Tungsten Isotopes, the Initial 182Hf/180Hf of the Solar System and the Origin of Enstatite Chondrites

Abstract

Hafnium (Hf) and tungsten (W) are both refractory elements whose relative proportions are strongly fractionated between silicate and metal. 182Hf decays with a half-life of 9 Myr to lS2w such that the W isotopic composition will be a function of the initial abundance of 182Hf in the early solar system and the timing of Hf/W fractionation, hence that of silicatemetal segregation (Lee and Halliday, 1995, 1996, 1997; Halliday et al., 1996; Lee et al., 1997). Previous studies have shown that iron meteorites and metal separates of ordinary chondrites contain W that is less radiogenic than chondritic, and this has been attributed to early metal segregation within their respective parent bodies during the lifetime of 182Hf (Lee and Halliday, 1995, 1996). In contrast, basaltic achondrites such as eucrites and SNC meteorites exhibit clear evidence of chondritic to radiogenic W, indicating rapid accretion, differentiation and core formation on asteroid 4 Vesta and Mars (Lee and Halliday, 1997). Despite a high Hf/W, the silicate Earth yields a W isotopic composition identical to that of carbonaceous chondrites (Lee and Halliday, 1995, 1996; Lee et al. , 1997), implying that the Earth's core did not form until = 50 Myr after the formation of the solar system, unless Earth's accretion was slower than generally modelled (Halliday e t al. , 1996). More interestingly, the Moon yields chondritic to slightly radiogenic W among various types of lunar samples (Lee et al. , 1997). This provides critical constraints regarding the age and origin of the Moon (Lee et al., 1997). Despite all the recent advances that have been made on the Hf-W system, the determination of the initial abundance of ~82Hf in the solar system was still hindered by analytical difficulties. Consequently, it has been estimated from the differences in W isotopic composition between the carbonaceous chondrites and the least radiogenic W yet measured (Lee and Halliday, 1995), which corresponds to a bulk solar system initial lS2HfflS~ of (2.4 _ 0.6) x 10 -4 (Lee and Halliday, 1996). Even though such a level of initial lS2Hf/lS~ was favoured by both experimental (Lee and Halliday, 1996) and theoretical (Wasserburg e t al. , 1996) considerations, it is critical to establish whether internal isochrons can be resolved in chondritic materials. Such a direct confirmation for the former presence of 182Hf in the solar system will not only serve as a cornerstone for future Hf-W studies, but also offer more precise constraints on the timing of planetary differentiation and astrophysical consideration for the production of r-process nuclides. Two ordinary chondrites, Forest Vale (H4) and Richardton (H5), and two enstatite chondrites, Indarch (EH4) and ALHA81021 (EL6), have been studied to test if intemal isochrons can be resolved. We have analysed only three fractions, metal, bulk silicates and whole rock in this preliminary study. Prior to crushing, each sample was leached with distilled HC1 (-1 N) to remove potential contamination from cutting. All the samples were crushed with an aluminium oxide mortar in a flow of filtered laminar air. A hand magnet was used to separate metal from silicates, and both were cleaned in an ultrasonic bath of distilled ethanol. The W isotopic data of metal separates from Forest Vale and Richardton have been previously reported (Lee and Halliday, 1996). However, to avoid systematic variations, the metal separate of Forest Vale was re-analysed. The result is consistent with the previous study. In addition to the bulk metal measurement, a portion of the metal separates from Forest Vale, Indarch and ALHA81021 were treated first with hot 6 N HCI until most of the metals were dissolved. The solution was then separated from the residue, which was undergoing additional dissolution steps. In general, a positive correlation is observed in each of the samples in a plot of Hf/W vs. W isotopic composition, phases with higher Hf/W exhibiting more radiogenic W. The best-fit line regressed through all the data of Forest Vale also intercepts the data of carbonaceous chondrites Murchison and Allende (Lee and Halliday, 1996), and corresponds to a slope of (1.87 _+ 0.16) x 10 -4 and an initial lS2w/184W of 0.86471. Similarly, the best-fit line regressed through the data of Richardton also