Alex N. Halliday

Core formation on Mars and differentiated asteroids

Meteorite chronometry based on the 182Hf–182W system can provide powerful constraints on the timing of planetary accretion and differentiation, although the full potential of this method has yet to be realized. For example, no measurements have been made on the silicate-rich portions of planets and planetesimals other than the Earth and Moon. Here we report tungsten isotope compositions for two eucrites, thought to be derived from asteroid 4 Vesta, and from eight other basaltic achondritic meteorites that are widely considered to be from Mars. The eucrites, which are among the oldest differentiated meteorites, yield exceedingly radiogenic tungsten, indicating rapid accretion, differentiation and core formation on Vesta within the first 5–15 Myr of Solar System history, whereas the range of radiogenic tungsten measurements on the martian meteorites points towards tungsten depletion via melting and core formation within the first 30 Myr of the Solar System. The survival of tungsten isotope heterogeneity in the martian upper mantle implies that no giant impacts or large-scale convective mixing took place since this time. These results contrast with those obtained for the Earth–Moon system, for which accretion and core formation related to giant impacts appears to have continued for at least an additional 20 Myr.

Nature of the Earth's earliest crust from hafnium isotopes in singledetrital zircons

Continental crust forms from, and thus chemically depletes, the Earths mantle. Evidence that the Earths mantle was already chemically depleted by melting before the formation of todays oldest surviving crust has been presented in the form of Sm–Nd isotope studies of 3.8–4.0 billion years old rocks from Greenland and Canada. But this interpretation has been questioned because of the possibility that subsequent perturbations may have re-equilibrated the neodymium-isotope compositions of these rocks. Independent and more robust evidence for the origin of the earliest crust and depletion of the Archaean mantle can potentially be provided by hafnium-isotope compositions of zircon, a mineral whose age can be precisely determined by U–Pb dating, and which can survive metamorphisms. But the amounts of hafnium in single zircon grains are too small for the isotopic composition to be precisely analysed by conventional methods. Here we report hafnium-isotope data, obtained using the new technique of multiple-collector plasma-source mass spectrometry, for 37 individual grains of the oldest known terrestrial zircons (from the Narryer Gneiss Complex, Australia, with U–Pb ages of up to 4.14 Gyr ( 10–13). We find that none of the grains has a depleted mantle signature, but that many were derived from a source with a hafnium-isotope composition similar to that of chondritic meteorites. Furthermore, more than half of the analysed grains seem to have formed by remelting of significantly older crust, indicating that crustal preservation and subsequent reworking might have been important processes from earliest times.

Early-middle archaean crustal evolution deduced from Lu-Hf and U-Pb isotopic studies of single zircon grains

Abstract We report detailed high precision combined single grain U-Pb and Lu-Hf studies of early zircons to obtain more reliable indications of the extent of mantle depletion and crustal recycling in the Archean. Despite the possibility that MC-ICPMS affords for precise Hf isotopic measurement of single zircons, the complexity of grain populations adds uncertainty to initial isotopic ratios. Multiple episodes of zircon growth and ancient Pb loss, common in early Archaean rocks, result in 207Pb/206Pb ages, and in some cases 176Hf/176Hf ratios, that are variable between and within zircon grains. In order to evaluate the role of heterogeneity of zircon populations and to obtain the most reliable eHf(T), we have analysed several abraded zircon grains (from two to eleven) from each of several samples for both Lu-Hf and U-Pb. Hf isotopic analyses with precision better than 1 to 1.5 e-units (2σ) were obtained from grains weighing between 3 and 10 micrograms. The observed internal variations in age, U-Pb discordance, and Hf isotopic composition have been tested against models of disturbance of isotopic systems in zircon. Application of the U-Pb and Lu-Hf methods to the same zircon grains and analysis of single grains appears to be crucial for finding closed geochemical systems and thereby obtaining reliable Hf isotope data from early Archaean rocks. The precision and accuracy of Hf isotopic data obtained with the approach presented here are limited mainly by the sensitivity of Hf isotopic analyses, and may be greatly improved with the progress of analytical techniques. Following our earlier study of the Jack Hill’s metaconglomerate in which we found no significant trace of depleted mantle-like Hf in >3.8 Ga zircons we have studied zircons from 15 rocks from four early-middle Archaean areas. Four ca. 3.6 Ga gneisses from the Acasta Gneiss Complex of Northwest Canada contain zircons with eHf(T) between +0.7 and −4.1, as well as xenocrystic zircon grains with unradiogenic Hf. Some older grains have reset U-Pb systems, but preserve their Hf isotopic composition. The 3.62 to 3.73 Ga Amitsoq gneisses, West Greenland, have eHf(T) between +1.4 and +2.6, while 3.52 to 3.32 Ga gneisses and felsic metavolcanics from Barberton and Pilbara yielded eHf(T) between +1.9 and +4.2. These early-middle Archaean complexes were formed from crust derived from previously depleted mantle. These complexes grew over the first 100 to 200 m.y. of their evolution mainly by addition of juvenile crust, while reworking of older crust was less significant. The higher eHf(T) values in the Barberton and Pilbara rocks compared to the Amitsoq gneisses are consistent with uniform or increasing mantle depletion during the early Archaean. The Acasta Gneiss Complex was probably formed from very old crust (3.8 to 4.0 Ga) that was extensively reworked during the Archaean. Both U-Pb and Hf isotopic data suggest the similarities between the evolution of the Acasta Gneiss Complex and the source of 4.2 to 3.4 Ga Jack Hills detrital zircons.

INCOMPATIBLE TRACE-ELEMENTS IN OIB AND MORB AND SOURCE ENRICHMENT IN THE SUB-OCEANIC MANTLE

The concentrations of incompatible trace elements in ocean island basalts (OIB) from the central Atlantic extend to relatively enriched and fractionated compositions in regions of older oceanic lithosphere. Certain trace element ratios normally considered to be uniform in the mantle, such as CePb, are particularly variable. However, other trace element ratios that are expected to be variable because of differences in bulk distribution coefficient, such as CeU, are relatively uniform. The CePb ratios in enriched OIB are correlated with unusually high UPb and low KU. These UPb ratios would have generated excessively radiogenic Pb if they were long-term (109 yr) features of the source such as might result from core formation or recycling of hydrothermally altered ocean floor basalts. However, volcanic centers with high UPb do have high 206Pb204Pb for their 207Pb204Pb, a feature that is most easily modelled by enrichment in U relative to Pb about 108 yr prior to melting, a time similar to the age of the lithosphere. We propose that the source regions of these magmas are enriched by the introduction of small degree partial melts soon after the formation of the oceanic lithosphere. Metasomatism of the uppermost mantle by small degree partial melts produced in equilibrium with a combination of residual upper mantle major silicate phases, together with minor amphibole (≤ 2%), sulfide (≤ 0.2%) and phlogopite (≤ 0.2%) at about the time of formation of the lithosphere, would generate a ‘near-surface fractionated’ (NSF) source with low KU and high UPb, Δ206Pb204Pb and CePb, while maintaining CeU, NbU, BaCe and BaNb that are only slightly fractionated relative to other OIB. An important feature of the modelling of NSF mantle is that U is more incompatible than Ba or Rb. This is confirmed by the variability in incompatible trace element ratios with U concentration for enriched OIB. However, this contrasts with the relative incompatibility deduced from UThRa disequilibrium data for MORB and OIB, endorsing the view that the variability in highly incompatible trace element ratios in enriched OIB is dominated by source enrichment effects that are distinct from the fractionation that takes place during the production of the erupted magmas. The CeU, BaCe and UPb ratios of all OIB, including enriched OIB from regions of old lithosphere, are uniform relative to data for MORB. This appears inconsistent with the degree of isotopic variability in OIB relative to MORB and is difficult to explain unless the variations in incompatible trace element ratios in MORB are dominated by effects other than melting. Ratio-element plots provide evidence that the incompatible element ratios of MORB are affected by OIB-component contamination in the source or in transit to the surface and this is consistent with covariation between trace element ratios and some isotopic compositions in MORB. The ratios and concentrations of highly incompatible trace elements in MORB vary as a consequence of this contamination, as well as degree of partial melting. The relative uniformity and near chondritic proportions of CeU and, to a lesser extent, BaCe in OIB compared with MORB are difficult to reconcile with recycling models that advocate material resembling present-day MORB or hydrothermally altered MORB as the dominant component of OIB sources but are consistent with NSF mantle recycling. Similarly, the Ba/U/Ce ratios are inconsistent with models in which the OIB source was affected by Ca perovskite fractionation in a magma ocean on the early Earth.

Closure temperatures of the Sm---Nd system in metamorphic garnets

Abstract Garnet-whole rock and garnet-mineral isochrons were determined on granulite facies gneisses and amphibolites from the Archean Pikwitonei Granulite Domain of the Superior Province, and the Proterozoic Central Gneiss Belt and Adirondack Highlands of the Grenville orogen. The Sm—Nd ages obtained from Archean garnets 0.1–0.5 cm in length are 30–110 Ma younger than the U—Pb ages obtained on the same garnets and also younger than the time of the last regional metamorphism, as determined by the growth ages of the youngest metamorphic garnets and zircons. Similarly, the Sm—Nd ages obtained from Proterozoic garnets with a diameter of 0.1–5 cm are younger than the time of the last regional metamorphism and similar or younger than cooling ages obtained on sphenes from the same sample or from the same geologic setting. Only the core of a garnet with a diameter of ca. 30 cm and without abundant inclusions may record the time of garnet growth. Comparison of the Sm—Nd ages with other geochronologic data and temperature estimates leads to the conclusion that the closure temperature for the Sm—Nd system in garnets analyzed in this study is ca. 600 ± 30°C. Only garnets with radii much larger than 5 cm may record Sm—Nd growth ages in upper amphibolite facies rocks from slowly cooled terranes. Garnets from higher grade terranes yield cooling ages that define the retrograde history of metamorphic terranes.

Isotopic record of lead pollution in lake sediments from the northeastern United States

Abstract Although it is common knowledge that Pb concentrations have increased in lake sediments in the northeastern United States over the last 150 years, the processes responsible have been the subject of debate. In this study, differences in lead isotopic compositions and concentrations in sediment from large lakes (Lake Erie, Ontario, and Michigan) and small ones (Deep Lake and Lake Andrus) are used to infer temporal changes in the source(s) of anthropogenic Pb in the Great Lakes region. A natural (background) component of Pb is present in sediment deposited prior to 1860 in Lake Erie and the other lakes as indicated from low Pb concentrations and uniform lead isotopic compositions. Changes in isotopic ratios of lake sediment reflect differing sources of anthropogenic Pb superimposed on the natural component such as regional deforestation from 1860–1890 followed by coal combustion and ore smelting through 1930. Combustion of leaded gasoline was the dominant anthropogenic Pb source to the atmosphere (and by inference to lake sediment) from 1930–1980. Temporal changes in lead isotopic compositions in lake sediment suggest that the source of the Pb used in gasoline additives gradually changed from 1930 to present. The best example is a distinct shift in lead isotopic ratios in lake sediment deposited after 1970 which corresponds to increased Pb production from the Viburnum Trend deposits in Missouri (present in all lakes except Ontario). However, the changes in lead isotopic compositions are much less variable than and do not parallel those calculated on the basis of annual United States mine production and imports. Rather, anthropogenic recycling of Pb as well as natural mixing processes during emission, transport, and deposition of Pb in lake sediment control most of the variation in lead isotope ratios. Differences in lead isotopic ratios in Lake Michigan, Erie, and Deep Lake sediment preserve regional differences in lead isotopic ratios from U.S. and Canadian sources first noted in aerosols by Sturges and Barrie (1987). More localized sources of Pb (such as point discharges) are needed to explain the results from Lake Ontario and Andrus.

Ir, Ru, Pt, and Pd in basalts and komatiites: New constraints for the geochemical behavior of the platinum-group elements in the mantle

Abstract The concentrations of the platinum-group elements (PGE) Ir, Ru, Pt, and Pd were determined in 18 mantle-derived basalts from a variety of tectonic settings and six komatiites from three locations. All analyses were performed using isotope dilution, Carius tube digestion, and the precise technique of multiple collector inductively coupled plasma mass spectrometry. Multiple analyses of two samples indicate external reproducibilities, based upon separate dissolutions, of approximately 2–9% in the ppt to ppb concentration range. Mid-ocean ridge basalts from the Kolbeinsey Ridge, tholeiites from Iceland and alkali basalts from the Cameroon Line define three individual sample suites that are characterized by distinct major, trace, and platinum-group element systematics. All three-sample suites display correlations of the PGE with MgO, Ni, and Cr. The new analytical results are employed to constrain the geochemical behavior of the PGE during the formation and differentiation of mantle–derived melts. The PGE are inferred to be compatible in sulfides during partial melting with sulfide-silicate melt partition coefficients of ∼1 × 104. The fractionated PGE patterns of mantle melts are a consequence of the incompatibility of Pd in nonsulfide phases, whereas Ir and Ru must be compatible in at least one other mantle phase. Model calculations indicate that PGE alloys or spinel may be responsible for the higher compatibility of the latter elements during partial melting. It is further demonstrated that the shape of the melting regime has a profound effect on the PGE systematics of mantle magmas. The systematic trends of the three sample suites in plots of PGE against Ni and Cr are the result of magma differentiation processes that involve fractional crystallization of silicate minerals and the concurrent segregation of an immiscible sulfide liquid. The behavior of the PGE during magma fractionation indicates that the segregated sulfides probably equilibrate with >90% of the silicate magma and that PGE scavenging by sulfides is best described by a combination of batch and fractional equilibrium partitioning.

Provenance of dust in the Pacific Ocean

Eolian dust preserved in deep-sea sediment cores provides a valuable indicator of past atmospheric circulation and continental paleoclimate. In order to identify the provenance of eolian dust, Nd and Sr isotopic compositions and Rb, Sr and rare earth element (REE) concentrations have been determined for the silicate fractions of deep-sea sediments from the north and central Pacific Ocean. Different regions of the Pacific Ocean are characterized by distinct air-borne inputs, producing a large range in eNd (--10 tO + 1), STSr//86Sr (0.705-0.721), La/Yb (5-15), EUN/EU ~ (0.6-1.0) and Sr/Nd (4-33). The average Nd isotopic composition of Pacific deep-sea sediments (ENO = --6), is more radiogenic than the average from the Atlantic (eNa = --8). In contrast, the average 1475m//14aNd ratio for Pacific sediments (0.114) is identical to that of Atlantic sediments and to that of global average riverine suspended material. The values of ENa and 147Sm/144Nd are positively correlated for the Pacific samples but negatively correlated for Atlantic samples, reflecting a fundamental difference between the dominant components in the end members with radiogenic Nd (island-arc components in the Pacific and LREE-enriched intraplate ocean island components in the Atlantic). Samples from the north central Pacific have distinctive unradiogenic eNd values of --10, 87Sr/86Sr > 0.715, high La/Yb (> 12), and low EuN/Eu ~ (0.6) and Sr/Nd (3-6). These data are virtually identical to the values for loess from Asia and endorse the use of these sediments as indicators of Asian paleoclimate and paleowind directions. Island-arc contributions appear to dominate in the northwest Pacific, resulting in higher end (--1 to + 1) and lower 87Sr/86Sr (= 0.705) and La/Yb (~-5). Sediments from the eastern Pacific tend to have intermediate Sr and Nd isotopic compositions but regionally variable Sr/Nd and REE patterns; they appear to be derived from the west margin of the North and South American continents, rather than from Asia. Our results confirm that dust provenance can be constrained by isotopic and geochemical analyses, which will facilitate reconstructions of past atmospheric circulation and continental paleoclimate.

APPLICATIONS OF MULTIPLE COLLECTOR-ICPMS TO COSMOCHEMISTRY, GEOCHEMISTRY, AND PALEOCEANOGRAPHY

Multiple collector-inductively coupled plasma mass spectrometry (MC-ICPMS) is a new technique for the measurement of isotopic compositions at high precision, and is of great relevance to planetary, earth, ocean, and environmental sciences. The method combines the outstanding ionization efficiency of the ICP source with the superior peak shapes achievable from the ion optical focal plane of a large dispersion magnetic sector mass spectrometer, utilizing simultaneous multiple collection to achieve the most precise isotopic measurements yet made for many elements—particularly those with high first ionization potential. The addition of a laser facilitates studies for which spatial resolution is required. This method is still in its infancy, yet diverse applications have already led to a number of important scientific developments. Here we review some of these accomplishments and the potential for further work. The Lu-Hf isotopic system, for many years considered analytically challenging, is now relatively straightforward and offers great promise in fields as diverse as garnet geochronology, hydrothermal fluxes to the oceans, and crustal evolution. The age of the Earth’s core, the Moon, and Mars have been measured using a new short-lived chronometer 182Hf-182W. Other such new chronometers will follow. High precision isotope dilution measurements of the Earth’s inventory of many poorly understood elements such as In, Cd, Te, and the platinum group elements are providing tests for models for the accretion of the inner solar system. The small natural isotopic variations in elements such as Cu and Zn, produced by mass dependent fractionation, are now measurable at high precision with this method, and entirely new fields of stable isotope geochemistry can be developed. Similarly, measurements of small nucleosynthetic isotopic anomalies should be made easier for some elements. Measurements of U and Th isotopic compositions at very high sensitivity and reproducibility are now possible, allowing the development of higher resolution Quaternary geochronology. Finally, using laser ablation, the first precise in situ Sr, Hf, W, and Pb isotopic measurements have been made in natural materials, opening up a range of microanalytical isotopic studies in petrology and marine geochemistry. MC-ICPMS offers exciting times ahead in areas well beyond the bounds of geochemistry. Indeed, MC-ICPMS is likely to become the method of choice for many isotopic measurements because it is a more user friendly and efficient method for the acquisition of high precision data. It is also much more versatile, permitting elements to be measured that were previously considered intractable, and allowing the acquisition of data in situ, all with a single mass spectrometer. The limiting factor on the sensitivity is the transmission which is ≤2% for all instruments thus far designed. If it is found possible to improve the transmission still further, thermal ionization mass spectrometry, the technique that has, thus far, provided the high precision measurements necessary for most of the vast field of radiogenic isotope geochemistry, may be relegated to specialized applications.

Silicon in the Earth’s core

Small isotopic differences between the silicate minerals in planets may have developed as a result of processes associated with core formation, or from evaporative losses during accretion as the planets were built up. Basalts from the Earth and the Moon do indeed appear to have iron isotopic compositions that are slightly heavy relative to those from Mars, Vesta and primitive undifferentiated meteorites (chondrites). Explanations for these differences have included evaporation during the ‘giant impact’ that created the Moon (when a Mars-sized body collided with the young Earth). However, lithium and magnesium, lighter elements with comparable volatility, reveal no such differences, rendering evaporation unlikely as an explanation. Here we show that the silicon isotopic compositions of basaltic rocks from the Earth and the Moon are also distinctly heavy. A likely cause is that silicon is one of the light elements in the Earth’s core. We show that both the direction and magnitude of the silicon isotopic effect are in accord with current theory based on the stiffness of bonding in metal and silicate. The similar isotopic composition of the bulk silicate Earth and the Moon is consistent with the recent proposal that there was large-scale isotopic equilibration during the giant impact. We conclude that Si was already incorporated as a light element in the Earth’s core before the Moon formed.