Ronny Schoenberg

New W-isotope evidence for rapid terrestrial accretion and very early core formation

The short-lived Hf-182-W-182-isotope system is an ideal clock to trace core formation and accretion processes of planets. Planetary accretion and metal/silicate fractionation chronologies are calculated relative to the chondritic Hf-182-W-182-isotope evolution. Here, we report new high-precision W-isotope data for the carbonaceous chondrite Allende that are much less radiogenic than previously reported and are in good agreement with published internal Hf-W chronometry of enstatite chondrites. If the W-isotope composition of terrestrial rocks, representing the bulk silicate Earth, is homogeneous and 2.24 epsilon(182W) units more radiogenic than that of the bulk Earth, metal/silicate differentiation of the Earth occurred very early. The new W-isotope data constrain the mean time of terrestrial core formation to 34 million years after the start of solar system accretion. Early terrestrial core formation implies rapid terrestrial accretion, thus permitting formation of the Moon by giant impact while Hf-182 was still alive. This could explain why lunar W-isotopes are more radiogenic than the terrestrial value. Copyright (C) 2002 Elsevier Science Ltd.

Tungsten isotope evidence from ∼ 3.8-Gyr metamorphosed sediments for early meteorite bombardment of the Earth

The ‘Late Heavy Bombardment’ was a phase in the impact history of the Moon that occurred 3.8–4.0 Gyr ago, when the lunar basins with known dates were formed. But no record of this event has yet been reported from the few surviving rocks of this age on the Earth. Here we report tungsten isotope anomalies, based on the 182Hf–182W system (half-life of 9 Myr), in metamorphosed sedimentary rocks from the 3.7–3.8-Gyr-old Isua greenstone belt of West Greenland and closely related rocks from northern Labrador, Canada. As it is difficult to conceive of a mechanism by which tungsten isotope heterogeneities could have been preserved in the Earths dynamic crust–mantle environment from a time when short-lived 182Hf was still present, we conclude that the metamorphosed sediments contain a component derived from meteorites.

Simplified method for the determination of Ru, Pd, Re, Os, Ir and Pt in chromitites and other geological materials by isotope dilution ICP-MS and acid digestion

A method for the determination of low Ru, Pd, Re, Os, Ir and Pt abundances in geological reference materials by isotope dilution inductively coupled plasma mass spectrometry (ICP-MS) after acid digestion in a high pressure asher (HPA-S) is presented. The digestion technique is similar to that using Carius tubes but easier to handle and reaches higher temperatures. Osmium can be determined as OsO4 with ICP-MS directly after digestion through a sparging technique. The remaining elements are preconcentrated by means of anion column chromatography. The resin is digested directly without elution leading to high yields but this causes problems if Zr is present at higher levels in the silicate rich materials. The analytical results for international platinum group element (PGE) reference materials, chromitite CHR-Bkg, basalt TDB-1 and gabbro WGB-1, are presented and compared with literature data, demonstrating the validity of the described method. Although higher in concentration, PGEs determined for reference material WGB-1 were worse than for TDB-1 indicating a more inhomogeneous distribution of the platinum group mineral phases. The low PGE abundance chromitite standard, CHR-Bkg, is likely to be homogeneous for Ru, Re, Os and Ir and is recommended as a reference material for the study of chromitites. Detection limits (3s x total procedure blank) range from 0.012 ng (Re and Os) to 0.77 ng (Pt), which could be further improved by applying higher quality acids.

Iron isotopes in the early marine diagenetic iron cycle

Measurements of Fe oxyhydroxides [reactive Fe(III)] in two shallow sediment profiles from reducing and oxic environments on the Arabian Sea margin demonstrate Fe isotope fractionation during early marine diagenesis. Reactive Fe(III) has δ 56 Fe values between −0.77‰ and −0.19‰. Values are lowest at the top of the sediment profile and considerably lower than bulk sediment Fe (δ 56 Fe ≈ 0‰). Preferential reduction and dissolution of light Fe isotopes during diagenesis leaves behind an increasingly smaller and heavier reactive Fe(III) residual. Initially, the isotopic composition of reactive Fe(III) evolves down-core according to the fractionation factor typical of microbial dissimilatory Fe reduction. Deeper in the profile, δ 56 Fe values remain unchanged despite further Fe reduction. Here, another process with a different fractionation factor becomes dominant, probably reduction by dissolved sulfide. The δ 56 Fe of the residual reactive Fe(III) suggests that ∼25% of the initially present reactive Fe(III) was reduced by microbial Fe reduction. When Fe is diagenetically recycled between reducing sediments at depth and an oxic top layer, the process, depending on recycling efficiency, may result in the accumulation of light Fe in the top layer while the complementary heavier residual is buried. Fe diffusing from the seafloor back into the ocean should reflect the low-δ 56 Fe diagenetic source of dissolved Fe.

Applications of accurate, high-precision Pb isotope ratio measurement by multi-collector ICP-MS

The isotope composition of Ph is difficult to determine accurately due to the lack of a stable normalisation ratio. Double and triple-spike addition techniques provide one solution and presently yield the most accurate measurements. A number of recent studies have claimed that improved accuracy and precision could also be achieved by multi-collector ICP-MS (MC-ICP-MS) Pb-isotope analysis using the addition of Tl of known isotope composition to Pb samples. In this paper, we verify whether the known isotope composition of Tl can be used for correction of mass discrimination of Pb with an extensive dataset for the NIST standard SRM 981, comparison of MC-ICP-MS with TIMS data, and comparison with three isochrons from different geological environments. When all our NIST SRM 981 data are normalised with one constant Tl-205/Tl-203 of 2.38869, the following averages and reproducibilities were obtained: Pb-207/Pb-206=0.91461+/-18; Pb-208/Ph-206 = 2.1674+/-7; and (PbPh)-Pb-206-Ph-204 = 16.941+/-6. These two sigma standard deviations of the mean correspond to 149, 330, and 374 ppm, respectively. Accuracies relative to triple-spike values are 149, 157, and 52 ppm, respectively, and thus well within uncertainties. The largest component of the uncertainties stems from the Ph data alone and is not caused by differential mass discrimination behaviour of Ph and Tl. In routine operation, variation of sample introduction memory and production of isobaric molecular interferences in the spectrometers collision cell currently appear to be the ultimate limitation to better reproducibility. Comparative study of five different datasets from actual samples (bullets, international rock standards, carbonates, metamorphic minerals, and sulphide minerals) demonstrates that in most cases geological scatter of the sample exceeds the achieved analytical reproducibility. We observe good agreement between TIMS and MC-ICP-MS data for international rock standards but find that such comparison does not constitute the ultimate. test for the validity of the MC-ICP-MS technique. Two attempted isochrons resulted in geological scatter (in one case small) in excess of analytical reproducibility. However, in one case (leached Great Dyke sulphides) we obtained a true isochron (MSWD = 0.63) age of 2578.3 +/- 0.9 Ma, which is identical to and more precise than a recently published U-Pb zircon age (2579 3 Ma) for a Great Dyke websterite [Earth Planet. Sci. Lett. 180 (2000) 1-12]. Reproducibility of this age by means of an isochron we regard as a robust test of accuracy over a wide dynamic range. We show that reliable and accurate Pb-isotope data can be obtained by careful operation of second-generation MC-ICP magnetic sector mass spectrometers

A refined solution to Earth’s hidden niobium: implications for evolution of continental crust and mode of core formation

New high-precision niobium (Nb) and tantalum (Ta) concentration data are presented for early Archaean metabasalts, metabasaltic komatiites and their erosion products (mafic metapelites) from SW Greenland and the Acasta gneiss complex, Canada. Individual datasets consistently show sub-chondritic Nb/Ta ratios averaging 15.1+/-11.6. This finding is discussed with regard to two competing models for the solution of the Nb-deficit that characterises the accessible Earth. Firstly, we test whether Nb could have sequestered into the core due to its slightly siderophile (or chalcophile) character under very reducing conditions, as recently proposed from experimental evidence. We demonstrate that troilite inclusions of the Canyon Diablo iron meteorite have Nb and V concentrations in excess of typical chondrites but that the metal phase of the Grant, Toluca and Canyon Diablo iron meteorites do not have significant concentrations of these lithophile elements. We find that if the entire accessible Earth Nb-deficit were explained by Nb in the core, only ca. 17% of the mantle could be depleted and that by 3.7 Ga, continental crust would have already achieved ca. 50% of its present mass. Nb/Ta systematics of late Archaean metabasalts compiled from the literature would further require that by 2.5 Ga, 90% of the present mass of continental crust was already in existence. As an alternative to this explanation, we propose that the average Nb/Ta ratio (15.1+/-11.6) of Earths oldest mafic rocks is a valid approximation for bulk silicate Earth. This would require that ca. 13% of the terrestrial Nb resided in the Ta-free core. Since the partitioning of Nb between silicate and metal melts depends largely on oxygen fugacity and pressure, this finding could mean that metal/silicate segregation did not occur at the base of a deep magma ocean or that the early mantle was slightly less reducing than generally assumed. A bulk silicate Earth Nb/Ta ratio of 15.1 allows for depletion of up to 40% of the total mantle. This could indicate that in addition to the upper mantle, a portion of the lower mantle is depleted also, or if only the upper mantle were depleted, an additional hidden high Nb/Ta reservoir must exist. Comparison of Nb/Ta systematics between early and late Archaean metabasalts supports the latter idea and indicates deeply subducted high Nb/Ta eclogite slabs could reside in the mantle transition zone or the lower mantle. Accumulation of such slabs appears to have commenced between 2.5 and 2.0 Ga. Regardless of these complexities of terrestrial Nb/Ta systematics, it is shown that the depleted mantle Nb/Th ratio is a very robust proxy for the amount of extracted continental crust, because the temporal evolution of this ratio is dominated by Th-loss to the continents and not Nb-retention in the mantle. We present a new parameterisation of the continental crust volume versus age curve that specifically explores the possibility of lithophile element loss to the core and storage of eclogite slabs in the transition zone

Precise Os isotope ratio and Re–Os isotope dilution measurements down to the picogram level using multicollector inductively coupled plasma mass spectrometry

Abstract This article describes analytical procedures to measure Os isotopic composition and to determine isotope dilution Re and Os concentrations accurately, precisely, and rapidly on a multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS). For Os measurements, a custom made apparatus to allow efficient direct OsO4 distillation into the plasma source is described. Static multi-Faraday cup measurements on 50 ng commercially available Os standard yield 2σ external reproducibilities of 0.016% for 187Os/188Os and 0.017% for 186Os/188Os ratios (n = 5). The mean values are indistinguishable within analytical uncertainty from those determined by negative thermal ionization mass spectrometry (N-TIMS). Further, amounts of 25–250 pg of spiked and unspiked analytes of the same Os standard have been determined by ion counting, which reproduced the 187Os/188Os ratio of the Faraday cup measurements within analytical uncertainty. Thereby, the in-run precision increases from 1.2% (25 pg, 2σ) to 0.14% (200 pg) with an overall external reproducibility of 0.32%. Measuring procedures for Re include Ir doping of the analytes for in-run mass fractionation correction, whereby Ir ion beams are detected on Faraday collectors and Re ion beams are simultaneously measured on ion counting electron multiplier collectors. The potential of the method was tested using Re spike calibration and Re-blank measurements, which showed that 0.2 pg Re can be accurately measured to a precision better than 1%, whereas larger samples (>10 pg) allow precisions that are significantly better than any other analytical uncertainty such as weighing errors. Thus, the feasibility of high precision Re–Os analyses at the pg level with multicollector ICP-MS is demonstrated.

Petrogenesis of crustal wehrlites in the Oman ophiolite: Experiments and natural rocks

In the Wadi Haymiliyah of the Oman ophiolite (Haylayn block), discordant wehrlite bodies ranging in size from tens to hundreds of meters intrude the lower crust at different levels. We combined investigations on natural wehrlites from the Wadi In the Wadi Haymiliyah of the Oman ophiolite (Haylayn block), discordant wehrlite bodies ranging in size from tens to hundreds of meters intrude the lower crust at different levels. We combined investigations on natural wehrlites from the Wadi Haymiliyah section with an experimental study on the phase relations in a wehrlitic system in order to constrain the petrogenesis of the crustal wehrlites of the Oman ophiolite. Secondary ion mass spectrometry analyses of clinopyroxenes from different wehrlite bodies imply that the clinopyroxenes were crystallized from tholeiitic, mid-ocean ridge (MORB)-type melts. The presence of primary magmatic amphiboles in some wehrlites suggests a formation under hydrous conditions. Significantly enhanced Sr-87/Sr-86 isotope ratios of separates from these amphiboles imply that the source of the corresponding magmatic fluids was either seawater or subduction zone-related. The experiments revealed that under wet conditions at relatively low temperatures, a MORB magma has the potential to produce wehrlite in the ocean crust by accumulation of early olivine and clinopyroxene. These show typically high Mg# which is a consequence of the oxidizing effect of the prevailing high aH(2)O. First plagioclases crystallizing after clinopyroxene under wet conditions are high in An content, in contrast to the corresponding dry system. Trace element compositions of clinopyroxenes of those wehrlites from the Moho transition zone are too depleted in HREE to be in equilibrium with present-day MORB, implying a genetic relation to the V2 lavas of the Oman ophiolite, which are interpreted to be the result of fluid-enhanced melting of previously depleted mantle. We present a model on the petrogenesis of the crustal wehrlites in an upper mantle wedge above an initial, shallow subduction zone at the beginning of the intraoceanic thrusting.

The Source of the Great Dyke, Zimbabwe, and Its Tectonic Significance: Evidence from Re‐Os Isotopes

Re‐Os data for chromite separates from 10 massive chromitite seams sampled along the 550‐km length of the 2.58‐Ga Great Dyke layered igneous complex, Zimbabwe, record initial 187Os/188Os ratios in the relatively narrow range between 0.1106 and 0.1126. This range of initial 187Os/188Os values is only slightly higher than the value for the coeval primitive upper mantle (0.1107) as modeled from the Re‐Os evolution of chondrites and data of modern mantle melts and mantle derived xenoliths. Analyses of Archean granitoid and gneiss samples from the Zimbabwe Craton show extremely low Os concentrations (3–9 ppt) with surprisingly unradiogenic present‐day 187Os/188Os signatures between 0.167 and 0.297. Only one sample yields an elevated 187Os/188Os ratio of 1.008. Using these data, the range of crustal contamination of the Great Dyke magma would be minimally 0%–33% if the magma source was the primitive upper mantle, whereas the range estimated from Nd and Pb isotope systematics is 5%–25%. If it is assumed that the primary Great Dyke magma derived from an enriched deep mantle reservoir (via a plume), a better agreement can be obtained. A significant contribution from a long‐lived subcontinental lithospheric mantle (SCLM) reservoir with subchondritic Re/Os to the Great Dyke melts cannot be reconciled with the Os isotope results at all. However, Os isotope data on pre‐Great Dyke ultramafic complexes of the Zimbabwe Craton and thermal modeling show that such an SCLM existed below the Zimbabwe Craton at the time of the Great Dyke intrusion. It is therefore concluded that large melt volumes such as that giving rise to the Great Dyke were able to pass lithospheric mantle keels without significant contamination in the late Archean. Because the ultramafic‐mafic melts forming the Great Dyke must have originated below the SCLM (which extends to at least a 200‐km depth), the absence of an SCLM signature precludes a subduction‐related magma‐generation process.

Climate change and tectonic uplift triggered the formation of the Atacama Desert’s giant nitrate deposits

The giant nitrate deposits of the hyperarid Atacama Desert (Chile) are one of the most extraordinary, yet enigmatic, mineral occurrences on Earth. These deposits are complex assemblages of highly soluble nitrates, chlorides, sulfates, perchlorates, iodates, and chromates, and their preservation is the result of prevalent hyperarid climate conditions in the Atacama Desert since the late Miocene, with average rainfall rates of <10 mm/yr in the past ~3 m.y. Although several hypotheses have been proposed since the mid-1800s, the formation of these extensive deposits still remains highly controversial despite the fact that recent studies have argued toward an atmospheric source for the nitrate, sulfate, and perchlorate components. In this report, we focus on the often overlooked and poorly studied iodine and chromium components of Atacama’s nitrates. We present the fi rst cosmogenic iodine ( 129 I) and stable chromium (δ 53/52 Cr) isotope data of nitrates showing that groundwater has played an unforeseen role in the formation of these massive deposits. The isotopic signature of I in the nitrates ( 129 I/I ~150–600 × 10 –15 ) share similarities with deep sedimentary (marine) pore waters and shales, deviating signifi cantly from atmospheric iodine ( 129 I/I ~1500 × 10 –15 ), while the positive and highly fractionated δ 53/52 Cr SRM979 values (+0.7‰ to +3‰) are indicative of intense Cr redox cycling due to groundwater transport. Our evidence points toward a multi-source genetic model for the Atacama Desert nitrate deposits, where these extensive accumulations were the result of long-lived, near-surface mineral precipitation driven by groundwater (i.e., chromates, iodates) coupled with dry atmospheric deposition (i.e., nitrates, perchlorates) and sea spray inputs (i.e., sulfates, chlorides), triggered by increasing aridity and tectonic uplift.