ScienceWeek

EARTH SCIENCE: ON THE ORIGIN OF THE EARTH

The following points are made by Alex N. Halliday (Nature 2004 427:505):

1) It has been proposed on the basis of new W isotopic measurements for chondrites(1-3) that the Earth formed slightly faster than was previously thought because the original W isotopic data for carbonaceous chondrites are now known to be inaccurate by 150-200 ppm. The mean life for accretion, tau, is the inverse of the time constant for exponentially decreasing growth and corresponds to the time taken for 63% of the planet to accrete(4,5). A value of 11 Myr is obtained with continuous core formation modelling using the revised Solar System parameters(1). This new "fast" result contrasts with the various values of 15 to 40 Myr defined by Pb isotopic data using the same models(5). These calculations of tau assume total equilibration of incoming metal and silicate with the bulk silicate Earth (BSE) and no change in parent/daughter ratio in the total and silicate Earth during accretion.

2) Of particular interest is whether accreting material really equilibrated isotopically with the silicate portion of the Earth -- an essential tenet of accretion rate models. Some accretion and core formation simulations provide evidence against this. If the incoming metallic core totally fragments into very small droplets upon impact because of Rayleigh-Taylor instabilities, say, then equilibration could be relatively efficient. If, however, some of the incoming core material forms large dense masses, then equilibration with silicate before core coagulation seems unlikely. A second issue is whether moderately volatile elements were lost during accretion. Much of the depletion in volatile elements in the terrestrial planets is thought to have occurred in the early solar nebula. Whether the additional dramatic differences between the Earth, Moon and Mars are the product of later losses during accretion has been less clear. Because Pb is moderately volatile, late changes in budgets will affect U-Pb but not Hf-W chronology. Lastly, changes in oxidation state in protoplanetary mantles can be deduced from time-integrated Hf/W ratios because of the change in core-mantle W partitioning. Therefore, W isotopes offer a fingerprint of protoplanetary environments, analogously to the recent modelling of Sr isotopes.

3) In summary: The degree to which efficient mixing of new material or losses of earlier accreted material to space characterize the growth of Earth-like planets is poorly constrained and probably changed with time. These processes can be studied by parallel modelling of data from different radiogenic isotope systems. The tungsten isotope composition of the silicate Earth yields a model timescale for accretion that is faster than current estimates based on terrestrial lead and xenon isotope data and strontium, tungsten, and lead data for lunar samples. A probable explanation for this is that impacting core material did not always mix efficiently with the silicate portions of the Earth before being added to the Earth's core. Furthermore, tungsten and strontium isotope compositions of lunar samples provide evidence that the Moon-forming impacting protoplanet Theia was probably more like Mars, with a volatile-rich, oxidized mantle. Impact-driven erosion was probably a significant contributor to the variations in moderately volatile element abundance and oxidation found among the terrestrial planets.

References (abridged):

1. Yin, Q. Z. et al. A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites. Nature 418, 949-952 (2002)

2. Kleine, T., Moenker, C., Mezger, K. & Palme, H. Rapid accretion and early core formation on asteroids and the terrestrial planets from Hf-W chronometry. Nature 418, 952-955 (2002)

3. Schoenberg, R., Kamber, B. S., Collerson, K. D. & Eugster, O. New W isotope evidence for rapid terrestrial accretion and very early core formation. Geochim. Cosmochim. Acta 66, 3151-3160 (2002)

4. Jacobsen, S. B. & Harper, C. L. Jr in Earth Processes: Reading the Isotope Code (eds Basu, A. & Hart, S.) 47-74 (AGU, Washington DC, 1996)

5. Halliday, A. N. Terrestrial accretion rates and the origin of the Moon. Earth Planet. Sci. Lett. 176, 17-30 (2000)

Nature http://www.nature.com/nature

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GEOCHEMISTRY: THE AGE OF PLANET EARTH

The following points are made by Stein B. Jacobsen (Science 2003 300:1513):

1) Recent reports on the tungsten (W) isotope composition of meteorites have led to a completely revised time scale for the formation of the terrestrial planets. The results show that most of planet Earth had formed within ~10 million years after the formation of the Solar System some 4567 million years ago (when the first solid grains formed in the solar nebula). The Moon-forming event happened ~30 million years after Solar System formation, when Earth was fully grown.

2) The decay of the hafnium isotope 182Hf (with a half-life of 9 million years) into 182W is the best "clock" we have for tracing the formation of terrestrial planets during the first 50 million years of Solar System history. The behavior of these elements during metal-silicate separation, which occurs during the formation of planetary cores, is well understood.

3) Hafnium is a lithophile element (it has a strong affinity for silicate liquid) and stays entirely in the silicate mantle (and crust) of the planet. Hence, the mantle is where radioactive decay of 182Hf to 182W occurs. In contrast, tungsten is siderophilic (it has a strong affinity for iron melt), and about 90 to 95% of it is partitioned into the metal when metal and silicate separate in the core-forming process. After 50 million years, the Hf-W chronometer is a dead clock because almost all 182Hf has decayed, but for the first 50 million years of solar system history, it is ideal for tracking a planet's growth.

4) In the earliest work on this chronometer, it was found that the Solar System's initial 182W/183W value was about 3 to 4 parts in 10,000 lower than the present terrestrial value, and inferred a relatively short time scale for the formation of Earth. This short time scale was challenged by Lee and Halliday (1996), who reported that Earth and chondritic meteorites have essentially identical 182W/183W values to within 20 parts per million --indicating that Earth formed relatively late, after the decay of 182Hf (when the Hf-W clock was dead). They reported an age of core formation within Earth corresponding to 60 +/- 10 million years after Solar System formation. This age has been widely cited. However, because the clock was dead by this time, it should have been reported as any time between 50 million years after Solar System formation and the present.

Science http://www.sciencemag.org

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AGE AND ORIGIN OF EARTH'S MOON

The most widely accepted theory for the origin of the Earth's Moon is that during the late stages of the Earth's accretion an impact with another planet at least the size of Mars occurred, and the impact generated both the hot debris that formed the Moon and the angular momentum of the Earth-Moon system.

In geology, the mantle of a planet or moon is the layer that lies between the crust and the core. Chondrites are a type of stony meteorite consisting of an agglomeration of millimeter-sized globules (chondrules) that are thought to be unchanged since the original condensation out of the nebula from which the sun and solar system formed, and "chondritic" is the term used to describe a rock composition similar to that of chondrites, which implies an age of 4.2 to 4.5 billion years.

The term "radiogenic", on the other hand, is used to describe a rock composition apparently resulting from varying isotope decays, and the oldest radiogenic compositions on Earth have been dated at 3.6 to 3.8 billion years.

A hafnium-tungsten chronometer is not an actual instrument but a method of radiometric age determination using the isotope ratios of the elements hafnium and tungsten. Hafnium is lithophilic (silicate-loving), which means it tends to associate with chondritic materials, while tungsten is siderophilic (metal-loving), which means it tends to associate with metal cores, and using these differing affinities of these elements, one can attempt a construction of the age and origin of the Moon by analysis of Moon rock samples and comparisons with Earth rocks.

Lee et al (Science 1997 278:1098) report a study of the age and origin of the Moon with the hafnium-tungsten chronometric method. The tungsten isotopic compositions of 21 lunar samples were found to range from chondritic to slightly radiogenic. The authors suggest this heterogeneity is probably the result of late radioactive decay within the Moon itself, and that the Moon formed 4.52 to 4.50 billion years ago and its mantle has since remained poorly mixed.

Science http://www.sciencemag.org

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