Mario Fischer-Gödde

Protracted core formation and rapid accretion of protoplanets.

The chronology of planetary embryos Protoplanets, or early planetary embryos such as iron meteorite parent bodies, formed in the early protoplanetary disk from dust, debris, and planetesimals. Defining the precise chronology of accretion and differentiation—including core formation—of these planetary embryos will aid in a richer understanding of the chemical evolution of the solar system. Through high-precision tungsten isotope measurements, Kruijer et al. show that the timing of accretion and core formation for iron meteorite groups falls within 0.6 to 2 million years of the age of the solar system (see the Perspective by Elliott). Differences of timing within this group are probably a function of volatile contents of the parent bodies or spatial and chemical heterogeneity within the protoplanetary disk. Science, this issue p. 1150; see also p. 1086 Tungsten isotopes record the chronology of accretion and core formation of iron meteorite groups. [Also see Perspective by Elliott] Understanding core formation in meteorite parent bodies is critical for constraining the fundamental processes of protoplanet accretion and differentiation within the solar protoplanetary disk. We report variations of 5 to 20 parts per million in 182W, resulting from the decay of now-extinct 182Hf, among five magmatic iron meteorite groups. These 182W variations indicate that core formation occurred over an interval of ~1 million years and may have involved an early segregation of Fe-FeS and a later segregation of Fe melts. Despite this protracted interval of core formation, the iron meteorite parent bodies probably accreted concurrently ~0.1 to 0.3 million years after the formation of Ca-Al–rich inclusions. Variations in volatile contents among these bodies, therefore, did not result from accretion at different times from an incompletely condensed solar nebula but must reflect local processes within the nebula.

Ruthenium isotopic evidence for an inner Solar System origin of the late veneer

The excess of highly siderophile elements in the Earth’s mantle is thought to reflect the addition of primitive meteoritic material after core formation ceased. This ‘late veneer’ either comprises material remaining in the terrestrial planet region after the main stages of the Earth’s accretion, or derives from more distant asteroidal or cometary sources. Distinguishing between these disparate origins is important because a late veneer consisting of carbonaceous chondrite-like asteroids or comets could be the principal source of the Earth’s volatiles and water. Until now, however, a ‘genetic’ link between the late veneer and such volatile-rich materials has not been established or ruled out. Such genetic links can be determined using ruthenium (Ru) isotopes, because the Ru in the Earth’s mantle predominantly derives from the late veneer, and because meteorites exhibit Ru isotope variations arising from the heterogeneous distribution of stellar-derived dust. Although Ru isotopic data and the correlation of Ru and molybdenum (Mo) isotope anomalies in meteorites were previously used to argue that the late veneer derives from the same type of inner Solar System material as do Earth’s main building blocks, the Ru isotopic composition of carbonaceous chondrites has not been determined sufficiently well to rule them out as a source of the late veneer. Here we show that all chondrites, including carbonaceous chondrites, have Ru isotopic compositions distinct from that of the Earth’s mantle. The Ru isotope anomalies increase from enstatite to ordinary to carbonaceous chondrites, demonstrating that material formed at greater heliocentric distance contains larger Ru isotope anomalies. Therefore, these data refute an outer Solar System origin for the late veneer and imply that the late veneer was not the primary source of volatiles and water on the Earth.

A rapid and efficient ion-exchange chromatography for Lu–Hf, Sm–Nd, and Rb–Sr geochronology and the routine isotope analysis of sub-ng amounts of Hf by MC-ICP-MS

The development and improvement of MC-ICP-MS instruments have fueled the growth of Lu–Hf geochronology over the last two decades, but some limitations remain. Here, we present improvements in chemical separation and mass spectrometry that allow accurate and precise measurements of 176Hf/177Hf and 176Lu/177Hf in high-Lu/Hf samples (e.g., garnet and apatite), as well as for samples containing sub-nanogram quantities of Hf. When such samples are spiked, correcting for the isobaric interference of 176Lu on 176Hf is not always possible if the separation of Lu and Hf is insufficient. To improve the purification of Hf, the high field strength elements (HFSE, including Hf) are first separated from the rare earth elements (REE, including Lu) on a first-stage cation column modified after Patchett and Tatsumoto (Contrib. Mineral. Petrol., 1980, 75, 263–267). Hafnium is further purified on an Ln-Spec column adapted from the procedures of Munker et al. (Geochem., Geophys., Geosyst., 2001, DOI: 10.1029/2001gc000183) and Wimpenny et al. (Anal. Chem., 2013, 85, 11258–11264) typically resulting in Lu/Hf < 0.0001, Zr/Hf < 1, and Ti/Hf < 0.1. In addition, Sm–Nd and Rb–Sr separations can easily be added to the described two-stage ion-exchange procedure for Lu–Hf. The isotopic compositions are measured on a Thermo Scientific Neptune Plus MC-ICP-MS equipped with three 1012 Ω resistors. Multiple 176Hf/177Hf measurements of international reference rocks yield a precision of 5–20 ppm for solutions containing 40 ppb of Hf, and 50–180 ppm for 1 ppb solutions (=0.5 ng sample Hf 0.5 in ml). The routine analysis of sub-ng amounts of Hf will facilitate Lu–Hf dating of low-concentration samples.

Ruthenium stable isotope measurements by double spike MC-ICPMS

We developed a new technique for precise measurements of ruthenium (Ru) stable isotope compositions by multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS). Instrumental mass bias and potential isotope fractionation induced during the chemical purification of Ru were corrected using a 98Ru–101Ru double spike added prior to sample digestion. The separation and purification of Ru from natural samples is achieved by cation exchange chromatography followed by distillation of Ru as volatile oxides. A series of analytical tests demonstrates that this method results in very pure Ru cuts and accurate and reproducible Ru isotope measurements. The Ru isotope results are expressed relative to an Alfa Aesar™ Ru standard solution as the permil deviations of the 102Ru/99Ru ratio (δ102/99Ru). The external reproducibility (2 s.d.) of the entire analytical procedure (including sample digestion, chemical purification and isotope measurement) that has been determined using several doped terrestrial rock standards is 0.05‰ for δ102/99Ru. We obtained Ru stable isotope data for three different Ru standard solutions and five chromitite samples from two different localities in the Shetland Ophiolite Complex and from the Bushveld complex. All three standard solutions are isotopically distinct with δ102/99Ru values up to ca. 0.9‰. The different chromitite samples also show different Ru stable isotope compositions with δ102/99Ru values between ca. 0.2‰ and ca. 0.7‰. These natural Ru stable isotope variations can be readily resolved using the 98Ru–101Ru double spike method presented here. Overall, these data show that Ru stable isotopes hold promise as a tracer of a wide range of geochemical and cosmochemical processes.