182Hf-182W Chronometry and the Evolution of
Early Solar System

Der-Chuen Lee Dept. of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1063, USA


Alex N. Halliday Dept. of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1063, USA

182Hf decays, with a half life of 9 myrs, to 182W and is expected to have been present in the early solar system. High precision W isotopic analyses have been performed on numerous samples, including carbonaceous chondrites, iron meteorites, lunar mare basalt and terrestrial samples, using multiple collector ICP mass spectrometry (MC-ICPMS). The carbonaceous chondrites (Allende and Murchison), the lunar mare basalt, the USGS standard AGV-1 and the NIST-3163 W standard all yield identical W isotopic compositions (Lee and Halliday, 1996). In contrast, four iron meteorites, Arispe and Goose Lake (IA), Coya Norte (IIA) and Kendall County (Anomalous), display clear deficits in 182W, ranging from -4.5±1.0 for Arispe to -2.8±0.5 ew for Goose Lake, relative to NIST-3163. These results are in excellent agreement with the NTIMS data reported for the iron meteorite Toluca (Jacobsen and Harper, 1996). The data suggest that the observed 182W deficit relative to terrestrial and chondritic W is probably a common feature for iron meteorites. Since the modification of W isotopes by neutron capture during space exposure is no more than 1 e unit (Masarik and Reedy, 1994), the observed 182W is most likely the consequence of metal/
silicate differentiation during the lifespan of 182Hf, since W is moderately siderophile while Hf is strongly lithophile. Moreover, the chondritic W isotopic composition of bulk silicate Earth provides compelling evidence that the Earth's core did not form during the lifespan of 182Hf. Terrestrial core formation must, therefore, postdate the age of the iron meteorite Arispe by at least 60 myrs. In addition, the
chondritic W isotopic composition of lunar mare basalt suggests that the Moon must also postdate the formation of iron meteorite Arispe by at least 60 myrs.

The variation of W isotopic composition observed in each iron meteorite class, as well as within the same class (Arispe vs Goose Lake), most likely reflects the different ages for each iron meteorites that had segregated from its chondritic (with respect to Hf/W) parent body. The Hf/W ratio in iron meteorite is sufficiently low that the observed W isotopic composition of each iron meteorite should remain unchanged since its segregation. The differences in W data among the iron meteorites, therefore, indicate an apparent time interval of ~2.8 myrs between Arispe and Coya Norte 9the same for Kendall County), and ~6.5 myrs between Arispe and Goose Lake. These results are comparable to those estimated using Pd-Ag (Chen and Wasserburg, 1990) and Mn-Cr systems (Lugmair and MacIsaac, 1995).

The data present here correspond to a bulk solar system initial (182Hf/180Hf)BSSI of (2.61±0.13) x 10-4, estimated from the differences of W isotope data between the carbonaceous chondrites and Arispe (largest 182W deficit of -4.5±1.0 ew). This is apparently the lower limit of (182Hf/180Hf)BSSI, even though this lower limit is still ~2 orders of magnitude greater than predicted in an AGB star model (Wasserburg et al., 1994). This (182Hf/180Hf)BSSI is also more abundant than any other short-lived nuclides of comparable half-life (Harper, 1996). The overall data are consistent with multiple nucleosynthetic events, or that there was greater input from a supernova that was significantly earlier than normally considered responsible for the "late spike".

Further experiments are underway to establish an accurate bulk solar system initial (182Hf/180Hf)BSSI, and to study the silicate fraction of differentiated meteorites that are predicted to have excess 182Hf.


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