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EARLY EARTH Report Summary

Project ID: 258658
Funded under: FP7-IDEAS-ERC
Country: Germany

Final Report Summary - EARLY EARTH (Early Earth Dynamics: Pt-Re-Os isotopic constraints on Hadean-Early Archean mantle evolution)

During the first 750 million years (Ma) of Earth history, known as the Hadean era (~4.56 - ~3.8 Ga), fundamental events shaped the Early Earth, precursor of the planet we live on today. The Hadean period may have seen the emergence of the first continents and was probably a defining period in the emergence of life.
There is however no Hadean rocks preserved at Earth’s surface that we can directly study to understand Earth’s infancy. Our knowledge hangs on few 100s of minerals, namely zircons (ZrO4), millimetric in size, possibly derived from these first continental crusts and recycled within younger, Archean (3.8-2.5 Ga) geological formations. These minerals support formation of Earth’s crust as early as ca.~ 4.4 Ga –ca. 160 Ma after the formation of our Solar System- from melting of the Earth’s mantle. Considering the genetic link between Earth’s mantle and crust, any event of crust formation should be mirrored by a melting event of Earth’s mantle. This is the rationale behind the ERC project “Early Earth”, which aimed for the first time ever at investigating the Hadean Earth history from the perspective of the Earth’s mantle.
Mantle rocks contain micrometric to nanometric sulfides of base metals (Fe-Ni-Cu) and alloys or sulfides of highly siderophile elements (HSE). They are the main hosts of HSE, especially osmium, rhenium and platinum; these 3 elements constituting two long-lived geochronometers based on the radioactive decay of 187Re into 187Os and of 190Pt into 186Os, with decay rates (i.e. half-lives) of ca. 42 and 470 Ga, respectively.
The ERC project “Early Earth” focused on dating the sulfides and alloys in the oldest mantle rocks available on our planet and integrating these in the detailed multidisciplinary (mineralogical and petrological) framework of the host mantle rock. The micrometric to nanometric size of our target minerals constituted a challenge both for their efficient extraction from the host rock, the analytical procedure and the measurement of the Os isotopic composition via negative thermal ionisation mass spectrometry but allowed us to open a new dimension (nanometric) in the size of the minerals we could study and what is analytically achievable for Re-Pt-Os geochronometers investigations.
Our target mantle rocks are Archean cratonic mantle peridotites associated with the North Atlantic (Greenland) and Kaapvaal-Zimbabwe (South Africa-Botswana-Zimbabwe) cratons (oldest continental masses preserved on Earth) and Archean chromite deposits.

1) Our investigations confirm that sulfides and alloys are robust time capsules and provide a high time resolution of the Earth’s mantle history [Coggon et al., 2013; 2015; Wainwright et al., 2015; 2016; van Acken et al., 2017], sometimes much older than the host mantle rocks. This sets new constraints on the timing and possibly formation models (i.e. using age distribution models) of the oldest continental masses. However, when sulfides contain alloys inclusions their age signatures does not reflect partial melting (or crust formation) but the age of the inclusions’s formation [Wainwright et al., 2016]. Extreme caution must then be exerted for their integration in the early crust formation models.

2) The oldest melting event, preserved in the Greenland Archean chromites, is 4.36 Ga old, extending the record of large mantle-melting events into the Hadean [Coggon et al., 2013]. This age is strikingly similar to the oldest crust formation event derived from zircons, providing undisputable evidences that Earth’s geological history even the earliest one is faithfully recorded in both its mantle and crust reservoirs.

3)Finally the similarity between the HSE composition of the Greenland chromite deposits and present-day chromites suggests that Earth was re-enriched in volatiles and HSE at least 200 millions years earlier than previously thought and thus could already have been habitable 4.36 billion years ago [Coggon et al., 2013].

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