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Archean Komatiite Volcanism Controlled by the Evolution of Early Continents

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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 distri- bution 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 informa- tion 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
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