Periodic Reporting for period 2 - MILESTONE (From mineral inclusions in zircon to continents: An in situ isotopic perspective on the evolution of the continental crust, the onset of plate tectonics and the development of a habitable Earth)
Período documentado: 2021-03-01 hasta 2022-08-31
The timing for the onset of plate tectonics is a key step in understanding the evolution of our planet. It is fundamental to resolving questions on the origin of life, the evolution and oxygenation of our atmosphere, past climates, mass extinctions, the thermal evolution of Earth, the interactions between the surficial and deep Earth, and the development of continents. The MILESTONE project brings new insights into when plate tectonics started and how continents formed and evolved through time.
Zircon lies at the core of Earth evolution studies, and yet our knowledge has remained restricted to the geochemical information that can be extracted from this mineral with current techniques. MILESTONE moves the debate to a different scale analytically, to the scale of tiny (i.e. <20 µm in size) mineral inclusions encapsulated within zircons. The integrated analysis of Sr and Pb isotopes of mineral inclusions, along with the trace elements, U-Pb, Hf and O isotopes analysis of their host zircons, for thousands of zircons of different ages and provenance, will provide new and different information to that available from the 'zircon only' record.
The overall objectives of MILESTONE are to:
i) Probe the inferred transition from intraplate- to subduction-related magmatism associated with the onset of plate tectonics;
ii) Date this transition and its duration precisely in different places;
iii) Develop a global model of continental crust evolution from the Hadean (i.e. >4 Ga) to the Present, in which the Earth has progressively, or more suddenly, become a habitable planet.
Zircon lies at the core of Earth evolution studies, and yet our knowledge has remained restricted to the geochemical information that can be extracted from this mineral with current techniques. MILESTONE moves the debate to a different scale analytically, to the scale of tiny (i.e. <20 µm in size) mineral inclusions encapsulated within zircons. The integrated analysis of Sr and Pb isotopes of mineral inclusions, along with the trace elements, U-Pb, Hf and O isotopes analysis of their host zircons, for thousands of zircons of different ages and provenance, will provide new and different information to that available from the 'zircon only' record.
The overall objectives of MILESTONE are to:
i) Probe the inferred transition from intraplate- to subduction-related magmatism associated with the onset of plate tectonics;
ii) Date this transition and its duration precisely in different places;
iii) Develop a global model of continental crust evolution from the Hadean (i.e. >4 Ga) to the Present, in which the Earth has progressively, or more suddenly, become a habitable planet.
The first step of the MILESTONE project was to build a state-of-the-art isotope geochemistry laboratory, in which ultra-high precision analyses could be achieved by laser ablation. This step was completed during the first year of the action, after which a number of analytical approaches have been developed:
i) The in situ U-Th-Pb dating of zircon with a spatial resolution better than 12x12 µm.
ii) The in situ measurement of Hf isotopes in zircon with a spatial resolution better than 25x25 µm.
iii) The in situ measurement of Pb isotopes in feldspar with a spatial resolution better than 12x12 µm.
iv) The in situ measurement of Sr isotopes in apatite with a spatial resolution better than 15x15 µm.
The second step (in progress) of MILESTONE is to sample and analyse zircons from the five continents, with crystallisation ages ranging from the Hadean (i.e. > 4 Ga) to the Phanerozoic (i.e. < 0.54 Ga). Amongst the most significant results achieved so far:
- Zircons from Palaeoarcheaean granitoids sampled across the Pilbara Craton, Western Australia, revealed a major juvenile crust formation event at 3.5 Ga. This event resulted in the differentiation, from a slightly depleted mantle source, of two crustal reservoirs that remained isolated from each other and from any new mantle input for over about 300 million years, until at least 3.23 Ga. This suggests that protracted intra-crustal remelting processes and differentiation have played a key role in the formation, evolution and maturation of the building blocks of continents during the Palaeoarchaean, and that 'vertical tectonics' dominated until at least ca. 3.2-3.0 Ga within the studied area.
- The statistical analysis of the Lu-Hf data of over 100,000 zircons from the 5 continents revealed that the depth of crystallisation of zircon-bearing magmas has changed with periodic oscillations of 500-600 million years. Such periodicity can be linked to the supercontinent cycles, and the presence of periodic oscillations from ca. 3 Ga may indicate the global onset of plate tectonics.
i) The in situ U-Th-Pb dating of zircon with a spatial resolution better than 12x12 µm.
ii) The in situ measurement of Hf isotopes in zircon with a spatial resolution better than 25x25 µm.
iii) The in situ measurement of Pb isotopes in feldspar with a spatial resolution better than 12x12 µm.
iv) The in situ measurement of Sr isotopes in apatite with a spatial resolution better than 15x15 µm.
The second step (in progress) of MILESTONE is to sample and analyse zircons from the five continents, with crystallisation ages ranging from the Hadean (i.e. > 4 Ga) to the Phanerozoic (i.e. < 0.54 Ga). Amongst the most significant results achieved so far:
- Zircons from Palaeoarcheaean granitoids sampled across the Pilbara Craton, Western Australia, revealed a major juvenile crust formation event at 3.5 Ga. This event resulted in the differentiation, from a slightly depleted mantle source, of two crustal reservoirs that remained isolated from each other and from any new mantle input for over about 300 million years, until at least 3.23 Ga. This suggests that protracted intra-crustal remelting processes and differentiation have played a key role in the formation, evolution and maturation of the building blocks of continents during the Palaeoarchaean, and that 'vertical tectonics' dominated until at least ca. 3.2-3.0 Ga within the studied area.
- The statistical analysis of the Lu-Hf data of over 100,000 zircons from the 5 continents revealed that the depth of crystallisation of zircon-bearing magmas has changed with periodic oscillations of 500-600 million years. Such periodicity can be linked to the supercontinent cycles, and the presence of periodic oscillations from ca. 3 Ga may indicate the global onset of plate tectonics.
Although more isotopic data have yet to be collected, current data show no compelling evidence for global plate tectonics in the first 1.5 billion years of Earth’s evolution. Our best estimate (so far) for when plate tectonics started is ca. 3 Ga.