Letters to Nature

Nature 409, 178-181 (11 January 2001) | doi:10.1038/35051557; Received 26 September 2000; Accepted 1 December 2000

Oxygen-isotope evidence from ancient zircons for liquid water at the Earth's surface 4,300Myr ago

Stephen J. Mojzsis1,2, T. Mark Harrison1 & Robert T. Pidgeon3

  1. Department of Earth and Space Sciences and IGPP Center for Astrobiology, University of California Los Angeles, Los Angeles, California 90095-1567, USA
  2. Department of Applied Geology, Curtin University of Technology, Perth, WA 6001, Australia
  3. Present address: Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309-0399, USA.

Correspondence to: Stephen J. Mojzsis1,2 Correspondence and requests for materials should be addressed to S.J.M. (e-mail: Email: Steve.Mojzsis@lasp.colorado.edu).

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Granitoid gneisses and supracrustal rocks that are 3,800–4,000Myr old are the oldest recognized exposures of continental crust1. To obtain insight into conditions at the Earth's surface more than 4Gyr ago requires the analysis of yet older rocks or their mineral remnants. Such an opportunity is presented by detrital zircons more than 4Gyr old found within 3-Gyr-old quartzitic rocks in the Murchison District of Western Australia2, 3. Here we report in situ U–Pb and oxygen isotope results for such zircons that place constraints on the age and composition of their sources and may therefore provide information about the nature of the Earth's early surface. We find that 3,910–4,280Myr old zircons have oxygen isotope (δ18O) values ranging from 5.4 ± 0.6‰ to 15.0 ± 0.4‰. On the basis of these results, we postulate that the ~4,300-Myr-old zircons formed from magmas containing a significant component of re-worked continental crust that formed in the presence of water near the Earth's surface. These data are therefore consistent with the presence of a hydrosphere interacting with the crust by 4,300Myr ago.

The Narryer Gneiss Complex, Western Australia, contains 3.73–3.30-Gyr-old granitic to tonalitic gneisses and about 3.0-Gyr-old metasedimentary rocks in the Mt Narryer and Jack Hills regions (Fig. 1). Previously, detrital zircons ranging in age from 4.27 to 3.05Gyr (refs 2,3,4,5) have been described from these two regions. The source rocks of these zircons have not been identified and may not have been preserved.

Figure 1: Schematic geological map of the Erawondoo region, Western Australia, showing the location of quartz-pebble conglomerate sample JH992 containing detrital zircons more than 4Gyr old.
Figure 1 : Schematic geological map of the Erawondoo region, Western Australia,
showing the location of quartz-pebble conglomerate sample JH992 containing
detrital zircons more than 4|[thinsp]|Gyr old. Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com

The Jack Hills and the Mt Narryer quartzitic units are part of an arcuate metasedimentary belt near the northwestern margin of the Yilgarn Craton (see inset).

High resolution image and legend (49K)

Because zircon is a common accessory phase in granitoids and their volcanic equivalents, these rocks are generally considered to be the dominant source of detrital zircons. But zircon can form in other rock types (albeit of low abundance in most cases) such as syenites, carbonatites, and mafic rocks6, and can also grow in hydrothermal veins and during metamorphism. Residual plagiogranites from Iceland-type melts more than 4Gyr old within a predominantly mafic crust have been suggested as the source of the oldest detrital zircons from the Narryer Gneiss Complex7, but this view has been challenged on trace-element and mineralogical grounds8.

Zircon has been extensively studied because it provides reliable U–Pb crystallization ages and can survive both high-grade metamorphism and sedimentary transport processes. Measurements of oxygen isotopes in rocks and minerals help us to understand the magmatic, fluid and thermal history of the crust. Exchange rates for oxygen are very slow9 in zircon, which may permit preservation of the protolith oxygen isotope composition through high-grade metamorphism10. Oxygen isotopes are used to discriminate among possible granitoid sources (mantle, metasedimentary, hybrid); thus, measurements of ancient zircons from the Narryer Gneiss Complex could allow us to directly explore crust–hydrosphere interactions before 4Gyr ago.

The zircon-bearing rocks in the Jack Hills form part of a thick (>2km) series of fan delta sequences deposited in a fault-bounded cratonic margin11 that were subsequently metamorphosed to upper greenschist facies8, 13. At least three distinct age groupings of ancient zircons have been described3, 8, 11, 13 (~4.3Gyr, 4.15Gyr and 3.9Gyr) that could represent discrete source terranes, although no rocks older than ~3.75Gyr have been found in the region4, 5, 12. We collected quartz-pebble conglomerates rich in heavy mineral indicators (such as Cr-spinel, fuchsite) from a locality in the Erawondoo region of the Jack Hills (Fig. 1) that was previously sampled for investigations of the trace element composition and inclusion mineralogy of detrital zircons more than 4Gyr old2, 3, 8, 13. In order to minimize the loss of zircon during sample preparation, we crushed, powdered and sieved 15kg of sample JH992 and processed all fractions through heavy liquids without pre-washing. This yielded about 80 zirconskg -1 in the size range 50–700µm, most with low aspect ratios. For each sample mount, we placed 100 zircon grains on adhesive tape together with standard zircon AS314 and filled an enclosing one-inch diameter mould with epoxy. After curing, the mount was cleaned and polished following our usual procedures15 and the internal features of the zircons were imaged using optical and back-scattered electron microscopy.

Because the very oldest grains are less than 10% of the total zircon population, we developed a new multi-collector ion microprobe technique to rapidly identify the oldest grains by determining 207Pb/206Pb ages by simultaneous measurement of 207Pb and 206Pb on adjacent electron multipliers. Using a primary O- beam of 3nA focused to an approximately 20µm diameter spot together with O2 flooding16, this new approach yielded precise (typically ±10Myr) 207Pb/206Pb ages in 3.5 minutes, permitting us quickly to survey 200 zircons. The oldest candidate grains were then targeted for conventional U–Pb ion microprobe measurements. Our search revealed 17 zircons more than 3.9Gyr old. Grains JH992_95 and JH992_48 (Table 1) showed very ancient and essentially concordant ages of 4,279 ± 5Myr(2σ) and 4,280 ± 5Myr(2σ), respectively, suggesting that ~1% of the zircons in our sample are about 4.3Gyr old.


We used a multi-collector method to obtain precise oxygen isotope compositions of microscale domains within the zircons. This new method provides the spatial resolution to determine isotopic heterogeneity at the sub-grain scale and avoid metamict zones or later metamorphic overgrowths that might complicate interpretation of the isotopic data. After the U–Pb measurements, the zircon mounts were re-polished, cleaned, and gold-coated and then analysed for oxygen isotopes using a 6-nA Cs+ primary beam focused to a 25µm diameter spot. In most cases, the oxygen isotope measurement was made directly adjacent to the spot on which a U–Pb age was determined. A mass resolving power of ~2,000 was sufficient to separate 18 O- and 16O- from molecular interferences. Each beam was focused into a Faraday cup with a typical count rate of 2GHz for 16O- and 4MHz for 18O-; the total integration time for each analysis was less than 6min. Backgrounds on the Faraday cup system were monitored periodically and the uncertainty in this correction was always less than 0.1‰. Instrumental mass fractionation was corrected by reference to standard zircons KIM5 (5.04 ± 0.05‰)17 and 91500 (9.8 ± 0.2‰; S. Claesson, personal communication), which were interposed between analyses of unknowns. Daily reproducibility determined from more than 15 spots was typically better than ±0.2‰ for both standards. Uncertainties in oxygen isotope data are reported at the 2σ level. Most zircons were examined for intracrystal homogeneity by analysing multiple spots. Two zircons (JH992_12, and JH992_42) were found to have core to rim variability over about 50µm that correlates with observed overgrowths (Table 1), emphasizing our need for the highest possible spatial resolution. The values of δ 18O for 3.90–4.28-Gyr-old zircon cores ranged from 5.4 ± 0.6‰ (JH992_76) to 9.0 ± 0.2‰ (JH992_42) for seven zircons from sample JH992. Figure 2 shows the data for these zircons plotted as δ18Ozircon versus 207Pb/206Pb age.

Figure 2: Ion microprobe δ18O data for individual zircon spot analyses versus 207Pb/206Pb zircon age (Supplementary Information available at http://www.nature.com).
Figure 2 : Ion microprobe |[delta]|18O data for individual zircon
spot analyses versus 207Pb/206Pb zircon age
(Supplementary Information available at http://www.nature.com).
 Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com

The right vertical axis shows the estimated δ18O data for the whole rock10,2518OWR) from which the zircon crystallized. High δ18OWR values are consistent with the incorporation of recycled crustal material that had interacted with low-temperature water in the magmatic source of the zircons.

High resolution image and legend (37K)

The δ18OSMOW of the mantle, estimated to be 5.5‰18, is unlikely to have changed over time as contemporary mid-ocean ridge and ocean island basalts have identical δ18O values of 5.7 ± 0.2‰ (refs 19 , 20). Crustal contamination can strongly affect oxygen isotopes in igneous rocks. Phanerozoic granitoids derived largely from orthogneiss protoliths (I-types) tend to have δ18O values below 9‰, whereas those derived by melting of clay-rich sedimentary rocks (S-types) have higher δ18O values21. Granitoids with δ18O values significantly less than 6‰ probably reflect interaction with meteoric water22. In general, S-type granitoids form by partial melting of metasediments enriched in 18O, as opposed to I-type granitoids, which form by melting of igneous rocks derived from arc processes23. As the average δ 18O values of Archean sedimentary rocks varies from 9 to 12‰ (ref. 24), only small inputs of this component to a primitive source magma will measurably raise its δ18O.

The δ18O values of the zircons we examined range from 5.4‰ to 15.0‰ (Table 1; Fig. 2). In the two most discordant grains (JH992_12, JH992_42), we identified core–rim relationships and analysed both. In these cases, rim compositions were systematically higher in δ18O than cores. In the five other zircons investigated, multiple analyses undertaken on optically continuous grains range in δ18O from 5.4 to 7.7‰. The oxygen isotope fractionation between zircon and a granitoid host rock is approximately -2‰ (refs 10, 25), permitting us to estimate the δ18O value of the melt from which the zircon crystallized. Source δ 18O values calculated in this fashion for all seven zircon cores analyzed range from about 7 to 11‰. These results indicate the presence in these zircons of recycled crustal material that had interacted with liquid water under surface, or near-surface, conditions. This conclusion is consistent with the results of a Hf isotope study of detrital zircons from the Narryer Gneiss Complex26 that shows that many Hadean zircons formed by re-melting of significantly older crust. If the rims of grains JH992_12 and JH992_42 also reflect crystallization from a melt, then a source δ 18O value as high as 17‰ is possible.

We note that mineral assemblages (including muscovite and monazite) suggestive of derivation from a peraluminous melt were found as inclusions in 4.2-Gyr-old zircons from the Narryer Gneiss Complex8. Peraluminous granitoids form mainly from melting of a graywacke protolith27; therefore, this observation is also consistent with the presence of a hydrosphere on Earth before 4.2Gyr ago.

The crystallization of Hadean zircons containing heavy oxygen isotopic compositions suggests the existence of a hydrosphere at or near the Earth's surface within about 200Myr of terrestrial core formation and the origin of the Moon at 4.50Gyr (ref. 28). Liquid water, a source of energy, and organic raw materials are assumed to be necessary for the origin and propagation of life. Our results thus raise the possibility that a biosphere could have arisen on Earth at least 400Myr earlier than is now thought29.

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References

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Supplementary Information

Supplementary information accompanies this paper.

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Acknowledgements

This work was supported by grants from NASA and NSF. Technical assistance from C. D. Coath is gratefully appreciated. We thank S. Claesson and J. Valley for providing the zircon oxygen standards. We also thank A. Halliday and C. Miller for comments on the manuscript. We are grateful to the McTaggart family of Mt Narryer station and the Broad family of Milly Milly station, Western Australia, for their hospitality in the field. We acknowledge support from the Instrumentation and Facilities Program of the National Science Foundation.