Sampling the deep graveyard of Earth's earliest crust

Date:
April 28, 2022
Source:
University of Cologne
Summary:
Scientists show that remnants of the roots of Earth's first crust
are still present in the terrestrial mantle and contribute to
magmas erupted at the surface over Earth's history.



FULL STORY ==========================================================================
In an international collaboration, Earth scientists at the University
of Cologne and Freie Universita"t Berlin discovered that some magmas
on Earth, which made their way through the deep terrestrial mantle and
erupted at Earth's surface, originate from mantle portions that contain remnants of Earth's earliest crust. This ancient material must have been
buried in a 'graveyard' of old and cold crust more than 4 billion years
ago and survived since then, maybe since the giant impact event forming
the Moon.


==========================================================================
This finding is unexpected because the plate tectonic regime of our
planet progressively recycles crustal material via large-scale mantle convection at much smaller time scales. Therefore, it has been assumed
that vestiges of early geological processes on Earth can only be found
as analogues, on other terrestrial planets (Mercury, Venus, and Mars), asteroids, or the Moon.

However, according to their study 'Long-term preservation of Hadean
protocrust in Earth's mantle', which has recently appeared in the
Proceedings of the National Academy of Sciences (PNAS), magmatic rocks
that erupted throughout Earth's history can still carry signatures
that provide detailed information about the nature of the first crust,
its long-term preservation in a graveyard in the lower-most mantle,
and its resurrection via younger volcanism.

For their study, the geologists investigated up to 3.55 billion years old
rocks from southern Africa. The analysis of these rocks revealed small anomalies in the isotope composition of the element tungsten (W). The
origin of these isotope anomalies, namely the relative abundance of 182W, relates to geological processes that must have occurred immediately
after the formation of the Earth more than 4.5 billion years ago.

Model calculations by the authors show that the observed 182W isotope
patterns are best explained by the recycling of Earth's earliest crust
into mantle material that ascends via plumes from the lower mantle to
generate lavas erupting at Earth's surface. Intriguingly, the study
shows that similar isotope patterns can be observed in distinct types
of modern volcanic rocks (ocean island basalts), which demonstrates that Earth's earliest crust is still buried in the lowermost mantle.

'We assume that the lower layers of the crust -- or the roots of the
primordial continents -- became heavier than their surroundings due
to a geological maturation process and therefore sank into the Earth's underlying mantle.

Similar to a lava lamp,' geochemist Dr Jonas Tusch from the University of Cologne's Institute of Geology and Mineralogy remarked. 'This fascinating insight provides a geochemical fingerprint of the young Earth, allowing
us to better understand how large continents formed over the history of
our planet.

It also explains how our current, oxygen-rich atmosphere evolved --
setting the stage for the origin of complex life,' Dr Elis Hoffmann of
Freie Universita"t Berlin added.

The geochemical fingerprint of the early Earth can also be compared with findings about other planets obtained during space missions. For example,
data from Mars missions and studies of Martian meteorites show that
Mars still has a very old surface due to the lack of plate tectonics,
and that its composition may correspond to that of the young Earth.


========================================================================== Story Source: Materials provided by University_of_Cologne. Note: Content
may be edited for style and length.


========================================================================== Related Multimedia:
* 3.48_billion-year-old_rocks ========================================================================== Journal Reference:
1. Jonas Tusch, J. Elis Hoffmann, Eric Hasenstab, Mario Fischer-Go"dde,
Chris S. Marien, Allan H. Wilson, Carsten Mu"nker. Long-term
preservation of Hadean protocrust in Earth's mantle. Proceedings
of the National Academy of Sciences, 2022; 119 (18) DOI:
10.1073/pnas.2120241119 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/04/220428142822.htm

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