Processes of crust formation in the early Earth imaged through Hf isotopes from the East Pilbara Terrane

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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Continental crust formation on early Earth controlled by intrusive magmatism

Nature ◽

10.1038/nature22042 ◽

2017 ◽

Vol 545 (7654) ◽

pp. 332-335 ◽

Cited By ~ 77

Author(s):

A. B. Rozel ◽

G. J. Golabek ◽

C. Jain ◽

P. J. Tackley ◽

T. Gerya

Keyword(s):

Continental Crust ◽

Early Earth ◽

Crust Formation

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Archaean Cratons Record a Duality of Melting Regimes During Early Continental Growth

10.21203/rs.3.rs-53939/v1 ◽

2020 ◽

Author(s):

S. Amrei Ladwig ◽

Priyadarshi Chowdhury ◽

Alex J. McCoy-West ◽

Oliver Nebel ◽

Peter Cawood

Keyword(s):

Rare Earth ◽

Plate Tectonics ◽

Direct Evidence ◽

Tectonic Setting ◽

Building Blocks ◽

Early Earth ◽

Crust Formation ◽

Mineral Assemblages ◽

Ree Patterns ◽

Continental Growth

Abstract The tectonic setting and pressure-temperature conditions responsible for the formation of felsic crust on the early Earth remain debated. Rare earth elements (REE) have been extensively used to study the formation of tonalite-trondhjemites-granodiorites (TTGs)- the building blocks of the early felsic crust, but conclusive interpretations based on the chondrite-normalized REE patterns have not materialised because of the inability to distinguish subtle differences. Here we apply a polynomial approach that quantifies the REE patterns by describing their slope and curvature using shaping coefficients to the TTG compositions from five different Archaean cratons. In combination with partial melting modelling, this enables an assessment of the effects of variations in pressure-temperature, degree of melting and residual mineral assemblages on the formation of TTGs. The REE composition of the Archaean TTGs display two distinct trends: (1) a horizontal-trend suggesting their formation in the presence of garnet-poor amphibolitic residues, possibly formed at the base of a thickened crust; and, (2) an inclined-trend consistent with their formation in equilibration with amphibole-poor, but garnet-rich residues at convergent settings (but not necessarily related to plate tectonics). These different melting regimes coexisted during the Paleoarchaean to Neoarchaean and provide direct evidence for a duality of petro-tectonic regimes of felsic crust formation on the early Earth.

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Rb-Sr, Sm-Nd and Lu-Hf isotope systematics of the lunar Mg-suite: the age of the lunar crust and its relation to the time of Moon formation

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

10.1098/rsta.2013.0246 ◽

2014 ◽

Vol 372 (2024) ◽

pp. 20130246 ◽

Cited By ~ 54

Author(s):

Richard W. Carlson ◽

Lars E. Borg ◽

Amy M. Gaffney ◽

Maud Boyet

Keyword(s):

Source Region ◽

Early Earth ◽

Lunar Crust ◽

Crust Formation ◽

Crustal Rocks ◽

Incompatible Elements ◽

Isotopic Analyses ◽

History Of ◽

Isotope Systematics ◽

Source Materials

New Rb-Sr, 146,147 Sm- 142,143 Nd and Lu-Hf isotopic analyses of Mg-suite lunar crustal rocks 67667, 76335, 77215 and 78238, including an internal isochron for norite 77215, were undertaken to better define the time and duration of lunar crust formation and the history of the source materials of the Mg-suite. Isochron ages determined in this study for 77215 are: Rb-Sr=4450±270 Ma, 147 Sm- 143 Nd=4283±23 Ma and Lu-Hf=4421±68 Ma. The data define an initial 146 Sm/ 144 Sm ratio of 0.00193±0.00092 corresponding to ages between 4348 and 4413 Ma depending on the half-life and initial abundance used for 146 Sm. The initial Nd and Hf isotopic compositions of all samples indicate a source region with slight enrichment in the incompatible elements in accord with previous suggestions that the Mg-suite crustal rocks contain a component of KREEP. The Sm/Nd— 142 Nd/ 144 Nd correlation shown by both ferroan anorthosite and Mg-suite rocks is coincident with the trend defined by mare and KREEP basalts, the slope of which corresponds to ages between 4.35 and 4.45 Ga. These data, along with similar ages for various early Earth differentiation events, are in accord with the model of lunar formation via giant impact into Earth at ca 4.4 Ga.

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