John W. Valley

Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago.

No crustal rocks are known to have survived since the time of the intense meteor bombardment that affected Earth between its formation about 4,550 Myr ago and 4,030 Myr, the age of the oldest known components in the Acasta Gneiss of northwestern Canada. But evidence of an even older crust is provided by detrital zircons in metamorphosed sediments at Mt Narryer and Jack Hills in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, where grains as old as ∼4,276 Myr have been found. Here we report, based on a detailed micro-analytical study of Jack Hills zircons, the discovery of a detrital zircon with an age as old as 4,404 ± 8 Myr—about 130 million years older than any previously identified on Earth. We found that the zircon is zoned with respect to rare earth elements and oxygen isotope ratios (δ18O values from 7.4 to 5.0‰), indicating that it formed from an evolving magmatic source. The evolved chemistry, high δ18O value and micro-inclusions of SiO2 are consistent with growth from a granitic melt with a δ18O value from 8.5 to 9.5‰. Magmatic oxygen isotope ratios in this range point toward the involvement of supracrustal material that has undergone low-temperature interaction with a liquid hydrosphere. This zircon thus represents the earliest evidence for continental crust and oceans on the Earth.

UWG-2, a garnet standard for oxygen isotope ratios: Strategies for high precision and accuracy with laser heating

UWG-2 is a new garnet standard for oxygen isotope analysis prepared from a single large porphyroblast that was homogeneous (±0.21‰) at the millimeter-scale before grinding. The δ 18O value of UWG-2 has been determined in seven laboratories using either a laser probe system or externally heated Ni reaction vessels. The raw laser probe value is 5.74‰ at the University of Wisconsin. If all data are normalized to NBS-28 = 9.59‰, then the UW value (5.89‰) is in good agreement with the average of all labs (5.78‰). There is no significant difference between garnet analyses made with the two techniques, nor among labs using different wavelengths of IR laser. UWG-2 is available for interlaboratory comparison, and for assessing the performance of microanalytical techniques including laser probes and ion microprobes with a recommended value of δ 18O = 5.8‰ SMOW. Multiple, daily analyses of UWG-2 at the University of Wisconsin provide an accurate evaluation of all components in our laser-probe, mass-spectrometer system, and allow analytical problems to be rapidly identified. With this standardization, the accuracy of a single laser probe analysis is better than ±0.10‰. Over 1000 analyses of UWG-2 have been made. The average of all uncorrected δ 18O values is 5.74 ± 0.15‰ (1 sd). The precision on a single day averages ±0.07‰ and is frequently better than 0.05‰. The uncertainty in the mean for all analyses is ±0.005‰ (1σ). A small drift of the daily average over time results from inevitable changes in the vacuum line which require careful attention and maintenance.

Oxygen Isotopes in Zircon

Isotopic and trace element analysis of zircons can provide reliable and robust estimates of age, compositions of coexisting minerals and melts, and constraints on the genesis and protoliths of host rocks. Recent technological developments facilitate analysis of oxygen isotope ratios in zircon with high accuracy and precision by laser heating/gas-source mass-spectrometry and in situ from thin sections or grain mounts by ion microprobe/secondary ion mass-spectrometer. A large number of studies have shown that non-metamict zircons preserve their δ18O value from the time of crystallization; hence oxygen isotope ratio can be correlated with age (U-Pb) or trace element composition. The zircon δ18O record is generally preserved despite other minerals that have been reset by high-grade metamorphism or intense hydrothermal alteration. Thus the refractory nature and robust inheritance of zircon offers a potential means to sort out magmatic equilibration and reequilibration, and post-magmatic alteration, an eternal problem for igneous rocks. New processes and interpretations for igneous events have been proposed when the effects of post-magmatic exchange are fully recognized. Crustal recycling can be recognized from magmatic values of δ18O(zircon), and if source rocks are igneous and young at the time of melting, δ18O will often be the best geochemical signature. Microanalytical techniques are increasingly useful for stable isotope analysis of silicates. Both the laser fluorination/mass-spectrometer and the ion microprobe/secondary ion mass-spectrometer offer significant advantages over conventional techniques for analysis of zircon. ### Laser fluorination Accurate analysis of δ18O in refractory minerals such as zircon is optimized by use of a CO2 laser (λ = 10.6 μm). The best precision has been obtained when zircons are powdered before analysis. Accuracy and precision of ±0.05 to ±0.1‰ (1 standard deviation = 1sd) are reported for homogeneous samples of ~2 mg (~0.5 mm3) (Valley et al. 1994, …

Herbivore tooth oxygen isotope compositions: Effects of diet and physiology

Abstract The applicability of rapid and precise laser probe analysis of tooth enamel for δ18O has been verified, and the method has been applied to different modern herbivores in East Africa. Sampling and pretreatment procedures involve initial bleaching and grinding of enamel to 95% apatite) can be analyzed reliably. Different East African herbivores exhibit previously unsuspected compositional differences. Average enamel δ18O values (V-SMOW) are approximately: 25‰ (goat), 27‰ (oryx), 28‰ (dikdik and zebra), 29‰ (topi), 30‰ (gerenuk), and 32‰ (gazelle). These compositions differ from generalized theoretical models, but are broadly consistent with expected isotope effects associated with differences in how much each animal (a) drinks, (b) eats C3 vs. C4 plants, and (c) pants vs. sweats. Consideration of diet, water turnover, and animal physiology will allow the most accurate interpretation of ancient teeth and targeting of environmentally-sensitive animals in paleoclimate studies.

Oxygen isotope geochemistry of zircon

The high-temperature and small sample size of an I.R. laser system has allowed the first detailed study of oxygen isotope ratios in zircon. Low-magnetism zircons that have grown during metamorphism in the Adirondack Mts., N.Y. preserve primary δ18O values and low-magnetism igneous zircons are likewise primary, showing no significant affect due to subsequent granulite facies metamorphism. The measured fractionation between zircon and garnet is Δ(Gt-Zrc) = 0.0 ± 0.2‰(1σ) for most low-magnetism zircons in meta-igneous rocks. The consistency of this value indicates equilibration at temperatures of 700–1100°C and little or no change in the equilibrium fractionation over this temperature range. In contrast, detrital low-magnetism zircons in quartzite preserve igneous compositions, up to 4‰ out of equilibrium with host quartz, in spite of granulite facies metamorphism. The oxygen isotope composition of zircon can be linked to UPb ages and can ‘see though’ metamorphism, providing a new tool for deciphering complex igneous, metamorphic and hydrothermal histories. Zircons separated by magnetic susceptibility show a consistent correlation. Low-magnetism zircons have the lowest uranium contents, the most concordant UPb isotopic compositions, and primary δ18O values. In contrast, high-magnetism zircons are up to 2‰ lower in δ18O than low-magnetism zircons from the same rock. The resetting of oxygen isotope ratios in high-magnetism zircons is caused by radiation damage which creates microfractures and enhances isotopic exchange. Zircons from the metamorphosed anorthosite-mangerite-charnockite-granite (AMCG) suite of the Adirondacks have previously been dated (1125–1157 Ma) and classified as igneous, metamorphic or disturbed based on their physical and UPb isotopic characteristics. Low-magnetism zircons from the AMCG suite have high, nearly constant values of δ18O that average 8.1 ± 0.4‰(1σ) for samples ranging from 39 to 75 wt% SiO2. Only olivine metagabbros have lower average values (6.4‰), consistent with the hypothesis that they represent nearly pristine samples of the anorthosites parent magma. Whole-rock values of δ18O are also high in the AMCG suite and increase with SiO2 content, as predicted for a process of assimilation and fractional crystallization. Taken together, these data suggest that the elevated values of oxygen isotope ratios result from partial melting and contamination involving metasediments in the deep crust, before the crystallization of zircon. More normal values elsewhere in the Grenville Province record deep-seated, pre-1150 Ma regional differences.

The petrologic case for a dry lower crust

Fluid pressure in the crust may be controlled by different mechanisms according to depth, temperature, and the mineralogy of the host rocks. Where rocks are fluid-saturated, fluid pressure may approach lithostatic or hydrostatic pressure depending on the ductility of the wall rocks and the connectivity of pores and fractures. However, if the host rocks contain minerals formed at temperatures higher than those currently prevailing, they will react with fluids to produce hydrated (or carbonated) retrograde minerals, and the fluid pressure will be limited by thermodynamic equilibrium between high-grade reactant minerals and retrograde products. The thermodynamically constrained parameter, water fugacity, may have a value of tens to hundreds of bars in the lower crust. In practice, this means that for typical igneous or high-grade metamorphic rocks now occurring in stable lower crust, notional fluid pressures are substantially (1 to 3 orders of magnitude) lower than lithostatic. No free, connected fluid phase can be present in deep stable crust, and alternative explanations must be sought for the relatively high electrical conductivity of such rocks. The proposal that high lower crustal conductivity is due to thin grain boundary films of graphite is also unlikely to be generally true because films of sufficient thickness would be readily visible on broken surfaces of hand specimens. An alternative explanation of the discrepancy between laboratory and field measurements of the conductivity of high-grade rocks is that laboratory measurements are not normally made under appropriate conditions of rock-buffered fluid pressure.

Oxygen isotope ratios and rare earth elements in 3.3 to 4.4 Ga zircons: Ion microprobe evidence for high δ18O continental crust and oceans in the Early Archean

Ion microprobe analyses of oxygen isotope ratios in Early Archean (Hadean) zircons (4.0- to 4.4-Ga) reveal variable magmatic δ18O values, including some that are high relative to the mantle, suggesting interaction between magmas and already-formed continental crust during the first 500 million yr of Earth’s history. The high average δ18O value of these zircons is confirmed by conventional analysis. A metaconglomerate from the Jack Hills in the Yilgarn Craton (Western Australia) contains detrital zircons with ages > 4.0 Ga (Compston and Pidgeon, 1986) and one crystal that is 4.40-Ga old (Wilde et al., 2001). The newly discovered 4.40-Ga grain is the oldest recognized terrestrial mineral. The Jack Hills metaconglomerate also contains a large 3.3- to 3.6-Ga-old zircon population with an average δ18O value of 6.3 ± 0.1‰ (1 s.e.,; n = 32 spot analyses). Two 4.15-Ga zircons have an average δ18O of 5.7 ± 0.2‰ (n = 13). In addition, a 4.13-Ga zircon has an average δ18O of 7.2 ± 0.3‰ (n = 8) and another 4.01-Ga zircon has an average δ18O of 6.8 ± 0.4‰ (n = 10). The oldest grain (4.40 Ga) is zoned with respect trace element composition (especially LREE), and intensity of cathodoluminescence, all of which correlate with oxygen isotope ratios (7.4‰ vs. 5.0‰). High LREE and high-δ18O values from the 4.01- to 4.40-Ga grains are consistent with growth in evolved granitic magmas (δ18O(WR) = 8.5 to 9.5‰) that had interacted with supracrustal materials. High δ18O values show that low-temperature surficial processes (i.e., diagenesis, weathering, or low-temperature alteration) occurred before 4.0 Ga, and even before 4.40 Ga, shortly following the hypothesized date of core differentiation and impact of a Mars-sized body to form the Moon at ∼4.45 Ga. This is the first evidence of continental crust as early as 4.40 Ga and suggests differentiation during the period of intense meteorite bombardment of the early Earth. The magnitude of water and rock interaction that would be necessary to cause the high δ18O values suggests the presence of liquid water and thus the possibility of an ocean at 4.40 Ga.

Oxygen isotope constraints on the sources of Hawaiian volcanism

We have measured oxygen isotope ratios in 99 separates of olivine and 14 separates of plagioclase or glass from Hawaiian lavas. These data confirm that the source(s) of some Hawaiian basalts are lower in δ^(18)O than peridotite xenoliths and the source region for mid-ocean ridge basalts (MORB). Our data document correlations between oxygen and radiogenic isotope ratios and consistent differences in δ^(18)O between volcanoes. Low values of δ^(18)O are associated with a ‘depleted’ component that is relatively high in ^(206)Pb/^(204)Pb, low in ^3He/^4He, and anomalously low in ^(207)Pb/^(204)Pb relative to ^(206)Pb/^(204)Pb. This component is preferentially sampled in lavas from the so-called Kea trend volcanoes (Kilauea, Mauna Kea, Kohala and Haleakala). Low δ^(18)O values in the ‘Kea’ component suggest that it is hydrothermally altered oceanic crust. The similarity of the Kea end member to Pacific MORB in terms of Sr, Nd, and Pb isotope ratios further suggests that this component is assimilated from the local Pacific plate in subcrustal magma chambers. Anomalous ^(206)Pb/^(204)Pb-^(207)Pb/^(204)Pb relationships indicate recent enrichment in U/Pb in this component and further support the hypothesis that this component is young ( < 10^8 yr) Pacific crust. The isotopic distinctions between Loa and Kea trend volcanoes implies a systematic difference in the magma supply and plumbing systems of volcanoes on these two trends. Samples from Lanai and Koolau have ‘enriched’ radiogenic isotope compositions (radiogenic Sr and non-radiogenic Nd and Pb) and higher δ^(18)O than typical upper mantle values, suggesting the incorporation of recycled sediment and/or oceanic crust in their sources. Other isotopic end members to Hawaiian lavas (e.g., high ^3He/^4He and post-erosional lavas) have δ^(18)O values within the range typical of the upper mantle.

Oxygen isotope variations in ocean island basalt phenocrysts

Oxygen isotope ratios are reported for olivine phenocrysts from sixty-seven samples of ocean island basalts (OlBs), mid-ocean ridge basalts (MORBs), and related peridotites, including representatives of the various isotopic endmembers defined by radiogenic isotope ratios. OIBs are more homogeneous in δ^(18)O by this analysis than suggested by previous studies of whole rocks and glasses. Most 0I13 samples have oxygen isotope ratios within a restricted range (δ^(18)O_(olivine) = 5.0–5.4‰), comparable to those of olivines in peridotites from ophiolites, in most peridotitic mantle xenoliths, and inferred for the sources of mid-ocean ridge basalts. The exceptions are EM2 lavas, which are enriched in ^(18)O (δ^(18)_(olivine) = 5.4–6.1‰), and a small number of samples characterized by low ^3He/^4He and distinctive lead isotope ratios, which are ^(18)O depleted (δ^(18)O = 4.7-5.1‰). The observed range in δ^(18)O and the correlations with radiogenic isotope ratios are similar to those observed in a detailed study of Hawaiian samples (Eiler et al., 1996b). These results indicate that recycled crust and/or sediments (or the imprint of extensive metasomatism by fluids derived from such materials) is present as at most a small mass fraction (⪅ l%) in the mantle sources of most OIBs. The results on most EM2 lavas are consistent with the presence of ∼2–6% recycled sediment in their source regions. Low δ^(18)O values in OIBs can be produced by assimilation of altered lavas from high-levels in the volcanic edifice, assimilation of the oceanic crust underlying the volcano, or incorporation of subducted oceanic crust in mantle sources. The only consistent correlatives with low δ^(18)O are low ^3He/^4He and anomalous ^(207)Pb/^(204)Pb-^(206)Pb/^(204)Pb relationships, and most of the low-δ^(18)O samples can be explained by contamination by the underlying oceanic crust or volcanic edifice. High-^3He/^4He lavas are indistinguishable from MORBs and most other OIBs in terms of δ^(18)O, suggesting that to the extent that the lower mantle is sampled by hotspot volcanism, there is no significant vertical stratification in oxygen isotope ratios in the mantle.

Chondrulelike Objects in Short-Period Comet 81P/Wild 2

The Stardust spacecraft returned cometary samples that contain crystalline material, but the origin of the material is not yet well understood. We found four crystalline particles from comet 81P/Wild 2 that were apparently formed by flash-melting at a high temperature and are texturally, mineralogically, and compositionally similar to chondrules. Chondrules are submillimeter particles that dominate chondrites and are believed to have formed in the inner solar nebula. The comet particles show oxygen isotope compositions similar to chondrules in carbonaceous chondrites that compose the middle-to-outer asteroid belt. The presence of the chondrulelike objects in the comet suggests that chondrules have been transported out to the cold outer solar nebula and spread widely over the early solar system.