. 12 % of Hadean zircons.
Hadean zircons have exclusively negative εHf values (e.g., Kemp et al., 2010; Bell et al., 2014; Reimink et al., 2020; Mulder et al., 2021), indicating residence in the felsic crust, which allowed preservation of the relatively unradiogenic Hf isotope composition. In contrast, Archean zircons have a more radiogenic Hf isotope composition, indicating input of juvenile mantle material. Such a difference reflects fundamentally different styles of crustal formation and evolution in the Hadean and Archean.
We addressed the problem of the geological transition occurring at the Hadean-Archean boundary on the example of nearly Hadean zircons discovered recently in metadacite of the Huliaipole Formation in the Azov Domain of the Ukrainian Shield. The oldest zircons found in these rocks were dated at 3970 Ma, and they revealed unradiogenic Hf isotope composition with εHf values ranging from -7 to -4. A series of younger zircons also revealed unradiogenic composition, remaining in the field defined by the Jack Hills zircons. At the same time, Eoarchean zircons found in the Azov and Dniester-Bouh domains of the Ukrainian Shield and dated at 3785 Ma, have radiogenic Hf isotope compositions with εHf reaching positive values. The chemical compositions of all these zircons confirm that they were derived from igneous rocks of dioritic-granodioritic composition.
Our data confirm that the “shadow” relics of the Hadean crust can be found sporadically in the Archean crustal domains. All these relics share similar isotope characteristics indicating very early isolation of felsic crustal domains. However, during the Hadean-Archean transition, these domains disappeared, and until ca. 3.0 Ga, during one billion years, Earth was dominated by juvenile igneous rocks. The reasons for such drastic differences between the Hadean and Archean remain unclear and their understanding is complicated by the rarity of Hadean material available for investigation. Application of the newly developed isotope methods such as Zr, Si, and Fe isotope measurement in zircon can potentially shed light on this mystery.
Dissemination of the research results
Obtained during the project implementation results were reported at the following conferences and scientific meetings:
List of publications
Project administrative assistant:
Mgr Eżbieta Gogacz
e-mail: e.gogacz@twarda.pan.pl
tel. + 48 22 697 87 00
Registered office:
ul. Twarda 51/55, 00-818 Warszawa
NIP: 5250008896, REGON: 000326345
Address for correspondence:
ul. Twarda 51/55, 00-818 Warszawa
