Thorsten Kleine

Rapid accretion and early core formation on asteroids and the terrestrial planets from Hf-W chronometry

The timescales and mechanisms for the formation and chemical differentiation of the planets can be quantified using the radioactive decay of short-lived isotopes. Of these, the 182Hf-to-182W decay is ideally suited for dating core formation in planetary bodies. In an earlier study, the W isotope composition of the Earths mantle was used to infer that core formation was late (≥60 million years after the beginning of the Solar System) and that accretion was a protracted process. The correct interpretation of Hf–W data depends, however, on accurate knowledge of the initial abundance of 182Hf in the Solar System and the W isotope composition of chondritic meteorites. Here we report Hf–W data for carbonaceous and H chondrite meteorites that lead to timescales of accretion and core formation significantly different from those calculated previously. The revised ages for Vesta, Mars and Earth indicate rapid accretion, and show that the timescale for core formation decreases with decreasing size of the planet. We conclude that core formation in the terrestrial planets and the formation of the Moon must have occurred during the first ∼30 million years of the life of the Solar System.

Late formation and prolonged differentiation of the Moon inferred from W isotopes in lunar metals.

The Moon is thought to have formed from debris ejected by a giant impact with the early ‘proto’-Earth and, as a result of the high energies involved, the Moon would have melted to form a magma ocean. The timescales for formation and solidification of the Moon can be quantified by using 182Hf–182W and 146Sm–142Nd chronometry, but these methods have yielded contradicting results. In earlier studies, 182W anomalies in lunar rocks were attributed to decay of 182Hf within the lunar mantle and were used to infer that the Moon solidified within the first ∼60 million years of the Solar System. However, the dominant 182W component in most lunar rocks reflects cosmogenic production mainly by neutron capture of 181Ta during cosmic-ray exposure of the lunar surface, compromising a reliable interpretation in terms of 182Hf–182W chronometry. Here we present tungsten isotope data for lunar metals that do not contain any measurable Ta-derived 182W. All metals have identical 182W/184W ratios, indicating that the lunar magma ocean did not crystallize within the first ∼60 Myr of the Solar System, which is no longer inconsistent with Sm–Nd chronometry. Our new data reveal that the lunar and terrestrial mantles have identical 182W/184W. This, in conjunction with 147Sm–143Nd ages for the oldest lunar rocks, constrains the age of the Moon and Earth to  Myr after formation of the Solar System. The identical 182W/184W ratios of the lunar and terrestrial mantles require either that the Moon is derived mainly from terrestrial material or that tungsten isotopes in the Moon and Earth’s mantle equilibrated in the aftermath of the giant impact, as has been proposed to account for identical oxygen isotope compositions of the Earth and Moon.

Hf-W Chronometry of Lunar Metals and the Age and Early Differentiation of the Moon

The use of hafnium-tungsten chronometry to date the Moon is hampered by cosmogenic tungsten-182 production mainly by neutron capture of tantalum-181 at the lunar surface. We report tungsten isotope data for lunar metals, which contain no 181Ta-derived cosmogenic 182W. The data reveal differences in indigenous 182W/184W of lunar mantle reservoirs, indicating crystallization of the lunar magma ocean 4.527 ± 0.010 billion years ago. This age is consistent with the giant impact hypothesis and defines the completion of the major stage of Earths accretion.

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.

NUCLEOSYNTHETIC TUNGSTEN ISOTOPE ANOMALIES IN ACID LEACHATES OF THE MURCHISON CHONDRITE: IMPLICATIONS FOR HAFNIUM-TUNGSTEN CHRONOMETRY

Progressive dissolution of the Murchison carbonaceous chondrite with acids of increasing strengths reveals large internal W isotope variations that reflect a heterogeneous distribution of s- and r-process W isotopes among the components of primitive chondrites. At least two distinct carriers of nucleosynthetic W isotope anomalies must be present, which were produced in different nucleosynthetic environments. The co-variation of 182W/184W and 183W/184W in the leachates follows a linear trend that is consistent with a mixing line between terrestrial W and a presumed s-process-enriched component. The composition of the s-enriched component agrees reasonably well with that predicted by the stellar model of s-process nucleosynthesis. The co-variation of 182W/184W and 183W/184W in the leachates provides a means for correcting the measured 182W/184W and 182W/183W of Ca-Al-rich inclusions (CAI) for nucleosynthetic anomalies using the isotopic variations in 183W/184W. This new correction procedure is different from that used previously, and results in a downward shift of the initial ?182W of CAI to ?3.51 ? 0.10 (where ?182W is the variation in 0.01% of the 182W/183W ratio relative to Earths mantle). This revision leads to Hf-W model ages of core formation in iron meteorite parent bodies that are ~2?Myr younger than previously calculated. The revised Hf-W model ages are consistent with CAI being the oldest solids formed in the solar system, and indicate that core formation in some planetesimals occurred within ~2?Myr of the beginning of the solar system.

A nucleosynthetic origin for the Earth’s anomalous 142Nd composition

A long-standing paradigm assumes that the chemical and isotopic compositions of many elements in the bulk silicate Earth are the same as in chondrites. However, the accessible Earth has a greater 142Nd/144Nd ratio than do chondrites. Because 142Nd is the decay product of the now-extinct 146Sm (which has a half-life of 103 million years), this 142Nd difference seems to require a higher-than-chondritic Sm/Nd ratio for the accessible Earth. This must have been acquired during global silicate differentiation within the first 30 million years of Solar System formation and implies the formation of a complementary 142Nd-depleted reservoir that either is hidden in the deep Earth, or lost to space by impact erosion. Whether this complementary reservoir existed, and whether or not it has been lost from Earth, is a matter of debate, and has implications for determining the bulk composition of Earth, its heat content and structure, as well as for constraining the modes and timescales of its geodynamical evolution. Here we show that, compared with chondrites, Earth’s precursor bodies were enriched in neodymium that was produced by the slow neutron capture process (s-process) of nucleosynthesis. This s-process excess leads to higher 142Nd/144Nd ratios; after correction for this effect, the 142Nd/144Nd ratios of chondrites and the accessible Earth are indistinguishable within five parts per million. The 142Nd offset between the accessible silicate Earth and chondrites therefore reflects a higher proportion of s-process neodymium in the Earth, and not early differentiation processes. As such, our results obviate the need for hidden-reservoir or super-chondritic Earth models and imply a chondritic Sm/Nd ratio for the bulk Earth. Although chondrites formed at greater heliocentric distances and contain a different mix of presolar components than Earth, they nevertheless are suitable proxies for Earth’s bulk chemical composition.

Crustal Evolution along the Early Ordovician Proto‐Andean Margin of Gondwana: Trace Element and Isotope Evidence from the Complejo Igneo Pocitos (Northwest Argentina)

The Paleozoic geodynamic evolution of the proto‐Andean margin of Gondwana was characterized by extensive subduction processes associated with the docking of several terranes. The major and trace element as well as Sr‐Nd‐Pb isotope composition of plutonic rocks from the Complejo Igneo Pocitos (CIP) are used here to constrain the processes of magma genesis in the Early Ordovician continental arc along the proto‐Andean margin of Gondwana in northern Chile and northwestern Argentina. U‐Pb sphene ages of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Tungsten isotopic constraints on the age and origin of chondrules

476\pm 2