HF-W Chronometry and Inner Solar System Accretion Rates |
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Authors: | Alex N Halliday |
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Institution: | (1) Department of Earth Sciences, Institute for Isotope Geology and Mineral Resources, ETH Zentrum, NO C61, CH-8049 Zürich, Switzerland |
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Abstract: | Models for the mechanisms of accretion of the terrestrial planets are re-examined using the experimental technique of high-precision
isotope ratio mass spectrometry of tungsten (W). The decay of 182Hf to 182W (via 182Ta) provides a new kind of radiometric chronometer of planet formation processes. Hafnium and W, the parent and daughter trace
elements, are highly refractory; however, Hf is lithophile and strongly partitioned into the silicate portion of a planet,
whereas W is moderately siderophile and preferentially partitioned into a coexisting metallic phase. More than 90% of terrestrial
W has gone into the Earth's core during its formation. The residual silicate portion, the Earth's primitive mantle, has a
Hf/W ratio in the range 10−40, an order of magnitude higher than chondritic (∼1.3). Tungsten isotopic data for the Earth and
the Moon suggest that we can date a major event of planet formation: The Moon formed about 50 Myrs after the start of the
solar system, providing strong support for the Giant Impact Theory of lunar origin. Recent simulations of this event imply
that the Earth was probably only half formed at the time. From this we can deduce the planetary accretion rate. Tungsten isotope
data for Mars provide evidence of a much shorter accretion interval, perhaps as little as 10 Myrs, but the rates for the Earth
over the same time interval could have been comparable. The large W isotopic heterogeneities on Mars could only have been
produced within the first 30 Myrs of the solar system. Large-scale mixing, e.g. from convective overturn, as is thought to drive the Earth's plates, must be absent from Mars.
Limitations of the method such as 1) cosmogenic 182Ta effects on lunar samples, 2) incomplete mixing of debris to cause W isotope heterogeneity on the Moon, and 3) initial 182Hf/180Hf heterogeneities of the early solar system are critically discussed.
This revised version was published online in June 2006 with corrections to the Cover Date. |
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