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Abstract
Sedimentary rocks and detrital minerals sample large areas of the
continental crust, and they are increasingly seen as a reliable archive
for its global evolution. This study presents two approaches to model
the growth of the continental crust through the sedimentary archive. The
first builds on the variations in U-Pb, Hf and O isotopes in global
databases of detrital zircons. We show that uncertainty in the Hf
isotope composition of the mantle reservoir from which new crust
separated, in the 176Lu/177Hf ratio of that new
crust, and in the contribution in the databases of zircons that
experienced ancient Pb loss(es), adds some uncertainty to the individual
Hf model ages, but not to the overall shape of the calculated
continental growth curves. The second approach is based on the variation
of Nd isotopes in 645 worldwide fine-grained continental sedimentary
rocks with different deposition ages, which requires a correction of the
bias induced by preferential erosion of younger rocks through an
erosion parameter referred to as K. This dimensionless
parameter relates the proportions of younger to older source rocks in
the sediment, to the proportions of younger to older source rocks
present in the crust from which the sediment was derived. We suggest
that a Hadean/Archaean value of K = 1 (i.e., no preferential erosion), and that post-Archaean values of K = 4–6,
may be reasonable for the global Earth system. Models built on the
detrital zircon and the fine-grained sediment records independently
suggest that at least 65% of the present volume of continental crust was
established by 3 Ga. The continental crust has been generated
continuously, but with a marked decrease in the growth rate at ~ 3 Ga.
The period from > 4 Ga to ~ 3 Ga is characterised by relatively high
net rates of continental growth (2.9–3.4 km3 yr− 1
on average), which are similar to the rates at which new crust is
generated (and destroyed) at the present time. Net growth rates are much
lower since 3 Ga (0.6–0.9 km3 yr− 1 on average),
which can be attributed to higher rates of destruction of continental
crust. The change in slope in the continental growth curve at ~ 3 Ga is
taken to indicate a global change in the way bulk crust was generated
and preserved, and this change has been linked to the onset of
subduction-driven plate tectonics. At least 100% of the present volume
of the continental crust has been destroyed and recycled back into the
mantle since ~ 3 Ga, and this time marks a transition in the average
composition of new continental crust. Continental crust generated before
3 Ga was on average mafic, dense, relatively thin (< 20 km) and
therefore different from the calc-alkaline andesitic crust that
dominates the continental record today. Continental crust that formed
after 3 Ga gradually became more intermediate in composition, buoyant
and thicker. The increase in crustal thickness is accompanied by
increasing rates of crustal reworking and increasing input of sediment
to the ocean. These changes may have been accommodated by a change in
lithospheric strength at around 3 Ga, as it became strong enough to
support high-relief crust. This time period therefore indicates when
significant volumes of continental crust started to become emergent and
were available for erosion and weathering, thus impacting on the
composition of the atmosphere and the oceans.
Original language | English |
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Pages (from-to) | 16-32 |
Number of pages | 17 |
Journal | Sedimentary Geology |
Volume | 357 |
Early online date | 6 Jun 2017 |
DOIs | |
Publication status | Published - 15 Jul 2017 |
Keywords
- Continental growth
- Hadean/Archaean
- Plate tectonics
- Zircon; Shale
- U-Pb/Hf/Nd/O isotopes
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Dive into the research topics of 'Continental growth seen through the sedimentary record'. Together they form a unique fingerprint.Projects
- 1 Finished
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The onset of plate tectonics 3 billion years ago - New insights from Ca-Sr-Pb-Nd isotopes and implications for large-scale crustal evolution.
Dhuime, B. P. M. (Principal Investigator)
1/10/13 → 30/09/17
Project: Research