Archean Komatiite Volcanism Controlled by the Evolution of Early Continents

Archean komatiite volcanism controlled by the evolution of early continents

David R. Molea,b,1,2, Marco L. Fiorentinia, Nicolas Thebauda, Kevin F. Cassidya, T. Campbell McCuaiga, Christopher L. Kirklandc, Sandra S. Romanoc, Michael P. Doubliera,c, Elena A. Belousovad, Stephen J. Barnese, and John Millera

aCentre for Exploration Targeting, Australian Research Council Centre of Excellence for Core to Crust Fluid Systems, School of Earth and Environment, University of Western Australia, Perth, WA 6009, Australia; bDepartment of Applied Geology, Curtin University, Bentley, WA 6102, Australia; cGeological Survey of Western Australia, Department of Mines and Petroleum, East Perth, WA 6004, Australia; dKey Centre for the Geochemical Evolution and Metallogeny of Continents, Australian Research Council Centre of Excellence for Core to Crust Fluid Systems, Macquarie University, North Ryde NSW 2109, Australia; and eEarth Science and Resource Engineering, Commonwealth Scientific and Industrial Research Organization (CSIRO), Kensington, Perth, WA 6151, Australia

Edited by Norman H. Sleep, Stanford University, Stanford, CA, and approved April 14, 2014 (received for review January 7, 2014) The generation and evolution of Earth’s continental crust has In the Yilgarn Craton of Western Australia (Fig. 1), two major played a fundamental role in the development of the planet. Its pulses of komatiite activity occurred at ∼2.9 Ga (southern Youanmi formation modified the composition of the mantle, contributed to Terrane; refs. 17–19) and 2.7 Ga (Kalgoorlie Terrane, Eastern the establishment of the atmosphere, and led to the creation of Goldfields Superterrane; refs. 5, 10). These represent two ecological niches important for early life. Here we show that in the separate plume events that impinged onto preexisting continental Archean, the formation and stabilization of continents also con- crust (20–23), with the resulting magmas heterogeneously dis- trolled the location, geochemistry, and volcanology of the hottest tributed across the craton (8, 10, 17–19, 23, 24). In this study, preserved lavas on Earth: komatiites. These magmas typically rep- we provide the first evidence of a fundamental relationship resent 50–30% partial melting of the mantle and subsequently between the spatiotemporal variation in komatiite abundance, record important information on the thermal and chemical evolu- geochemistry, and volcanology and the evolution of an Archean tion of the Archean–Proterozoic Earth. As a result, it is vital to microcontinent, reflected in the changing isotopic composition of

constrain and understand the processes that govern their localiza- the crust. EARTH, ATMOSPHERIC, tion and emplacement. Here, we combined Lu-Hf isotopes and We used Lu-Hf and U-Pb isotopic techniques on multiple AND PLANETARY SCIENCES U-Pb geochronology to map the four-dimensional evolution of the magmatic and inherited zircon populations from granitoid rocks Yilgarn Craton, Western Australia, and reveal the progressive de- and felsic volcanic units, which represent the exposed Archean velopment of an Archean microcontinent. Our results show that in crust of the Yilgarn Craton. All zircon grains were dated using the early Earth, relatively small crustal blocks, analogous to modern the sensitive high-resolution ion microprobe (SHRIMP), before microplates, progressively amalgamated to form larger continen- in situ laser ablation inductively coupled plasma mass-spectrometry tal masses, and eventually the first cratons. This cratonization pro- (LA-ICP-MS) analysis for Lu-Hf isotopes. The Lu-Hf isotopic cess drove the hottest and most voluminous komatiite eruptions data are expressed as eHf, which denotes the derivation of the to the edge of established continental blocks. The dynamic evolu- 176Hf/177Hf ratio of the sample from the contemporaneous ratio tion of the early continents thus directly influenced the addition of deep mantle material to the Archean crust, oceans, and atmo- Significance sphere, while also providing a fundamental control on the distribution of major magmatic ore deposits. Komatiites are rare, ultra-high-temperature (∼1,600 °C) lavas that were erupted in large volumes 3.5–1.5 bya but only very crustal evolution | lithosphere | architecture | mantle plumes | rarely since. They are the signature rock type of a hotter early Ni-Cu-PGE deposits Earth. However, the hottest, most extensive komatiites have a very restricted distribution in particular linear belts within olcanism on Earth is the dynamic surface expression of our preserved Archean crust. This study used a combination of dif- Vplanet’s thermal cycle, with heat created from radioactive ferent radiogenic isotopes to map the boundaries of Archean decay and lost through mantle convection (1). In the Archean microcontinents in space and time, identifying the microplates eon (>2.5 bya), Earth’s heat flux was significantly higher than that form the building blocks of Precambrian cratons. Isotopic that observed today (1, 2) due to the combined effects of a more mapping demonstrates that the major komatiite belts are lo- radioactive mantle (1, 3) and residual heat from planetary ac- cated along these crustal boundaries. Subsequently, the evo- cretion (4). This resulted in the eruption of komatiites: ultra-high lution of the early continents controlled the location and temperature, low-viscosity lavas with MgO >18% and eruption extent of major volcanic events, crustal heat flow, and major temperatures >1,600 °C (5) formed from mantle plumes (1, 2, 6). ore deposit provinces. These rare, ancient magmas are dominantly restricted to the early history of the planet (3.5–1.5 Ga; ref. 7) and represent the Author contributions: D.R.M., M.L.F., and J.M. designed the research project; D.R.M., N.T., remnants of huge volcanic flow fields (8) consisting of the hottest S.S.R., and M.P.D. performed research; D.R.M., M.L.F., K.F.C., T.C.M., C.L.K., and S.J.B. analyzed data; D.R.M. wrote the paper; N.T., K.F.C., T.C.M., C.L.K., and J.M. performed lavas preserved on Earth (5, 9, 10). These now-extinct volcanic regional geological analysis; S.S.R. and M.P.D. performed regional geological analysis and systems and flow complexes had the potential to cover significant mapping; E.A.B. provided assistance with operation of analytical equipment and data portions of the early continents, and were likely analogous to reduction; and S.J.B. provided access to the CSIRO komatiite database. large igneous provinces in size and magma volume (11, 12). The authors declare no conflict of interest. Komatiites are vital to our understanding of Earth’s thermal This article is a PNAS Direct Submission. – – evolution (1 3, 7, 13 16), and represent a window into the dy- 1Present address: Earth Science and Resource Engineering, Commonwealth Scientific and namic secular development of the mantle throughout the early Industrial Research Organization (CSIRO), Kensington, Perth, WA 6151, Australia. history of our planet (5). Subsequently, understanding the physical 2To whom correspondence should be addressed. E-mail: [email protected]. and chemical processes that govern their localization, volcanology, This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. and geochemistry is vital in deciphering this information. 1073/pnas.1400273111/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1400273111 PNAS Early Edition | 1of6 Downloaded by guest on October 2, 2021 displayed in Fig. S4]. In these maps, point data representing the median eHf value of granites and felsic volcanics are plotted as contour maps that show the spatial extent of “blocks” of specific Lu- Hf isotopic character and their evolution through time. This method is based on previous isotopic mapping of the Yilgarn Craton using the analogous Sm-Nd system (27). Importantly, the Lu-Hf data presented here replicate the features of the Sm-Nd work (27, 28), with the added ability to look further back in time due to the in situ analysis of abundant inherited zircons (21). The variable isotopic signatures of the crust (Figs. 2–4) can be interpreted as proxies for lithospheric thickness (Figs. 5 and 6; ref. 29), where young, juvenile eHf values (eHf > 0) indicate relatively thin lithosphere and old, reworked values (eHf < 0) reflect thicker lithosphere (29, 30); a pattern observed in the modern-day western United States (29–32). Here, this information is combined to document the four-dimensional lithospheric architecture of the Yilgarn Craton and development of an Archean microcontinent. Results

The first time slice (T1 − 3,050–2,820 Ma; Fig. 2) shows the litho- Fig. 1. Map of the Archean Yilgarn Craton showing the basic granite- spheric architecture at the time of ∼2.9 Ga komatiite emplacement greenstone bedrock geology and location of the ∼2.9 and 2.7 Ga komatiite in the southern Youanmi Terrane. The Lu-Hf mapping identifies localities. Individual terranes/domains (39, 40) are labeled. Greenstone belts three lithospheric blocks: Marda, Hyden, and Lake Johnston. The are labeled as follows: MD, Marda–Diemals; SC, Southern Cross; FO, For- Marda and Hyden blocks dominantly comprise reworked older – restania; LJ, Lake Johnston; RAV, Ravensthorpe; AW, Agnew Wiluna; and crust, with eHf −6.0 and −4.0, respectively. In contrast, the Lake KAL, Kalgoorlie/Kambalda. Komatiite localities are from Barnes and Fior- entini (10) (Table S4). Johnston block comprises younger, more juvenile material, with eHf +2.0. This protocratonic lithospheric architecture exerts a first-order control on the localization of the ∼2.9 Ga high-MgO of the chondritic uniform reservoir (CHUR), multiplied by 104. komatiites of the Forrestania (17) and Lake Johnston (10, 19, 33) The term “juvenile” refers to crustal material that plots on or greenstone belts. These komatiites occur in the juvenile Lake close to the depleted mantle evolution line, suggesting derivation Johnston block, primarily along the margin of the older Hyden from a depleted mantle source. In contrast, “reworked” refers block (Fig. 2). Within the Marda block to the north, komatiites to the remobilization of preexisting crust by partial melting are largely absent and the greenstone stratigraphy is almost ex- and/or erosion and sedimentation (25, 26). Complete sample in- clusively comprised of basalt (24). No known volcanic units from formation, methodology, and geochemical datasets (U-Pb, Lu-Hf, this time are preserved in the Hyden block. and komatiite) are available in the Supporting Information, The second time slice (T2 − 2,820–2,720 Ma; Fig. 3) images Figs. S1–S3,andTables S1–S4. the lithospheric architecture of the craton at a time when no The Lu-Hf data are displayed as a series of time-slice contour significant evidence of komatiite magmatism is recorded. Six maps, which show “snapshots” of the changing source and age of crustal blocks can be identified: Barlee, Marda, Hyden, Corrigin, the crust at 3,050–2,820; 2,820–2,720; and 2,720–2,600 Ma [Figs. 2– Lake Johnston, and Eastern Goldfields. The Barlee block shows 4; intervals based on the work of Mole et al. (21); full Hf dataset a bimodal eHf distribution, with peaks at 0 and +3.0. In relation

Fig. 2. Lu-Hf (eHf) map of the Yilgarn Craton at 3,050–2,820 Ma. (A) Hf isotope map with the location of sample sites and komatiite localities. (B) Interpretive map of the area, showing the individual crustal blocks identified from the Hf isotope map and corresponding probability density plots. The blue curve

represents the median eHf for discrete temporal groups (ng), whereas the red curve represents all of the individual grain analyses (na). Dark gray polygons shown in the background of all maps represent supracrustal belts (Fig. 1).

2of6 | www.pnas.org/cgi/doi/10.1073/pnas.1400273111 Mole et al. Downloaded by guest on October 2, 2021 Fig. 3. Lu-Hf (eHf) map of the Yilgarn Craton at 2,820–2,720 Ma. (A) Hf isotope map with the location of sample sites and komatiite localities. (B) Interpretive map of the area, showing the individual crustal blocks identified from the Hf isotope map and corresponding probability density plots. The blue curve

to the first time slice, the Hyden block has extended north and Goldfields Superterrane (10, 33). Two major lithospheric blocks are merged with the Marda block, although its western extent present at this time: the Eastern Goldfields and the West Yilgarn e

appears to have been rejuvenated by mantle input (Corrigin (Fig. 4). The Eastern Goldfields block comprises a bimodal Hf EARTH, ATMOSPHERIC, block – eHf ∼0). The result is a narrow block of old, reworked distribution, with the majority of material at +2.0, and a notable AND PLANETARY SCIENCES crust with eHf peaks at −4.3 and −2.3. The Marda block remains minor peak at −2.0. The West Yilgarn is the result of the pro- old and unradiogenic, with a major eHf peak at −4.0. The gressive cratonization of the individual blocks identified in time – Lake Johnston block has been significantly reworked since 3,050 slices T1 and T2.IthasaeHf distribution with peaks at −2.4 2,820 Ma, as demonstrated by the lower eHf at −0.5 (Fig. 3). The (major) and +1.8, −5.5, and −8.0 (minor). These variable sources Eastern Goldfields is now identifiable as a crustal block, and is reflect the complex history of the West Yilgarn as well as the more juvenile than the blocks to the west, displaying a bimodal intracratonic signature of the individual blocks from which it eHf distribution with peaks at +2.0 and +4.0. Overall, the derives (Fig. 4). Overall, it appears that the majority of the West second time slice shows reworking throughout the core area of the Yilgarn formed from a source ∼800 Ma (Marda) to 200 Ma (Lake West Yilgarn and addition of juvenile material at the northern, Johnston) older than that of the Eastern Goldfields (Fig. S4). western, and eastern edges. Discussion Thethirdtimeslice(T3 − 2,720–2,600 Ma; Fig. 4) represents the lithospheric architecture at the time of voluminous ∼2.7 Ga In the Yilgarn Craton, high-flux, thick channelized komatiites with komatiite volcanism in the Kalgoorlie Terrane of the Eastern average whole-rock MgO contents >30% only occur in greenstone

Fig. 4. Lu-Hf (eHf) map of the Yilgarn Craton at 2,720–2,600 Ma. (A) Hf isotope map with the location of sample sites and komatiite localities. (B) Interpretive map of the area, showing the individual crustal blocks identified from the Hf isotope map and corresponding probability density plots. The blue curve

Mole et al. PNAS Early Edition | 3of6 Downloaded by guest on October 2, 2021 belts located at the interface between juvenile and reworked crustal unchannelized, cumulate-poor, and unmineralized (18). We suggest domains (Figs. 2 and 4). Preexisting lithospheric weaknesses at that, due to the unique location of the Southern Cross greenstone these craton margins (10, 34, 35) led to crustal attenuation, belt between the reworked Hyden and Marda blocks (Fig. 2), major intracontinental rifts, and high-flux magma transport magmas were focused enough to form high-MgO komatiites, but through associated translithospheric conduits (10, 34). Ultra- not to the extent needed for high-flux, continuous eruptions. mafic magmas rapidly ascended from mantle to surface without The heterogeneous nature of komatiite magmatism at ∼2.9 significant ponding or differentiation in the lithosphere (Figs. 5 and 2.7 Ga is consistent with the impingement of a mantle plume and 6) (34, 35), and formed giant, high-flux komatiite flow fields onto lithosphere of variable thickness (Figs. 5 and 6). The iso- that contain large nickel-sulfide ore reserves (5, 33). At ∼2.9 Ga topic architecture constrained in this study indicates that the in the juvenile Lake Johnston block, channelized high-MgO ∼2.9 and 2.7 Ga komatiites were emplaced in similar geodynamic komatiites erupted to form thick olivine cumulate bodies that are settings, at the margins of thick crustal domains (Figs. 2–6). now preserved in the Forrestania (17), Lake Johnston (10, 19, Consequently, the melting dynamics of the underlying mantle 33), and Ravensthorpe (36) greenstone belts. Their setting is and the petrogenetic signature of the komatiites would also be consistent with the occurrence of a paleocraton margin at the expected to be similar. However, although both Munro- and Hyden–Lake Johnston block boundary at ∼2.9 Ga (Figs. 2 and Barberton-type komatiites were erupted in the Lake Johnston 5). Between ∼2.9 and 2.7 Ga, the focus of hot and voluminous block at ∼2.9 Ga (17–19, 36), only Munro-type komatiite magma- komatiite magmatism shifted to the east toward the Kalgoorlie tism is recorded in the Kalgoorlie Terrane at ∼2.7 Ga (10, 19, 33). Terrane of the Eastern Goldfields (Fig. 6), following the margin Barberton-type komatiites contain notably depleted aluminum of the growing continent. concentrations (Al2O3/TiO2 ∼10) in relation to Munro-type mag- Conversely, plume-derived magmas that erupted in “intra- mas (Al2O3/TiO2 ∼20). These compositional differences reflect continent” settings (Figs. 5 and 6) formed abundant basalts and the conditions under which the melts separated from their plume only low-flux, thin komatiites with average MgO values typically sources (2, 5). Barberton-type komatiites formed in the garnet <30% (10, 33), as demonstrated by the greenstone belts of the stability zone, at a depth of ∼450–300 km (37); whereas, Munro- Marda block at ∼2.9 Ga (24) and in the Kurnalpi Terrane at ∼2.7 type komatiites segregated from their mantle sources at <300 km Ga (10). In this setting, magmas rose through thick reworked depth, although high-percentage partial melts (>30%) only formed lithosphere (Marda) or thinner juvenile (Kurnalpi) crust that was at <150 km depth (2, 5). A possible explanation for the occurrence not adjacent to an older, thicker crustal block at the time of of Barberton-type komatiites in the ∼2.9 Ga sequences is the magmatism (Figs. 2 and 4). Consequently, eruptions were not secular cooling of the Earth; melt generation at >300 km is sufficiently voluminous or continuous to form the giant komatiite only possible in the hotter, older mantle (13). This hypothesis is flow fields and related nickel mineralization that is associated supported by the well-constrained global decline of Barberton- with craton margins. type komatiite magmatism from 3.5 to 2.7 Ga (2, 5). However, it The komatiites of the Southern Cross greenstone belt (18) form is unlikely that sufficient global cooling occurred between ∼2.9 and an intermediate group between the intracontinent (low-MgO, un- 2.7 Ga to affect the source depth of the Yilgarn komatiites (1, 2). channelized) and the continent edge (high-MgO, channelized) type An alternative scenario considers the spatiotemporal vari- komatiites. These magmas have high-MgO contents (18), but are ability in komatiite type as a consequence of petrogenetic

Fig. 5. Isotopic cross-section and interpreted lithospheric architecture during the emplacement of ∼2.9 Ga komatiites in the southern Youanmi Terrane. (A) eHf map showing the isotopic architecture at the time of the ∼2.9 Ga plume emplacement, with the approximate extent of the plume head (red) and tail (yellow) shown for scale; (B) isotopic cross-section (A– A′) documenting the changing eHf of the crust from east to west (circles represent median; squares represent individual analyses) together with the occurrence and MgO content (Ta- ble S4) of ultramafic–mafic magmatism; and (C) interpreted lithospheric architecture based on the changing isotopic properties of the crust. The white ellipses represent the types of magma available in a particular area and the dashed lines show the approximate limits of their source

regions. TiO2 vs. Al2O3 data (17, 19) are shown for komatiites of the relevant greenstone belts, demonstrating the progressive eastward homogenization of Barberton-type melts. The location of the continent core and continent edge are shown based on the Lu-Hf data. The eruption of komatiite would likely have been facilitated by plume-related extension at this interface. Approximate thickness values for de- veloped Archean lithosphere (∼250–150 km) were taken from Boyd et al. (41), Artemieva and Mooney (42), and Begg et al. (43). The approximate scale of the plume head (∼1,600 km), tail (200–100 km), and thickness (∼150–100 km) were taken from Campbell et al. (15). Note that the plume-tail material moves above the plume head, despite impacting the lithosphere later, as it is hotter, more buoyant, and subsequently emplaced at higher flux (15).

4of6 | www.pnas.org/cgi/doi/10.1073/pnas.1400273111 Mole et al. Downloaded by guest on October 2, 2021 Fig. 6. Isotopic cross-sections and interpreted lithospheric architecture during the emplacement of the ∼2.7 Ga komatiites in the Eastern Goldfields (Kalgoorlie Terrane). (A) eHf map showing the isotopic architecture at the time of the ∼2.7 Ga plume emplacement, with the approximate extent of the plume head (red) and tail (yellow) shown for scale; (B) isotopic cross-section (B–B′) documenting the changing eHf of the crust from east to west (circles represent median; squares represent individual analyses) together with the occurrence and MgO content (Table S4) of ultramafic–mafic magmatism; and (C) interpreted lithospheric architecture based on the changing isotopic properties of the crust. The white ellipses represent the types of magma available in a particular area and the dashed lines show the approximate limits of their source

regions. TiO2 vs. Al2O3 data (9) are shown for komatiites of the Eastern Goldfields, demonstrating the eastward dilution and removal of Barberton-type melts. The location of the continent core and continent edge are shown based on the Lu-Hf dataset. The eruption of komatiite would likely have been facilitated by plume-related extension at this interface. The dashed red lines shown in C account for the potential variation in lithospheric architecture within the Lake Johnston block based on the Lu-Hf data. External data used to construct these diagrams are the same as for Fig. 5. Note that the plume-

tail material moves above the plume head, despite impacting EARTH, ATMOSPHERIC, the lithosphere later, as it is hotter, more buoyant, and sub- AND PLANETARY SCIENCES sequently emplaced at higher flux (15).

filtering by thick lithosphere. The relative architecture between architecture over the stationary plume, resulting in variations in the ∼2.9 Ga Hyden−Lake Johnston and ∼2.7 Ga West Yilgarn− the character, frequency, and isotopic signature of volcanism (29, Eastern Goldfields blocks is fundamentally similar (Figs. 5 and 6). 31, 32). Under the younger, thinner accreted terranes in the west, However, the less-reworked nature of the Lake Johnston the plume ascends to higher levels allowing the formation of block at ∼2.7 Ga (Fig. 4) suggests that the ∼2.9 Ga Hyden large volumes of decompression melt (29, 31, 32, 34, 35) and a block was thicker than this part of the ∼2.7 Ga West Yilgarn juvenile (eNd +4, eHf +10; ref. 29) volcanic sequence domi- block (Figs. 5 and 6); an inference supported by magnetotelluric nated by basaltic (including the Columbia River basalts; ref. 32) data (38). Consequently, at ∼2.9 Ga, the >150-km-thick litho- and minor felsic magmatism. This setting is a modern analog to sphere of the Hyden block prevented the segregation of high- the high-MgO, voluminous continent-edge–type komatiite vol- percentage (∼50–30%) partial melt Munro-type magmas (MH) canism observed in the Yilgarn Craton. To the east, where the and restricted melt generation to Barberton- and low-percentage Yellowstone plume impinges on thick, old Archean lithosphere, partial melt (<30%) Munro-type magmas (ML; Fig. 5). The lack less decompression melting occurs. This relatively small amount of large quantities of Munro-type melt, together with the close of mantle melt infiltrates the overlying lithosphere, where it is proximity of the craton margin (Fig. 5), ensured Barberton-type contaminated by more unradiogenic continental material. This melts were erupted in the Forrestania greenstone belt (17) be- process leads to evolved (eNd −11, eHf −10; ref. 29) felsic-only fore significant dilution by the upper high-percentage partial melt volcanism, similar to that recorded in the Marda block at ∼2.7 Munro-type magmas could occur. The eastward position of the Ga (24) (eHf −7to−2; Table S2). This architecture is a modern Lake Johnston greenstone belt allowed homogenization of the analog to the intracontinent Archean settings, where komatiite Barberton–Munro melts, resulting in magmas with intermediate magmatism is largely absent. compositions (19) (Fig. 5). This study demonstrates that the dynamic evolution of the In contrast, at ∼2.7 Ga in the Eastern Goldfields, due to the early continents controlled the location, geochemistry, metal- shallower nature of the Lake Johnston block, Barberton-type logeny, and volcanology of komatiites. An analogous process magmas generated under the Hyden block would have had to travel continues to operate in the modern Earth (30, 32, 34), and has ∼300–200 km laterally through both low- and high-percentage been fundamental to the transfer of deep mantle material to partial-melt Munro-type sources (Fig. 6). Subsequently, the the continental crust, oceans, and atmosphere throughout the Barberton-type signature was diluted to the extent that only history of the planet. Munro-type komatiites were erupted in the Eastern Goldfields (9). Materials and Methods A Phanerozoic analog for the relationship between lithospheric architecture and magmatism in the Yilgarn Craton at We report here on new U-Pb geochronology (36 samples) and Lu-Hf isotopic ∼2.9 and 2.7 Ga is the ∼17 Ma (31, 32) continental plume setting data (84 samples) from the Yilgarn Craton of Western Australia (Tables S2 at Yellowstone in the western United States. The overlying litho- and S3). The U-Pb zircon geochronology was performed on the sensitive high-resolution ion microprobes at the John de Laeter Centre of Mass sphere comprises the old, thick Archean Wyoming Craton to the – Spectrometry at Curtin University, Western Australia. Following precise east and the thinner Mesozoic Paleozoic-accreted oceanic ter- dating of the magmatic and inherited zircon populations from sampled ranes to the west (29–31). The westward movement of the North granites and felsic volcanics, >900 in situ Lu-Hf isotopic analyses were carried American plate (∼2 cm/y; ref. 31) traversed this lithospheric out at the Centre for the Geochemical Evolution and Metallogeny of

Mole et al. PNAS Early Edition | 5of6 Downloaded by guest on October 2, 2021 Continents at Macquarie University in Sydney, Australia. These data were Australia. S.J.B.’s contribution is supported by the Commonwealth Sci- then processed and plotted as time-slice contour maps using ArcGIS (Figs. 2– entific and Industrial Research Organization (CSIRO) Minerals Down Un- 4 and Fig. S5). For the complete methodology we refer the reader to the der National Research Flagship. The Lu-Hf analytical data were obtained using instrumentation funded by Department of Education Science and Supporting Information. Training (DEST) Systemic Infrastructure grants, ARC Linkage Infrastructure, Equipment and Facilities (LIEF), National Collaborative Research Infra- ACKNOWLEDGMENTS. Adam Wilson and Alex Clarke-Hale are thanked for structure Strategy (NCRIS), industry partners, and Macquarie University. field support. This project was funded by Australian Research Council TheU-Pbzircongeochronologywasperformed on the sensitive high- (ARC) Linkage Grants LP0776780 and LP100100647 with BHP Billiton resolution ion microprobes at the John de Laeter Centre of Mass Spectrom- Nickel West, Norilsk Nickel, St Barbara, and the Geological Survey of etry (Curtin University). This is contribution 456 from the ARC Centre of Western Australia (GSWA). The GSWA is acknowledged for sample pro- Excellence for Core to Crust Fluid Systems (CCFS) and 939 from the Centre vision and technical advice. C.L.K., S.S.R., and M.P.D. publish with per- for the Geochemical Evolution and Metallogeny of Continents (GEMOC) mission of the Executive Director of the Geological Survey of Western Key Centre.

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