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A Cool Early Earth, (2002) A cool early Earth John W. Valley* William H. Peck* Elizabeth M. King* Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA Simon A. Wilde School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, Australia ABSTRACT No known rocks have survived from the ®rst 500 m.y. of Earth history, but studies of single zircons suggest that some continental crust formed as early as 4.4 Ga, 160 m.y. after accretion of the Earth, and that surface temperatures were low enough for liquid water. Surface temperatures are inferred from high d18O values of zircons. The range of d18O values is constant throughout the Archean (4.4±2.6 Ga), suggesting uniformity of processes and conditions. The hypothesis of a cool early Earth suggests long intervals of relatively temperate surface conditions from 4.4 to 4.0 Ga that were conducive to liquid- water oceans and possibly life. Meteorite impacts during this period may have been less frequent than previously thought. Keywords: Archean, zircon, oxygen, isotopes, meteorites, impacts. INTRODUCTION ``hell-like,'' in which any crust was destroyed and Table DR-11). Figure 2 shows histograms The young Earth (4.5±3.8 Ga) was highly by meteorite impact or melting, all water was of d18O for these samples in comparison to energetic. Sources of heat, both internal and ex- vaporized, and the atmosphere was steam rich olivine and zircon from Earth's mantle. Sev- ternal, were signi®cantly greater than at any (see Pollack, 1997). eral conclusions can be made. There is no sys- time since. The combined energy of planetary The only terrestrial materials that are tematic trend for d18O versus the age of the accretion, gravitational settling of Earth's me- known to be older than 4 Ga are detrital and Archean magmas that represent 40% of the tallic core, meteorite impacts, and possibly col- xenocrystic zircons from Western Australia age of Earth. Values average close to the prim- lision with a Mars-size body was suf®cient to (see Compston and Pidgeon, 1986; Wilde et itive value of Earth's mantle (5.3½ 6 0.3½ melt the entire Earth during its accretion, dated al., 2001; Peck et al., 2001; Mojzsis et al., based on zircon xenocrysts in kimberlite, Fig. as 4.56±4.45 Ga (Halliday, 2000). Furthermore, 2001), including a recently discovered crystal 2B; Valley et al., 1998), in high-temperature before 4.4 Ga, the heat produced by radioactive from the Jack Hills, Western Australia, with equilibrium with the value of mantle-derived decay was approximately ®ve times higher than an age of 4.404 Ga (Wilde et al., 2001). At olivine from peridotite xenoliths and from today, not including the effects of short-lived present, these isolated zircon crystals are the oceanic island basalts (Fig. 2A; Mattey et al., nuclides (Pollack, 1997). Although these events only direct evidence of conditions on the ear- 1994; Eiler et al., 1997). The average value surely caused massive melting before 4.45 Ga, liest Earth. for Archean igneous zircons from four conti- estimates of surface temperatures after 4.45 Ga nents, exclusive of the Jack Hills (Fig. 2C), is depend critically on the magnitudes of mete- ZIRCONS 5.6½, which would be in high-temperature orite bombardment and atmospheric insulation, Zircon is a common trace mineral in granitic equilibrium with a basalt (whole rock) of which are uncertain. rocks that preserves detailed records of magma ;6½ or a granite of 6.5½±7.5½. The distri- The most direct evidence of past tempera- genesis. Zircons also form in ma®c rocks or bution is slightly skewed toward higher val- tures on the surface of Earth comes from ox- during metamorphism. Zircons provide reliable ues. High values of 7½±8½ are found even 18 ygen isotope compositions (d O) in rocks; crystallization ages, magmatic d18O values, and in the oldest zircon crystal (Fig. 1). Thus, the however, no rocks have been identi®ed from magmatic rare earth element (REE) composi- processes affecting the d18O of magmatic zir- the earliest Earth. Dating of lunar rocks (Sny- tions, and may contain inclusions of other co- cons show no evidence of change during this der et al., 2000) indicates a crust by ca. 4.4 genetic minerals. These characteristics can be period. The relative constancy of d18OinAr- Ga, but no such evidence exists on Earth. The preserved even when a crystal has been re- chean zircons is in contrast to zircons younger oldest known rocks on Earth formed at 4.0 Ga moved from its host rock by weathering, trans- than 2 Ga, which have magmatic values from (Bowring and Williams, 1999), and the oldest ported as a detrital grain, deposited, hydrother- 0½ to 13.5½ due to evolution of Earth's crust water-laid sediments are 3.8±3.6 Ga (Nutman mally altered, and metamorphosed (Valley et to contain more high d18O sediments, evolved et al., 1997; Whitehouse et al., 1999). The ab- al., 1994). In well-de®ned situations, zircons ``S-type'' magmas, and hydrothermally altered sence of rocks older than ca. 4 Ga, taken with provide information of even earlier events, be- rocks (Peck et al., 2000). other evidence, has been interpreted to suggest fore magmatic crystallization. Thus, studies of a hot early Earth, sometimes called Hadean or zircon can provide important ground truth for models of the early Earth. 1GSA Data Repository item 2002034, Table *E-mail: ValleyÐ[email protected]. Pre- DR1, Values of d18O and U-Pb ages of zircons from sent addresses: PeckÐDepartment of Geology, Col- Archean igneous rocks, is available from Docu- gate University, Hamilton, New York 13346, USA; Archean Zircon Record ments Secretary, GSA, P.O. Box 9140, Boulder, CO 18 KingÐDepartment of Geography-Geology, Illinois Figure 1 shows the d O of zircons with 80301-9140, [email protected], or at www. State University, Normal, Illinois 61790, USA. measured U-Pb ages of 2.6 to 4.4 Ga (see text geosociety.org/pubs/ft2002.htm. q 2002 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; April 2002; v. 30; no. 4; p. 351±354; 3 ®gures; Data Repository item 2002034. 351 Figure 1. Crystallization age (U-Pb) and d18O for Archean magmatic zircons. Distribution of magmatic d18O values does not change throughout Archean. Most magmas had primitive d18O value similar to that in mantle today (mantle zircon), but some zircon values are as high as 7.5½. High-d18O zircons and host magmas resulted from melting of protoliths that were altered by interaction with liquid water at low temperatures near surface of Earth (see text). Time line (inset, lower right) shows: (1) accretion of Earth, (2) formation of Moon and Earth's core, (3) minimum age of liquid water based on high d18O zircon, (4) Acasta gneiss, and (5) Isua metasedimentary rocks. VSMOW is Vienna standard mean ocean water. Jack Hills Detrital Zircons Any model should consider the features of The distribution of d18O values measured in this 4.4 Ga zircon: ®nely concentric magmatic Jack Hills detrital zircons (Peck et al., 2001; zoning as revealed by cathodoluminescence; Wilde et al., 2001; Mojzsis et al., 2001) REE patterns that include negative Eu and matches that of younger Archean samples positive Ce anomalies, and heavy-REE enrich- (Fig. 1). The only unusual aspect of the oldest ments to ;10 000 times chondrites; zoning in 18 zircons is their age. The higher Jack Hills d O that correlates to trace element zoning Figure 2. Histograms of d18O. A: Olivine from 18 d O values (6½±8½) are the same as those (7.4½ vs. 5.0½); inclusions of SiO2 that in- mantle xenoliths (Mattey et al., 1994) and Ha- of zircons from syntectonic to posttectonic sa- dicate a quartz-saturated granitic magma; and waiian basalts (Eiler et al., 1997). B: Zircon nukitoids (see King et al., 1998) and plutons a Pb-Pb age of 4.404 6 8 Ga with .99% xenocrysts from kimberlites in South Africa intruded into metasedimentary rocks (Fig. 2, concordant U-Pb ages (Peck et al., 2001; (Valley et al., 1998). C: Zircons from igneous 18 rocks of Superior province, Canada (King et C and D). In younger granites, such high-d O Wilde et al., 2001). al., 1998). D: Ion microprobe analyses of sin- zircons are known to form in magmas with Could the 4.4 Ga Jack Hills zircon be ex- gle zircons from Jack Hills, Western Austra- d18O (whole rock) values to 10½ (Valley et traterrestrial in origin? Zircons as old as 4.32 lia (Peck et al., 2001). Superior province his- 18 al., 1994), indicating burial and melting of Ga occur in lunar granophyre (Meyer et al., togram (C) peaks at mantle d O value with tail toward higher values. Jack Hills zircons rocks that were subjected to low-temperature 1996; Snyder et al., 2000); however, the com- (D) are higher in d18O than mantle. VSMOW 18 alteration, weathering, or diagenesis (Taylor bination of high d O values, quartz satura- is Vienna standard mean ocean water. and Sheppard, 1986; Gregory, 1991). It is sig- tion, and enriched REE compositions are not ni®cant that such alteration requires liquid wa- consistent with formation on the Moon or in ter near the surface of Earth. High d18O values any known meteorite. Furthermore, meteoritic Jack Hills sample. Furthermore, at magmatic are routinely found in younger granitoids and zircons are very rare on Earth. The close sim- temperatures, the d18O of a zircon in equilib- interpreted to result from interaction of low- ilarity of the 4.4 Ga crystal to other zircons in rium with such an evolving magma would be temperature liquid water with their protoliths.
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