Isotope Ratio Measurements Firm Up Knowledge of Earth's Formation
Physics Today
January 2003
New measurements on primitive meteorites suggest that Earth's core
formed earlier than was previously thought. -- Richard Fitzgerald
A complicated interplay of processes occurred 4.6 billion years ago
in the early stages of the Solar System as material from the initial
solar nebula condensed and collided to form aggregates,
planetesimals, and eventually planets. And on Earth as well as the
other terrestrial planets and the larger asteroids, heat from
accretion and from the decay of short-lived isotopes such as aluminum-
26 was sufficient to melt--at least partially--the assembled
material, which led to the segregation of the iron-rich core and the
silicate mantle.
Determining when Earth's core formed can provide constraints on
models of the planet's formation. Studying Earth's isotopic
composition can provide such constraints, but this approach has led
to conflicting answers.
Recently, three reports--by Ronny Schoenberg, Ken Collerson, and
coworkers at the University of Queensland and the University of
Bern;[1] by Qingzhu Yin, Stein Jacobsen, and colleagues at Harvard
University and the Ecole Normale Sup�rieure in Lyon, France;[2] and
by
Thorsten Kleine, Klaus Mezger, and coworkers at the University of
M�nster and the University of Cologne[3]--have independently
established a chronometry, based on careful measurements of tungsten
isotopic compositions, that appears to have settled the timescales.
The core of the matter
As the Solar System formed, according to prevalent models, Earth
accreted rapidly--over a period of a few million to a few tens of
millions of years.[4] But confirming such models requires resolving
what happened in the first few tens of millions of years in the 4.6-
billion-year-old Solar System.
The hafnium-tungsten (Hf-W) system is well suited for dating the
formation of Earth's core. One Hf isotope, 182Hf, decays to stable
182W with a halflife of 9 million years. Produced like other heavy
elements (including 182W) in supernova explosions, 182Hf was present
in the original solar nebula, but the initial supply of 182Hf has
long since decayed. Thus, its concentration can't be extrapolated
back, unlike the concentrations of uranium-235 and uranium-238--whose
decays to lead-207 and lead-206 form another chronometer of the early
Solar System. The erstwhile presence of 182Hf is revealed, however,
in the isotopic composition of W. Today's deviations from the average
Solar System ratio of 182W to other stable, nonradiogenic W isotopes
reflect variations in the Hf/W ratio in the earliest period of the
Solar System.
The chemistry of the Hf-W system also is vital to dating core
formation. Lithophile ("stone loving") Hf stayed in the silicate
mantle when the metallic core formed. Tungsten, in contrast, is
moderately siderophile ("iron loving"), preferentially dissolving in
the metallic core. Thus, following the core formation, the mantle had
a higher Hf/W ratio than the core. And so, if Earth's mantle and core
have different 182W abundances, mantle-core separation must have
occurred before the 182Hf vanished.
Tungsten samples are easily obtained from the silicate Earth--just
break open an incandescent light bulb--but not from the ferrous core.
Fortunately, there is another source of W samples that can be tapped:
meteorites. Carbonaceous chondrites are a class of primitive
meteorites that are widely thought to have the same chemical
composition (except for volatile elements such as hydrogen, helium,
and the alkalis) as the solar nebula from which Earth and the other
planets formed. Such meteorites can thus serve as proxies for
establishing a baseline from which to look for deviations in W
isotope ratios. (For more on what can be learned from meteorites, see
the article by Thomas Bernatowicz and Robert Walker in Physics Today,
December 1997, page 26.)
Measuring W isotope ratios in chondrites is challenging, however. The
assays require a sensitive mass spectrometer to detect differences of
less than one part in 104 in W isotope abudances, and the chondrites
themselves contain W only at the parts-per-billion level.
Furthermore, corrections must be made for instrument effects and for
background signals from other elements.
Following reports[5] in the mid-1990s by Jacobsen and Charles Harper
(Harvard) that iron meteorites--thought to be the cores of early
planetesimals--showed a deficiency in 182W, Der-Chuen Lee and Alex
Halliday (then at the University of Michigan) reported the first
measurements of the 182W abundance in carbonaceous chondrites; they
found essentially no difference from the composition of the silicate
Earth.[6] The apparent absence of any significant deviation indicated
that the fractionation of Hf and W due to core formation must have
occurred after the 182Hf was extinct. Lee and Halliday thus concluded
that, unless planetary accretion was a slow process occurring over
tens of millions of years, Earth's core must have formed at least 50
million years after the birth of the Solar System.
The three recent experimental efforts, which reexamine the Hf-W
chronometry based on new measurements of chondrites, have challenged
the late formation time. The figure shows the new results for the
abundance of 182W in various chondrite and terrestrial samples. These
results each reveal a clear difference in the 182W content, with the
chondrites having about 2 parts in 104 less than Earth's mantle and
crust.
At the 2002 Goldschmidt Conference, held last August in Davos,
Switzerland, Halliday, now at ETH Z�rich, reported finding
chondritic W concentrations that agree with the new sets of results,
rather than with his earlier measurements, and said he has reproduced
the M�nster results. Thus the question of the 182W abundance in
chondrites appears to be settled.
A new consistency
The new findings point to an earlier formation of Earth's core, when
there was still 182Hf left in the hafnium-enriched mantle to decay
and thereby increase the 182W fraction. Determining how much earlier
requires additional information: the relative abundances of Hf and W,
and the initial ratio of 182Hf to other Hf isotopes at the birth of
the Solar System.
Each team drew on existing estimates of 12-18 for the Hf/W ratio in
bulk silicate Earth. The M�nster group inferred an initial
182Hf/180Hf ratio of about 1.0 � 10-4 from W isotope measurements
of various phases in a well-dated chondrite that formed very soon
after the beginning of the Solar System. The Harvard group found the
same ratio from its study of chondritic meteorites. The Queensland
researchers obtained a value 50% higher based on other reported
measurements on iron meteorites, but found a value comparable to that
of the other groups when considering their own measurements on iron
meteorites.
Conclusions about the core formation date also rest on assumptions
regarding how the core formed. Models commonly assume that small
planetesimals formed early and had differentiated cores. As those
objects collided to form bigger ones, the accreting material was at
least partly molten, allowing some degree of equilibration between
the silicate and metal melts before the metal sank to Earth's core.
This equilibration is what the Hf-W studies date, but the extent of
equilibrium varies from model to model.
Putting all the isotope ratios together, and assuming fully
equilibrated element distribution between the mantle and core, the
three teams reported a consistent value of about 30 million years as
the latest time after the birth of the Solar System for Earth's core
to have formed in a single event. Such an earlier formation time is
in better agreement with other chronometries[7] and with models.
Arguing that Earth's core is more likely to have segregated
continuously as the planet grew, the Harvard group calculated 11
million years as the mean core formation time.
The new chronometry also has implications for other parts of the
Solar System. For example, the Moon, which has a chemical composition
similar to that of Earth's mantle, is commonly thought to have been
formed by a giant impact with Earth. Based on their data, the Harvard
and M�nster groups estimate that the Moon formed 25-33 million
years after the beginning of the Solar System, comparable--and
perhaps related--to the date they obtained for Earth's core
formation.
Richard Fitzgerald
References
1. R. Schoenberg, B. S. Kamber, K. D. Collerson, O. Eugster, Geochim.
Cosmochim. Acta 66, 3151 (2002).
2. Q. Yin, S. B. Jacobsen, K. Yamashita, J. Blichert-Toft, P.
T�louk, F. Albar�de, Nature 418, 949 (2002).
3. T. Kleine, C. M�nker, K. Mezger, H. Palme, Nature 418, 952
(2002).
4. See, for example, G. W. Wetherill in Origin of the Moon, W. K.
Hartmann, R. J. Phillips, G. J. Taylor, eds., Lunar & Planetary
Institute, Houston, Texas (1986), p. 519; J. E. Chambers, Icarus 152,
205 (2001).
5. C. L. Harper, S. B. Jacobsen, Geochim. Cosmochim. Acta 60, 1131
(1996).
6. D.-C. Lee, A. N. Halliday, Nature 378, 771 (1995); Science 274,
1876 (1996); Nature 388, 854 (1997).
7. See, for example, G. W. Lugmair, A. Shukolyukov, Geochim.
Cosmochim. Acta 62, 2863 (1998); G. Srinivasan, J. Goswami, N.
Bhandari, Science 284, 1348 (1999).
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