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.
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Halliday, A.N. HF-W Chronometry and Inner Solar System Accretion Rates. Space Science Reviews 92, 355–370 (2000). https://doi.org/10.1023/A:1005280220751
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DOI: https://doi.org/10.1023/A:1005280220751