Colin M. Graham

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.

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.

Short Paper: The depositional age of the Dalradian Supergroup: U-Pb and Sm-Nd isotopic studies of the Tayvallich Volcanics, Scotland

Zircons from a keratophyre associated with the Tayvallich Volcanics in the Dalradian rocks of the SW Scottish Highlands have been dated by U-Pb methods, yielding an age of 595 ± 4 Ma. This age indicates that most or all of the Dalradian is Precambrian in age, and that Dalradian sedimentation may have lasted for about 200 million years. The age also constrains the time interval between cessation of Dalradian sedimentation and subsequent crustal thickening and regional metamorphism during the Grampian Orogeny. Sm-Nd isotopic data for the Tayvallich Volcanins and related metadolerite sills yield initial ºNd values of +2 to +4, which are thought to reffect the effects of melting of lithospheric mantle.

Slow oxygen diffusion rates in igneous zircons from metamorphic rocks

Abstract Empirical tests of oxygen exchange rate in zircon crystals from amphibolite- and granulite-facies metamorphic rocks of the Grenville Province demonstrate preservation of igneous δ18O through protracted igneous and metamorphic histories, forming the basis of quantitative estimates of diffusion rate. Granitic orthogneisses, which cooled slowly after granulite-facies metamorphism, show no consistent relationship between zircon size and δ18O, indicating slow oxygen diffusion. Detrital zircon crystals from granulite-facies quartzites are out of equilibrium with their host rocks, and no consistent correlation is seen between δ18O and grain size in high-precision analyses by laser fluorination of multiple grains, sieved for size. In a single sample, individual detrital zircon crystals preserve grain-to-grain variability in δ18O (determined by ion microprobe), ranging from 5.0 to 9.5‰. The inherited cores of some zircon crystals are up to 5.6‰ lower than igneous overgrowths, showing that gradients of 5.6‰ can be preserved over 50 mm even at magmatic conditions. All of these lines of evidence show that oxygen diffusion in zircon in these rocks was slow both during metamorphism and during slow cooling of 1-3 °/m.y. Calculations based on the measurements indicate that the oxygen diffusion rate in zircon (D) must be ≥ 10-22 cm2/s at 600 °C to explain δ18O(zircon) values measured from Grenville quartzite and orthogneiss. This value is consistent with the experimentally determined value of D = 2 × 10-27 cm2/s for dry diffusion experiments extrapolated to 600 °C (Watson and Cherniak 1997). These results indicate that oxygen-isotope analysis of zircon may be used to see through granulite-facies metamorphism and anatexis, and to unravel crustal recycling processes in igneous rocks.

Ion microprobe analysis of oxygen isotope ratios in granulite facies magnetites: diffusive exchange as a guide to cooling history

Ion microprobe analysis of magnetites from the Adirondack Mountains, NY, yields oxygen isotope ratios with spatial resolution of 2–8 μm and precision in the range of 1‰ (1 sigma). These analyses represent 11 orders of magnitude reduction in sample size compared to conventional analyses on this material and they are the first report of routinely reproducible precision in the 1 per mil range for analysis of δ18O at this scale. High precision micro-analyses of this sort will permit wide-ranging new applications in stable isotope geochemistry. The analyzed magnetites form nearly spherical grains in a calcite matrix with diopside and monticellite. Textures are characteristic of granulite facies marbles and show no evidence for retrograde recrystallization of magnetite. Magnetites are near to Fe3O4 in composition, and optically and chemically homogeneous. A combination of ion probe plus conventional BrF5 analysis shows that individual grains are homogeneous with δ18O=8.9±1‰ SMOW from the core to near the rim of 0.1–1.2 mm diameter grains. Depth profiling into crystal growth faces of magnetites shows that rims are 9‰ depleted in δ18O. These low δ18O values increase in smooth gradients across the outer 10 μm of magnetite rims in contact with calcite. These are the sharpest intracrystalline gradients measured to date in geological materials. This discovery is confirmed by bulk analysis of 150–350 μm diameter magnetites which average 1.2‰ lower in δ18O than coarse magnetites due to low δ18O rims. Conventional analysis of coexisting calcite yields °18O=18.19, suggesting that bulk Δ18O (Cc-Mt)=9.3‰ and yielding an apparent equilibration “temperature” of 525° C, over 200° C below the temperature of regional metamorphism. Consideration of experimental diffusion data and grain size distribution for magnetite and calcite suggests two contrasting cooling histories. The data for oxygen in calcite under hydrothermal conditions at high P(H2O) indicates that diffusion is faster in magnetite and modelling of the low δ18O rims on magnetite would suggest that the Adirondacks experienced slow cooling after Grenville metamorphism, followed by a brief period of rapid cooling, possibly related to uplift. Conversely, the data for calcite at low P(H2O) show slower oxygen diffusion than in magnetite. Modelling based on these data is consistent with geochronology that shows slow cooling through the blocking temperature of both minerals, suggesting that the low δ18O rims form by exchange with late, low temperature fluids similar to those that infiltrated the rock to serpentinize monticellite and which infiltrated adjacent anorthosite to form late calcite veinlets. In either case, the ion microprobe results indicate that two distinct events are recorded in the post-metamorphic exchange history of these magnetites. Recognition of these events is only possible through microanalysis and has important implications for geothermometry.

Experimental hydrogen isotope studies—I. Systematics of hydrogen isotope fractionation in the systems epidote-H2O, zoisite-H2O and AlO(OH)-H2O

HD Fractionation factors between epidote minerals and water, and between the AlO(OH) dimorphs boehmite and diaspore and water, have been determined between 150 and 650°C. Small water mineral ratios were used to minimise the effect of incongruent dissolution of epidote minerals. Waters were extracted and analysed directly by puncturing capsules under vacuum. Hydrogen diffusion effects were eliminated by using thick-walled capsules. HD Exchange rates are very fast between epidote and water (and between boehmite and water), complete exchange taking only minutes above 450°C but several months at 250°C. Exchange between zoisite and water (and between diaspore and water) is very much slower, and an interpolation method was necessary to determine fractionation factors at 450 and below. For the temperature range 300–650°C, the HD equilibrium fractionation factor (αe) between epidote and water is independent of temperature and Fe content of the epidote, and is given by 1000 In αepidote-H2Oe = −35.9 ± 2.5, while below 300°C 1000 In αepidote-H2Oe = 29.2(106T2) − 138.8, with a ‘cross-over’ estimated to occur at around 185°C. By contrast, zoisite-water fractionations fit the relationship 1000 In αzoisite-H2Oe = − 15.07 (106T2) − 27.73. All studied minerals have hydrogen bonding. Fractionations are consistent with the general relationship: the shorter the O-H -- O bridge, the more depleted is the mineral in D. On account of rapid exchange rates, natural epidotes probably acquired their H-isotope compositions at or below 200°C, where fractionations are near or above 0%.; this is in accord with the observation that natural epidotes tend to concentrate D relative to other coexisting hydrous minerals.

Experimental hydrogen isotope studies III: Diffusion of hydrogen in hydrous minerals, and stable isotope exchange in metamorphic rocks

Diffusion parameters for hydrogen diffusion in epidote-group minerals and micas have been measured under hydrothermal conditions, or calculated from existing experimental data, for bulk hydrogen isotope exchange experiments between hydrous minerals and water. Activation energies in the range 14 to 31 kcals/g-atom H are comparable to those derived by application of kinetic theory to experimental hydrogen isotope exchange data, and to those for oxygen diffusion in minerals under hydrothermal conditions. Diffusion of hydrogen in epidote is about four orders of magnitude faster than in muscovite, and about two orders of magnitude faster than in zoisite. Hydrogen diffusion in micas is about five orders of magnitude faster than oxygen diffusion, and hydrogen transport occurs dominantly parallel to the layers rather than parallel to the c-axis as for oxygen.Rapid hydrogen transport in minerals may proceed by hydrolysis of Si-O and Al-O bonds, followed by exchange of hydrolyzed oxygens with slower-diffusing (OH) or H2O. Water appears to be essential for stable isotope exchange between minerals in slowly cooling metamorphic rocks.Stable isotope data for regional metamorphic mineral assemblages suggests that water is usually present in small amounts during cooling of prograde regional metamorphic systems, and estimated closure temperatures for cessation of stable isotope exchange are often more comparable to those calculated from diffusion data than to likely temperatures of metamorphism.Alpine deformation of the Hercynian Monte Rose Granite (Frey et al. 1976) permitted access of water and initiated stable isotope exchange amongst coexisting minerals. The diffusional behaviour of species in relict Hercynian muscovites is consistent with available experimental diffusion data.

A composite C-isotope profile for the Neoproterozoic Dalradian Supergroup of Scotland and Ireland

Abstract: The Neoproterozoic Dalradian Supergroup is a dominantly siliciclastic metasedimentary succession in the Caledonian orogenic belt of Scotland and Ireland. Despite polyphase deformation and greenschist- to upper amphibolite-facies metamorphism, carbonate units distributed throughout the Dalradian record marked δ13Ccarbonate excursions that can be linked to those associated with key environmental events of Neoproterozoic time. These include: (1) tentative correlation of the Ballachulish Limestone with the c. 800 Ma Bitter Springs anomaly; (2) the presence of the pre-Marinoan Trezona anomaly and 635 Ma Marinoan-equivalent cap carbonate sequence in rocks of the middle Easdale Subgroup; (3) the terminal Proterozoic (c. 600–551 Ma) Wonoka–Shuram anomaly in the Girlsta Limestone on Shetland. These linkages strengthen previously inferred correlations of the Stralinchy–Reelan formations and the Inishowen–Loch na Cille–MacDuff ice-rafted debris beds to the respectively 635 Ma Marinoan and 582 Ma Gaskiers glaciations, and suggest that the oldest Dalradian glacial unit, the Port Askaig Formation, represents one of the c. 750–690 Ma Sturtian glacial episodes. These δ13C data and resulting correlations provide more robust constraints on the geological evolution of the Dalradian Supergroup than anything hitherto available and enhance its utility in helping refine understanding of Neoproterozoic Earth history.

Hydrogeochemical changes before and after a major earthquake

Hydrogeochemical changes were detected by monitoring ice age meteoric waters before and after a magnitude (M) 5.8 earthquake on 16 September 2002 in the Tjornes Fracture Zone, northern Iceland. Significant Cu, Zn, Mn, and Cr anomalies reached our sampling station 1, 2, 5, and ≥10 weeks before the earthquake, respectively. By comparison with published experimental, geophysical, and geochemical studies, we suggest stress-induced source mixing and leakage of fluid from an external (hotter) basalt-hosted source reservoir, where fluid-rock interaction was more rapid. Rapid 12%–19% increases in the concentrations of B, Ca, K, Li, Mo, Na, Rb, S, Si, Sr, Cl, and SO 4 , and decreases in Na/Ca, δ 18 O, and δD, occurred 2–9 days after the earthquake. The rapidity of these changes is consistent with time scales of fault sealing due to coupled deformation and fluid flow. We interpret fluid-source switching in response to fault sealing and unsealing, with the newly tapped aquifer containing chemically and isotopically distinct ice age meteoric water. Variation in Na/Ca ratio appears to be sensitive to the changing stress state associated with M > 4 earthquakes. This study highlights the potential of hydrogeochemical change in earthquake-prediction studies.

An ion microprobe study of anhydrous oxygen diffusion in anorthite: a comparison with hydrothermal data and some geological implications

The diffusion rate of 18O tracer atoms in anorthite (An97Ab03) under anhydrous conditions has been measured using SIMS techniques. The tracer source was 18O2 98.4% gas at 1 bar, in the temperature range 1300° C−850° C. The measured diffusion constants are D0=1−0.6+1×10−9 m2s−1Q=236±8 kJ mol−1 Comparison of these values with published data for 18O diffusion in anorthite under hydrothermal conditions shows that “dry” oxygen diffusivities are orders of magnitude lower than equivalent “wet” values at similar temperatures. The effect of these differences on oxygen isotope equilibration during cooling is discussed.