Mafic Archean continental crust prohibited exhumation of orogenic UHP eclogite

Abstract The absence of ultrahigh pressure (UHP) orogenic eclogite in the geological record older than c. 0.6 Ga is problematic for evidence of subduction having begun on Earth during the Archean (4.0–2.5 Ga). Many eclogites in Phanerozoic and Proterozoic terranes occur as mafic boudins encased within low-density felsic crust, which provides positive buoyancy during subduction; however, recent geochemical proxy analysis shows that Archean continental crust was more mafic than previously thought. Here, we show via petrological modelling that secular change in the composition of upper continental crust (UCC) would make Archean continental terranes negatively buoyant in the mantle before reaching UHP conditions. Subducted or delaminated Archean continental crust passes a point of no return during metamorphism in the mantle prior to the stabilization of coesite, while Proterozoic and Phanerozoic terranes remain positively buoyant at these depths. UHP orogenic eclogite may thus readily have formed on the Archean Earth, but could not have been exhumed, weakening arguments for a Neoproterozoic onset of subduction and plate tectonics. Further, isostatic balance calculations for more mafic Archean continents indicate that the early Earth was covered by a global ocean over 1 kilometre deep.

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Changing exhumation potential of (U)HP eclogite through geological time

10.5194/egusphere-egu2020-2635 ◽

2020 ◽

Author(s):

Richard Palin

Keyword(s):

Continental Crust ◽

Plate Tectonics ◽

Global Network ◽

Ultrahigh Pressure ◽

Compositional Variation ◽

Uhp Metamorphism ◽

Geological Time ◽

Geological Record ◽

Mafic Intrusions ◽

Archean Crust

<p>Ultrahigh-pressure (UHP) metamorphism is defined by achieving P&#8211;T conditions sufficient to transform quartz to coesite (~26&#8211;28 kbar at ~500&#8211;900 &#176;C), which occurs at ~90-100 km depth within the Earth under lithostatic conditions. Thus, the occurrence of UHP metamorphism is often taken as being a diagnostic indicator of subduction having operated in the geological record, and hence plate tectonics. Yet, the oldest such coesite-bearing rocks belong to the Pan-African belt in northern Mali, and formed at 620 Ma, although there exist multiple lines of evidence to show that a global network of subduction had been operative on Earth for billions of years beforehand. Why, then, are these key geodynamic indicators missing from the majority of the rock record? Here, I show how secular cooling of the Earth's mantle since the Mesoarchean (c. 3.2 Ga) has affected the exhumation potential of UHP (and HP) eclogite through time due to time-dependent compositional variation of both oceanic and continental crust. Petrological modeling of density changes during metamorphism of Archean, Proterozoic, and Phanerozoic composite continental terranes shows that more mafic Archean crust reaches a point-of-no-return during transport into the mantle at shallower depths than less MgO-rich modern-day crust, regardless of whether this occurs via subduction of stagnant lid-like vertical 'drip' tectonics. Thus, while Alpine- and Himalayan-type (U)HP orogenic eclogites represented by metamorphosed mafic intrusions into continental crust may readily have formed during the Precambrian, they would have lacked the buoyancy required for exhumation and preservation in the geological record.</p>

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Two types of Archean continental crust: Plume and plate tectonics on early Earth

American Journal of Science ◽

10.2475/10.2010.01 ◽

2010 ◽

Vol 310 (10) ◽

pp. 1187-1209 ◽

Cited By ~ 119

Author(s):

M. J. Van Kranendonk

Keyword(s):

Continental Crust ◽

Plate Tectonics ◽

Early Earth

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Hydrated Peridotite – Basaltic Melt Interaction Part I: Planetary Felsic Crust Formation at Shallow Depth

Frontiers in Earth Science ◽

10.3389/feart.2021.640464 ◽

2021 ◽

Vol 9 ◽

Author(s):

Anastassia Y. Borisova ◽

Nail R. Zagrtdenov ◽

Michael J. Toplis ◽

Wendy A. Bohrson ◽

Anne Nédélec ◽

...

Keyword(s):

Continental Crust ◽

Plate Tectonics ◽

Mafic Magma ◽

Aqueous Fluid ◽

Early Earth ◽

Magma Ocean ◽

Crust Formation ◽

Basaltic Melt ◽

Low Pressures ◽

Direct Melting

Current theories suggest that the first continental crust on Earth, and possibly on other terrestrial planets, may have been produced early in their history by direct melting of hydrated peridotite. However, the conditions, mechanisms and necessary ingredients for this crustal formation remain elusive. To fill this gap, we conducted time-series experiments to investigate the reaction of serpentinite with variable proportions (from 0 to 87 wt%) of basaltic melt at temperatures of 1,250–1,300°C and pressures of 0.2–1.0 GPa (corresponding to lithostatic depths of ∼5–30 km). The experiments at 0.2 GPa reveal the formation of forsterite-rich olivine (Fo90–94) and chromite coexisting with silica-rich liquids (57–71 wt% SiO2). These melts share geochemical similarities with tonalite-trondhjemite-granodiorite rocks (TTG) identified in modern terrestrial oceanic mantle settings. By contrast, liquids formed at pressures of 1.0 GPa are poorer in silica (∼50 wt% SiO2). Our results suggest a new mechanism for the formation of the embryonic continental crust via aqueous fluid-assisted partial melting of peridotite at relatively low pressures (∼0.2 GPa). We hypothesize that such a mechanism of felsic crust formation may have been widespread on the early Earth and, possibly on Mars as well, before the onset of modern plate tectonics and just after solidification of the first ultramafic-mafic magma ocean and alteration of this primitive protocrust by seawater at depths of less than 10 km.

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The inception of plate tectonics: a record of failure

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2017.0414 ◽

2018 ◽

Vol 376 (2132) ◽

pp. 20170414 ◽

Cited By ~ 13

Author(s):

Craig O'Neill ◽

Simon Turner ◽

Tracy Rushmer

Keyword(s):

Subduction Zones ◽

Plate Tectonics ◽

Initial Conditions ◽

Numerical Models ◽

Early History ◽

Early Earth ◽

Strong Nonlinearity ◽

Plate Tectonic ◽

Geological Record ◽

History Of

The development of plate tectonics from a pre-plate tectonics regime requires both the initiation of subduction and the development of nascent subduction zones into long-lived contiguous features. Subduction itself has been shown to be sensitive to system parameters such as thermal state and the specific rheology. While generally it has been shown that cold-interior high-Rayleigh-number convection (such as on the Earth today) favours plates and subduction, due to the ability of the interior stresses to couple with the lid, a given system may or may not have plate tectonics depending on its initial conditions. This has led to the idea that there is a strong history dependence to tectonic evolution—and the details of tectonic transitions, including whether they even occur, may depend on the early history of a planet. However, intrinsic convective stresses are not the only dynamic drivers of early planetary evolution. Early planetary geological evolution is dominated by volcanic processes and impacting. These have rarely been considered in thermal evolution models. Recent models exploring the details of plate tectonic initiation have explored the effect of strong thermal plumes or large impacts on surface tectonism, and found that these ‘primary drivers’ can initiate subduction, and, in some cases, over-ride the initial state of the planet. The corollary of this, of course, is that, in the absence of such ongoing drivers, existing or incipient subduction systems under early Earth conditions might fail. The only detailed planetary record we have of this development comes from Earth, and is restricted by the limited geological record of its earliest history. Many recent estimates have suggested an origin of plate tectonics at approximately 3.0 Ga, inferring a monotonically increasing transition from pre-plates, through subduction initiation, to continuous subduction and a modern plate tectonic regime around that time. However, both numerical modelling and the geological record itself suggest a strong nonlinearity in the dynamics of the transition, and it has been noted that the early history of Archaean greenstone belts and trondhjemite–tonalite–granodiorite record many instances of failed subduction. Here, we explore the history of subduction failure on the early Earth, and couple these with insights from numerical models of the geodynamic regime at the time. This article is part of a discussion meeting issue ‘Earth dynamics and the development of plate tectonics'.

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Early earth geodynamics: cross examining the geological testimony

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2018.0169 ◽

2018 ◽

Vol 376 (2132) ◽

pp. 20180169 ◽

Cited By ~ 3

Author(s):

Anthony I. S. Kemp

Keyword(s):

Plate Tectonics ◽

Input Data ◽

Detrital Zircons ◽

Early Earth ◽

Complex Nature ◽

Careful Selection ◽

Geological Record ◽

Multi Scale ◽

Earth Dynamics ◽

Lines Of Evidence

Many studies link the presence of continents on Earth to the operation of plate tectonics. Radiogenic isotope data have, however, long consigned the bulk of crust generation and preservation to the murky realm of the Precambrian Earth, where the prevailing geodynamic systems are highly uncertain due to the sparse and complex nature of the geological record of these early eons. The purpose of this paper is to examine the nature of this geological record, considering the biases and artefacts that may undermine its fidelity, and to assess what are the most robust lines of evidence from which meaningful geodynamic inferences can be drawn. This is pursued with reference to Hadean detrital zircons, Archean gneiss complexes and Archean granite–greenstone terranes, and by considering isotopic proxies of crust–mantle interaction. The evidence reinforces long held views that the formation of some of the oldest continental nuclei involved a distinctive mode of planetary geodynamics that rests uneasily within definitions of modern style plate tectonics. A detailed interrogation of the oldest rocks, integrating multi-scale information from the best preserved whole-rock and mineral archives, and emphasizing careful selection at the sampling and analytical stages, will lead to the most robust input data for petrological and thermodynamic models of early Earth processes. This article is part of a discussion meeting issue ‘Earth dynamics and the development of plate tectonics’.

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Crustal evolution and mantle dynamics through Earth history

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2017.0408 ◽

2018 ◽

Vol 376 (2132) ◽

pp. 20170408 ◽

Cited By ~ 30

Author(s):

Jun Korenaga

Keyword(s):

Continental Crust ◽

Mantle Convection ◽

Plate Tectonics ◽

Indirect Evidence ◽

Quantitative Model ◽

Early Earth ◽

Mantle Dynamics ◽

Earth History ◽

Geological Processes ◽

Earth Dynamics

Resolving the modes of mantle convection through Earth history, i.e. when plate tectonics started and what kind of mantle dynamics reigned before, is essential to the understanding of the evolution of the whole Earth system, because plate tectonics influences almost all aspects of modern geological processes. This is a challenging problem because plate tectonics continuously rejuvenates Earth's surface on a time scale of about 100 Myr, destroying evidence for its past operation. It thus becomes essential to exploit indirect evidence preserved in the buoyant continental crust, part of which has survived over billions of years. This contribution starts with an in-depth review of existing models for continental growth. Growth models proposed so far can be categorized into three types: crust-based, mantle-based and other less direct inferences, and the first two types are particularly important as their difference reflects the extent of crustal recycling, which can be related to subduction. Then, a theoretical basis for a change in the mode of mantle convection in the Precambrian is reviewed, along with a critical appraisal of some popular notions for early Earth dynamics. By combining available geological and geochemical observations with geodynamical considerations, a tentative hypothesis is presented for the evolution of mantle dynamics and its relation to surface environment; the early onset of plate tectonics and gradual mantle hydration are responsible not only for the formation of continental crust but also for its preservation as well as its emergence above sea level. Our current understanding of various material properties and elementary processes is still too premature to build a testable, quantitative model for this hypothesis, but such modelling efforts could potentially transform the nature of the data-starved early Earth research by quantifying the extent of preservation bias.This article is part of a discussion meeting issue ‘Earth dynamics and the development of plate tectonics’.

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