Remixed mantle suggests early start of plate tectonics

Date:
June 12, 2020
Source:
Curtin University
Summary:
New research on the remixing of Earth's stratified deep interior
suggests that global plate tectonic processes, which played a
pivotal role in the existence of life on Earth, started to operate
at least 3.2 billion years ago.



FULL STORY ==========================================================================
New Curtin University research on the remixing of Earth's stratified deep interior suggests that global plate tectonic processes, which played a
pivotal role in the existence of life on Earth, started to operate at
least 3.2 billion years ago.


========================================================================== Published in Nature's Scientific Reports, researchers from Curtin
University's Earth Dynamics Research Group re-analysed global data to
detect sudden changes in the chemical characteristics of basalt and
komatiite lava rocks, believed to have been derived from Earth's upper
and lower mantle layers and erupted to the surface between two and four
billion years ago.

Lead researcher PhD Candidate Mr Hamed Gamal El Dien, from Curtin's
School of Earth and Planetary Sciences, said there was much scientific
debate over the exact start date of plate tectonics on Earth.

"Some scientists believe plate tectonics only began to operate from
around 800 million years ago, whereas others think it could go as far
back as four billion years ago, soon after the formation of our planet,"
Mr Gamal El Dien said.

"So far nearly all the evidence used in this debate came from scarcely preserved surface geological proxies, and little attention has been paid
to the record kept by Earth's deep mantle -- this is where our research
comes in.

"For the first time, we were able to demonstrate that a significant shift
in mantle composition (or a major mantle remixing) started around 3.2
billion years ago, indicating a global recycling of the planet's crustal materials back in to its mantle layer, which we believe shows the start
of global plate tectonic activity." During the earliest stages of Earth's planetary differentiation, the planet was divided into three main layers:
the core, the mantle and the crust. Scientists believe there would have
been very little remixing between the lighter crust and the much denser
mantle, until the onset of plate tectonics.



========================================================================== However through the ongoing process of subduction, some lighter crustal materials are carried back into the denser deep Earth and remixed with
the mantle. The question the researchers then asked was, when did this
global and whole-mantle remixing process start? "Keeping the basic
process of subduction in mind, we hypothesise that ancient rock samples
found on the crust, that are ultimately sourced from the deep mantle,
should show evidence of the first major 'stirring up' in the mantle
layer, marking the start of plate subduction as a vital component of
plate tectonic processes," Mr Gamal El Dien said.

To complete this research, the team looked at the time variation of the isotopic and chemical composition of approximately 6,000 mantle-derived basaltic and komatiitic lava rocks, dated to be between two and four
billion years old.

Research co-author John Curtin Distinguished Professor and Australian
Laureate Fellow Professor Zheng-Xiang Li, head of the Earth Dynamics
Research Group, said the research is highly significant in understanding
the dynamic evolution of our planet.

"Plate tectonic activity on the planet is responsible for the formation
of mineral and energy resources. It also plays a vital role for the very existence of humankind. Plate tectonics are found uniquely operative on
Earth, the only known habitable planet," Professor Li said.

"Through our retrospective analysis of mantle-derived samples, we
discovered that after the initial chemical stratification and formation
of a hard shell in the first billion years of Earth's 4.5 billion year
history, there was indeed a major chemical 'stir up' some 3.2 billion
years ago.

"We take this 'stir up' as the first direct evidence from deep Earth
that plate tectonics started over 3 billion years ago, leading to a
step change in mantle composition, followed by the oxygenation of our atmosphere and the evolution of life."

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


========================================================================== Journal Reference:
1. Hamed Gamal El Dien, Luc S. Doucet, J. Brendan Murphy, Zheng-Xiang
Li.

Geochemical evidence for a widespread mantle re-enrichment
3.2 billion years ago: implications for global-scale
plate tectonics. Scientific Reports, 2020; 10 (1) DOI:
10.1038/s41598-020-66324-y ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200612111430.htm

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