ArticlePublisher preview available

Giant impacts and the origin and evolution of continents

August 2022
Nature 608(7922):330-335

DOI:10.1038/s41586-022-04956-y

Authors:

Tim E. Johnson

Curtin University

Yong-jun Lu

Geological Survey of Western Australia

Show all 6 authorsHide

Earth is the only planet known to have continents, although how they formed and evolved is unclear. Here using the oxygen isotope compositions of dated magmatic zircon, we show that the Pilbara Craton in Western Australia, Earth’s best-preserved Archaean (4.0–2.5 billion years ago (Ga)) continental remnant, was built in three stages. Stage 1 zircons (3.6–3.4 Ga) form two age clusters with one-third recording submantle δ18O, indicating crystallization from evolved magmas derived from hydrothermally altered basaltic crust like that in modern-day Iceland1,2. Shallow melting is consistent with giant impacts that typified the first billion years of Earth history3–5. Giant impacts provide a mechanism for fracturing the crust and establishing prolonged hydrothermal alteration by interaction with the globally extensive ocean6–8. A giant impact at around 3.6 Ga, coeval with the oldest low-δ18O zircon, would have triggered massive mantle melting to produce a thick mafic–ultramafic nucleus9,10. A second low-δ18O zircon cluster at around 3.4 Ga is contemporaneous with spherule beds that provide the oldest material evidence for giant impacts on Earth11. Stage 2 (3.4–3.0 Ga) zircons mostly have mantle-like δ18O and crystallized from parental magmas formed near the base of the evolving continental nucleus12. Stage 3 (<3.0 Ga) zircons have above-mantle δ18O, indicating efficient recycling of supracrustal rocks. That the oldest felsic rocks formed at 3.9–3.5 Ga (ref. 13), towards the end of the so-called late heavy bombardment4, is not a coincidence. Oxygen isotope compositions of dated magmatic zircon show that the Pilbara Craton in Western Australia, Earth’s best-preserved Archaean continental remnant, was built in three stages initiated by a giant meteorite impact.

Zircon oxygen isotope data for magmatic samples superimposed on the geological map of the Pilbara Craton, Western Australia WPT, West Pilbara Terrane; CPTZ, Central Pilbara Tectonic Zone; MCB, Mosquito Creek Basin. The median zircon δ¹⁸O values of 26 samples are shown as contoured circles. Map from ref. ⁵⁵, reproduced under a CC BY 4.0 licence.

…

Zircon δ¹⁸O (‰) versus age (Ga) for individual magmatic zircon grains from the Pilbara Craton All data have concordant (>90%) U–Pb systematics. A LOESS curve (red line) with 2.5 and 97.5 percentile (bottom and top dashed red lines) uncertainties is fitted to the data. The horizontal grey band shows the array of δ¹⁸O in mantle zircon (5.3 ± 0.6‰, 2 s.d.). The vertical grey bands subdivide the data into three stages, as discussed in the text. The pink boxes represent age spans within which several high-energy impact deposits (spherule layers) are recognized in the Pilbara Craton and more widely. Uncertainties on individual data points (1 s.d.) are indicated, as are median δ¹⁸O values with uncertainties (2 s.d.) for each of the individual stages.

…

Geodynamic cartoons illustrating the three-stage evolution of the Pilbara Craton a–c, A giant meteorite impacting hydrothermally altered primary crust (a) produces low δ¹⁸O shallow melts and triggers formation of a mafic–ultramafic continental nucleus by decompression melting of the mantle (b,c). d–f, The nucleus fractionates to produce low-MgO basaltic protoliths (d) that melt to form TTG magmas (e), which themselves melt to form granite (f). Further details are provided in the figure panels and main text.

…

Zircon δ¹⁸O (‰) versus age (Ga) for individual magmatic zircon grains from the Yilgarn Craton and Acasta Gneiss Complex a,b, All data have concordant (>90%) U–Pb systematics. Although data from the Yilgarn Craton (a) has been filtered for ¹⁶O¹H/¹⁶O to eliminate altered grains, that from the Acasta Gneiss Complex (b; data from ref. ⁵⁰) has not. The horizontal grey band shows the array of δ¹⁸O in mantle zircon (5.3 ± 0.6‰, 2 s.d.).

…

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330 | Nature | Vol 608 | 11 August 2022

Article

Giant impacts and the origin and evolution

of continents

Tim E. Johnson1,2 ✉, Christopher L. Kirkland1, Yongjun Lu3,4, R. Hugh Smithies1,3,

Michael Brown5 & Michael I. H. Hartnady1

Earth is the only planet known to have continents, although how they formed and

evolved is unclear. Here using the oxygen isotope compositions of dated magmatic

zircon, we show that the Pilbara Craton in Western Australia, Earth’s best-preserved

Archaean (4.0–2.5 billion years ago (Ga)) continental remnant, was built in three

stages. Stage 1 zircons (3.6–3.4 Ga) form two age clusters with one-third recording

submantle δ18O, indicating crystallization from evolved magmas derived from

hydrothermally altered basaltic crust like that in modern-day Iceland1,2. Shallow

melting is consistent with giant impacts that typied the rst billion years of Earth

history3–5. Giant impacts provide a mechanism for fracturing the crust and

establishing prolonged hydrothermal alteration by interaction with the globally

extensive ocean6–8. A giant impact at around 3.6 Ga, coeval with the oldest low-δ18O

zircon, would have triggered massive mantle melting to produce a thick mac–

ultramac nucleus9,10. A second low-δ18O zircon cluster at around 3.4 Ga is

contemporaneous with spherule beds that provide the oldest material evidence for

giant impacts on Earth11. Stage 2 (3.4–3.0 Ga) zircons mostly have mantle-like δ18O and

crystallized from parental magmas formed near the base of the evolving continental

nucleus12. Stage 3 (<3.0 Ga) zircons have above-mantle δ18O, indicating ecient

recycling of supracrustal rocks. That the oldest felsic rocks formed at 3.9–3.5 Ga

(ref. 13), towards the end of the so-called late heavy bombardment4, is not a coincidence.

Three-quarters or more of the present-day volume of continental crust

was produced in the Archaean Eon (4.0–2.5 billion years ago(Ga))

when many think Earth’s surface was almost entirely covered in water

6,15

a critical ingredient in its manufacture

. The oldest preserved conti-

nental crust mostly comprises sodic granites of the tonalite–trond-

hjemite–granodiorite (TTG) series, derived through partial melting of

hydrated basaltic rocks (amphibolites) at depths of around 25–50 km

(ref. 12). However, how the first continental nuclei formed and evolved

into stable cratons is poorly understood.

During the billion years following accretion, Earth witnessed a

barrage of bolide impacts that caused large-scale melting and recy-

cling of the crust

5,17

. Whether or not this bombardment ended with

a cataclysm (the Late Heavy Bombardment), crater densities on the

Moon and other inner solar system bodies show that the impact rate

declined sharply between 3.9 and 3.5 Ga (ref.

). With a surface area

more than ten times that of the Moon, and a gravity-well more than

20 times as deep, Earth would have endured an impact flux 20–300

times that of its satellite

. That the ages of the oldest continental crust

in most cratons also span the time period 3.9–3.5 Ga (ref. 13) begs the

question of whether this is coincidence or if there is a causal relation-

ship. Although an origin for cratons through giant impacts (that is,

collisions with asteroids several tens to hundreds of kilometres in

diameter) is not a new hypothesis

18,19

, a paucity of direct evidence

means the idea has garnered little support.

Interrogation of time-constrained geochemical tracers of source

material is key to resolving how the nascent continents formed and

evolved. Oxygen isotopes, which are fractionated only by relatively

low-temperature (<400 °C) fluid–rock interactions, offer a power-

ful means of differentiating juvenile from evolved magmas and fin-

gerprinting both reworking (partial melting and/or assimilation) and

recycling (reincorporation into the mantle) of near-surface and suprac-

rustal rocks, which in turn can be linked to geodynamic processes

20–22

The oxygen isotopic compositions of dated magmatic zircon grains,

coupled with the whole-rock composition of their host rocks, provide

a robust time-encoded signature of these processes20,21.

The Pilbara Craton

The Pilbara Craton in Western Australia is among the oldest, best

exposed and most pristine ancient continental fragments on Earth23.

Here, we present insitu

O isotope data (δ

O normalized to Vienna

Standard Mean Ocean Water) from zircons in variably deformed and

metamorphosed igneous rocks, which range in age from 3.6 to 2.9 Ga

and in composition from mafic (hornblende-rich amphibolite) to felsic

https://doi.org/10.1038/s41586-022-04956-y

Received: 29 April 2021

Accepted: 9 June 2022

Published online: 10 August 2022

Check for updates

1School of Earth and Planetary Sciences, the Institute for Geoscience Research, Timescales of Mineral Systems Group, Curtin University, Perth, Western Australia, Australia. 2Centre for Global

Tectonics, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China. 3Department of Mines, Industry Regulation and Safety,

Geological Survey of Western Australia, Perth, Western Australia, Australia. 4Centre for Exploration Targeting and Australian Research Council Centre of Excellence for Core to Crust Fluid

Systems, School of Earth Sciences, The University of Western Australia, Crawley, Western Australia, Australia. 5Laboratory for Crustal Petrology, Department of Geology, University of Maryland,

College Park, MD, USA. ✉e-mail: tim.johnson@curtin.edu.au

Content courtesy of Springer Nature, terms of use apply. Rights reserved

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Jan 2022
EARTH PLANET SC LETT

The formation of stable buoyant continental crust during the Archaean Eon was fundamental in establishing the planet's geochemical reservoirs. However, the processes that created Earth's first continents and the timescales over which they formed are debated. Here, we report the Pb isotope compositions of K-feldspar grains from 52 Paleoarchaean to Neoarchaean granites from the Pilbara Craton in Western Australia, one of the world's oldest and best-preserved granite–greenstone terranes. The Pb isotope composition of the Pilbara K-feldspars is variable, implying the granites were derived from crustal precursors of different age and/or variable time-integrated ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb compositions. Trends to sub-mantle ²⁰⁷Pb/²⁰⁶Pb ratios preclude the influence of 4.3 Ga crustal precursors. In order to estimate crustal residence times we derive equations to calculate source model ages in a linearized Pb isotope evolution system. The best agreement between the feldspar Pb two-stage source model ages and those derived from zircon initial Hf isotope compositions requires crustal precursors that separated from a chondritic mantle source between 3.2 and 3.8 Ga, and rapidly differentiated to continental crust with ²³⁸U/²⁰⁴Pb and ²³²Th/²³⁸U ratios of ∼14 and 4.2–4.5, respectively. The preservation of Pb isotope variability in the Pilbara Paleoarchaean granites indicates their early continental source rocks were preserved for up to 500 Ma after their formation. The apparent longevity of these early continental nuclei is consistent with the incipient development of buoyant melt-depleted cratonic lithosphere during the Eoarchaean to Paleoarchaean.

The coupled Hf-Nd isotope record of the early Earth in the Pilbara Craton

Article

Aug 2021
EARTH PLANET SC LETT

Initial Hf and Nd isotope compositions of Earth's oldest rocks provide essential information on the differentiation of the Earth into enriched crustal and depleted mantle reservoirs in its early history. The majority of Eo-Paleoarchean rocks worldwide, however, have isotope compositions that appear to be decoupled: initial Hf isotope compositions, determined on zircon, are broadly chondritic with little variation; initial Nd isotopes on bulk rocks, in contrast are highly variable with both supra- and sub-chondritic compositions. Most of these studies are from polymetamorphic terranes where the potential for disturbance of the isotope system is high. This is particularly true for the Sm-Nd system where more easily altered REE-rich accessory phases are the major repositories for these elements. In order to better understand crust-mantle evolution during the Archean—and to address the issue of Hf and Nd isotope decoupling—we examine a suite of well-preserved Paleoarchean granites from the Pilbara Craton. Our approach integrates the initial Hf isotope composition and U-Pb ages of zircon, the initial Nd isotope compositions of titanite and apatite, and U-Pb ages of titanite by laser ablation split stream (LASS) analysis. The zircon and titanite U-Pb data yield crystallization ages of 3.47 to 3.28 Ga, in good agreement with the combined apatite-titanite-WR Sm-Nd isochron ages of each sample, demonstrating that both the U-Pb and Sm-Nd systems have not been modified since igneous crystallization. The initial Hf isotope compositions of zircon from all samples are broadly chondritic with εHf(i) values of −0.3 to +0.8, in agreement with the bulk-rock Hf. The initial Nd isotope compositions of the titanite and apatite are also broadly chondritic (εNd(i) titanite, −1.0 – +2.0; apatite, −0.6 – +0.9) and agree with the Nd isotope composition of the bulk-rock (εNd(i) = +0.2 to +1.2) and the initial ¹⁴³Nd/¹⁴⁴Nd ratios determined from the titanite-apatite-WR isochrons (εNd(i) −0.9 to +1.3). From these data, we make two fundamental observations. First, the granites in this study were derived from a source that was chondritic with respect to both Hf and Nd isotopes from 3.47 to 3.28 Ga; neither system supports the presence of either a strongly depleted mantle or enriched crustal source. Second, the Lu-Hf and Sm-Nd isotope systems in the Pilbara samples are in full agreement. This stands in stark contrast to the record of rocks from Eo-Paleoarchean terranes of higher metamorphic grade, where the Hf and Nd isotope compositions have been “decoupled”. This further underscores the importance of recognizing potential effects of high-grade metamorphism on the Sm-Nd bulk-rock record. The integrated age-isotope approach taken here illustrates a way to assess the integrity of bulk-rock Nd isotope data through examination of the Sm-Nd isotope systematics of the LREE-rich accessory minerals in rocks.

Combined Sm-Nd, Lu-Hf, and 142Nd study of Paleoarchean basalts from the East Pilbara Terrane, Western Australia

Article

May 2021
CHEM GEOL

The chemistry of the major reservoirs in the silicate Earth reflects a long history of differentiation into- and interaction between- incompatible element enriched crust and depleted mantle. Evidence from ¹⁴²Nd variability in ancient mantle-derived rocks imply such differentiation began very early in Earth's history, while the short lived ¹⁴⁶Sm (half-life 103 Ma) was extant, but the size and composition of primitive reservoirs in the Hadean and early Archean is debated. Therefore, pivotal to the study of Earth evolution is the recognition of well-preserved ancient magmatic rocks that can provide robust constraints on source compositions to test geodynamic models. We present the first combined petrological, geochemical and isotope (¹⁴⁶⁻¹⁴⁷Sm-¹⁴²⁻¹⁴³Nd and ¹⁷⁶Lu¹⁷⁶Hf) study of basaltic rocks from the East Pilbara Terrane, Pilbara Craton, Western Australia. The basaltic pillow lavas, from the ca. 3470 Ma Mount Ada Basalt of the Pilbara Supergroup, include quartz-phyric quartz-normative tholeiites, plagioclase-phyric olivine-normative tholeiites and clinopyroxene spinifex textured olivine-normative tholeiites and include samples that classify as siliceous high Mg basalts. Their major and trace-element compositions suggest that the basalts are high-degree partial melts (20–30%) produced at shallow mantle depths of 0.5–1 GPa, which is consistent with formation of both quartz-normative and olivine-normative parental magmas. Isotopic data from this sample suite defines SmNd and LuHf isochrons with dates of 3484 ± 113 Ma and 3463 ± 50 Ma, respectively. The samples yield coupled initial εNd = +1.1 and εHf = +2.1, and μ¹⁴²Nd values indistinguishable from the modern mantle. The uniform initial εNd and εHf in samples from this study and other Mount Ada Basalt basaltic samples across four greenstone belts indicates that this portion of the Paleoarchean mantle was well mixed at the scale of melt generation and evolved with slightly suprachondritic Sm/Nd and Lu/Hf. Absence of ¹⁴²Nd variations suggests the portion of the Paleoarchean Pilbara mantle sampled by these basalts retains no signature of Hadean Sm/Nd fractionation.