Maria Schönbächler

Heterogeneous Accretion and the Moderately Volatile Element Budget of Earth

Earths Silver Lining The age of the oldest rocks on Earths surface is controversial, but, even if they are at their oldest estimate, hundreds of millions of years in our planets earliest history are still missing. However, in some rocks that until relatively recently resided in the mantle, the isotopic signature from the time of Earths formation is still preserved. Schönbächler et al. (p. 884) exploited this preservation to constrain models that describe the early material that assembled together to form Earth. Because the isotopic profile of silver in these rocks is nearly identical to that measured in a class of primitive meteorites, the earliest material probably had high volatile content. However, the fractionation of other isotopes suggests that the volatile content probably decreased over time in subsequent accretion events. With these isotopic model constraints, it is possible that one of the last major collisions—the Moon-forming giant impact—added considerable amounts of water and other volatile elements to Earth. Silver isotopes from mantle rocks suggest that Earth assembled from materials with variable volatile contents. Several models exist to describe the growth and evolution of Earth; however, variables such as the type of precursor materials, extent of mixing, and material loss during accretion are poorly constrained. High-precision palladium-silver isotope data show that Earth’s mantle is similar in 107Ag/109Ag to primitive, volatile-rich chondrites, suggesting that Earth accreted a considerable amount of material with high contents of moderately volatile elements. Contradictory evidence from terrestrial chromium and strontium isotope data are reconciled by heterogeneous accretion, which includes a transition from dominantly volatile-depleted to volatile-rich materials with possibly high water contents. The Moon-forming giant impact probably involved the collision with a Mars-like protoplanet that had an oxidized mantle, enriched in moderately volatile elements.

Measurement of zinc stable isotope ratios in biogeochemical matrices by double-spike MC-ICPMS and determination of the isotope ratio pool available for plants from soil

AbstractAnalysis of naturally occurring isotopic variations is a promising tool for investigating Zn transport and cycling in geological and biological settings. Here, we present the recently installed double-spike (DS) technique at the MAGIC laboratories at Imperial College London. The procedure improves on previous published DS methods in terms of ease of measurement and precisions obtained. The analytical method involves addition of a 64Zn–67Zn double-spike to the samples prior to digestion, separation of Zn from the sample matrix by ion exchange chromatography, and isotopic analysis by multiple-collector inductively coupled plasma mass spectrometry. The accuracy and reproducibility of the method were validated by analyses of several in-house and international elemental reference materials. Multiple analyses of pure Zn standard solutions consistently yielded a reproducibility of about ±0.05‰ (2 SD) for δ66Zn, and comparable precisions were obtained for analyses of geological and biological materials. Highly fractionated Zn standards analyzed by DS and standard sample bracketing yield slightly varying results, which probably originate from repetitive fractionation events during manufacture of the standards. However, the δ66Zn values (all reported relative to JMC Lyon Zn) for two less fractionated in-house Zn standard solutions, Imperial Zn (0.10 ± 0.08‰: 2 SD) and London Zn (0.08 ± 0.04‰), are within uncertainties to data reported with different mass spectrometric techniques and instruments. Two standard reference materials, blend ore BCR 027 and ryegrass BCR 281, were also measured, and the δ66Zn were found to be 0.25 ± 0.06‰ (2 SD) and 0.40 ± 0.09‰, respectively. Taken together, these standard measurements ascertain that the double-spike methodology is suitable for accurate and precise Zn isotope analyses of a wide range of natural samples. The newly installed technique was consequently applied to soil samples and soil leachates to investigate the isotopic signature of plant available Zn. We find that the isotopic composition is heavier than the residual, indicating the presence of loosely bound Zn deposited by atmospheric pollution, which is readily available to plants. FigureZinc isotope ratio pools of bulk soil and the associated acid leach (estimated plant available pool) as measured by double-spike MC-ICPMS. δxZnLyon-JMC=(Rsample/RJMC-Lyon -1)x103, where Rsample and RJMC-Lyon denote the xZn/64Zn isotope ratio of the sample and standard (JMC-Lyon), respectively, and where x denotes either 66 or 68.

Zirconium isotope evidence for incomplete admixing of r-process components in the solar nebula

Abstract Isotopic anomalies in Mo and Zr have recently been reported for bulk chondrites and iron meteorites and have been interpreted in terms of a primordial nucleosynthetic heterogeneity in the solar nebula. We report precise Zr isotopic measurements of carbonaceous, ordinary and enstatite chondrites, eucrites, mesosiderites and lunar rocks. All bulk rock samples yield isotopic compositions that are identical to the terrestrial standard within the analytical uncertainty. No anomalies in 92Zr are found in any samples including high Nb/Zr eucrites and high and low Nb/Zr calcium–aluminum-rich inclusions (CAIs). These data are consistent with the most recent estimates of

NEW TITANIUM ISOTOPE DATA FOR ALLENDE AND EFREMOVKA CAIs

We measured the titanium (Ti) isotope composition, i.e., 50Ti/47Ti, 48Ti/47Ti, and 46Ti/47Ti, in five calcium-rich-aluminum-rich refractory inclusions (CAIs) from the oxidized CV3 chondrite Allende and in two CAIs from the reduced CV3 chondrite Efremovka. Our data indicate that CAIs are enriched in 50Ti/47Ti and 46Ti/47Ti and are slightly depleted in 48Ti/47Ti compared to normal Ti defined by ordinary chondrites, eucrites, ureilites, mesosiderites, Earth, Moon, and Mars. Some CAIs have an additional 50Ti excess of ~8e relative to bulk carbonaceous chondrites, which are enriched in 50Ti by ~2e relative to terrestrial values, leading to a total excess of ~10e. This additional 50Ti excess is correlated with nucleosynthetic anomalies found in 62Ni and 96Zr, all indicating an origin from a neutron-rich stellar source. Bulk carbonaceous chondrites show a similar trend, however, the extent of the anomalies is either less than or similar to the smallest anomalies seen in CAIs. Mass balance calculations suggest that bulk Allende Ti possibly consists of a mixture of at least two Ti components, anomalous Ti located in CAIs and a normal component possibly for matrix and chondrules. This argues for a heterogeneous distribution of Ti isotopes in the solar system. The finding that anomalous Ti is concentrated in CAIs suggests that CAIs formed in a specific region of the solar system and were, after their formation, not homogeneously redistributed within the solar system. Combining the CAI data with improved model predictions for early solar system irradiation effects indicates that a local production scenario for the relatively short lived radionuclides can be excluded, because the production of, e.g., 10Be, 26Al, and 41Ca, would result in a significant collateral shift in Ti isotopes, which is not seen in the measured data.

Ion exchange chromatography and high precision isotopic measurements of zirconium by MC-ICP-MS

This paper presents a new technique for the precise and accurate determination of Zr isotopic compositions in geological samples. Following the separation of Zr from the geological matrix with a two-stage anion-exchange procedure the isotopic compositions are measured by multiple collector ICP-MS. Replicate dissolutions of the carbonaceous chondrite Allende with <100 ng Zr yield a long-term reproducibility of ±39 ppm for 91Zr/90Zr, ±25 ppm for 92Zr/90Zr, and ±82 ppm for 96Zr/90Zr. Analyses of synthetic standards solutions show that isobaric interferences of Mo and Ru can be adequately corrected for Mo/Zr ≤ 0.5 × 10−2 and Ru/Zr ≤ 1 × 10−2 and such elemental ratios are readily achieved for geological samples following the anion-exchange procedure. It is furthermore shown that the chemical separation technique effectively isolates Zr from Ti, Cr, and Fe. This is important because the Zr isotope data can be readily biased by the argides of these elements. The presented method has been successfully applied to terrestrial igneous rocks, meteorites and mineral separates including samples with high Ti contents.

The rates of accretion, core formation and volatile loss in the early Solar System

Nuclides with half–lives of 105–108 yr permit the elucidation of nebula time–scales and the rates of accretion of planetesimals. However, the 182Hf–182W system with a half–life of 9_2 Myr also provides new and very useful constraints on the formation of the terrestrial planets. This technique allows one to address the timing of metal–silicate equilibration in objects as different as chondrites and the Earth. With improvements in sensitivity and precision, very small time differences in metal segregation in asteroids should be resolvable from measuring iron meteorites. It is already clear that the formation and differentiation of some asteroidal–sized objects was completed in less than 10 Myr. Accretion and core formation were protracted in the case of the Earth (greater than 50 Myr) relative to Mars (probably less than 20 Myr). Indeed, the Martian mantle appears to retain both chemical and isotopic heterogeneities that are residual from the process of core formation. Such early features appear to have been eliminated from the Earths mantle presumably because of 4.5 Gyr of relatively efficient convective mixing. Tungsten isotope data provide compelling support for the ‘giant impact’ theory of lunar origin. The Moon is a high Hf/W object that contains a major component of chondritic W. This is consistent with a time of formation of greater than 50 Myr after the start of the Solar System. New highly precise oxygen isotope data are unable to resolve any difference between the source of components in the Earth and Moon. Therefore, the giant impact itself may have produced some of the differences in moderately volatile element budgets between these objects. This finds support in precise Sr isotopic data for early lunar samples. The data are consistent with the proto–Earth and Theia (the impactor) having Rb/Sr ratios that were not very different from that of present day Mars. Therefore, the extended history of accretion, rather than nebular phenomena, may be responsible for some of the major differences between the terrestrial planets.

High-precision measurement of W isotopes in Fe–Ni alloy and the effects from the nuclear field shift

We present an analytical protocol for high-precision measurements of W isotopes by multi-collector ICPMS in metal alloy samples, with a particular emphasis on the least abundant W isotope (180W). The external reproducibility, based on replicate analysis of an NIST Fe–Ni steel (SRM 129c), for e180W (6/4) is ± 0.49. This is an improvement of at least a factor of ≈2.4 compared to previous studies using MC-ICPMS. External precisions for other isotope ratios are comparable to previous studies of metallic samples. In addition, we observed resolvable deviations relative to the measurement reference standard (NIST SRM 3163) in isotope ratios that contain 183W. Similar effects have been reported in several previous studies of W isotopes. Our new data demonstrate that these effects are induced during chemical purification of W in the laboratory and that the isotopic variations are consistent with nuclear field shift effects. Such effects have been reported for various other elements but not yet for W. Possible bias introduced by this effect has implications for the use of W isotopes in early solar system chronology.

A new method for high-precision palladium isotope analyses of iron meteorites and other metal samples

This paper presents a new method for high precision Pd isotope analyses in iron meteorites. First, Pd is separated from the sample matrix by a novel two-stage anion exchange procedure after which isotopic measurements are carried out using MC-ICP-MS. Analyses of doped standard solutions show that isobaric interference from Ru and Cd can be adequately corrected for Ru/Pd < 0.0005 and Cd/Pd < 0.025. This is frequently achieved using the presented separation method. The purified Pd fraction after ion exchange chromatography is also sufficiently devoid of Ni (Ni/Pd < 0.04), Zr (Zr/Pd < 0.0002) and Zn (Zn/Pd < 0.06) for precise and accurate measurements because these elements produce molecular interference on the masses of the Pd isotopes. An external reproducibility of 1.29 for e102Pd, 0.22 for e104Pd, 0.11 for e106Pd, and 0.27 for e110Pd is calculated based on the repeated analyses of five independently processed aliquots of the IAB iron meteorite Toluca. The method was verified by the analysis of three metals from IVB iron meteorites and the results show excellent agreement with previous data. The new method enables accurate analysis of all Pd isotopes, and in particular 102Pd, which is of major interest for cosmochemical applications.

The initial abundance and distribution of 92Nb in the Solar System

Abstract Niobium-92 is an extinct proton-rich nuclide, which decays to 92Zr with a half-life of 37 Ma. This radionuclide potentially offers a unique opportunity to determine the timescales of early Solar System processes and the site(s) of nucleosynthesis for p-nuclei, once its initial abundance and distribution in the Solar System are well established. Here we present internal Nb–Zr isochrons for three basaltic achondrites with known U–Pb ages: the angrite NWA 4590, the eucrite Agoult, and the ungrouped achondrite Ibitira. Our results show that the relative Nb–Zr isochron ages of the three meteorites are consistent with the time intervals obtained from the Pb–Pb chronometer for pyroxene and plagioclase, indicating that 92Nb was homogeneously distributed among their source regions. The Nb–Zr and Pb–Pb data for NWA 4590 yield the most reliable and precise reference point for anchoring the Nb–Zr chronometer to the absolute timescale: an initial 92Nb/93Nb ratio of ( 1.4 ± 0.5 ) × 10 − 5 at 4557.93 ± 0.36 Ma , which corresponds to a 92Nb/93Nb ratio of ( 1.7 ± 0.6 ) × 10 − 5 at the time of the Solar System formation. On the basis of this new initial ratio, we demonstrate the capability of the Nb–Zr chronometer to date early Solar System objects including troilite and rutile, such as iron and stony-iron meteorites. Furthermore, we estimate a nucleosynthetic production ratio of 92Nb to the p-nucleus 92Mo between 0.0015 and 0.035. This production ratio, together with the solar abundances of other p-nuclei with similar masses, can be best explained if these light p-nuclei were primarily synthesized by photodisintegration reactions in Type Ia supernovae.

Late metal–silicate separation on the IAB parent asteroid: Constraints from combined W and Pt isotopes and thermal modelling

Abstract The short-lived 182Hf–182W decay system is a powerful chronometer for constraining the timing of metal–silicate separation and core formation in planetesimals and planets. Neutron capture effects on W isotopes, however, significantly hamper the application of this tool. In order to correct for neutron capture effects, Pt isotopes have emerged as a reliable in-situ neutron dosimeter. This study applies this method to IAB iron meteorites, in order to constrain the timing of metal segregation on the IAB parent body. The e 182 W values obtained for the IAB iron meteorites range from −3.61 ± 0.10 to −2.73 ± 0.09. Correlating e i Pt with e 182 W data yields a pre-neutron capture e 182 W of −2.90 ± 0.06. This corresponds to a metal–silicate separation age of 6.0 ± 0.8 Ma after CAI for the IAB parent body, and is interpreted to represent a body-wide melting event. Later, between 10 and 14 Ma after CAI, an impact led to a catastrophic break-up and subsequent reassembly of the parent body. Thermal models of the interior evolution that are consistent with these estimates suggest that the IAB parent body underwent metal–silicate separation as a result of internal heating by short-lived radionuclides and accreted at around 1.4 ± 0.1 Ma after CAIs with a radius of greater than 60 km.