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A Short Timescale for Terrestrial Planet Formation from Hf–W Chronometry of Meteorites

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A Short Timescale for Terrestrial Planet Formation from Hf–W Chronometry of Meteorites letters to nature .............................................................. to 2 Myr of the age of Allende CAIs (the oldest solid objects of the A short timescale for terrestrial Solar System). The chondrite isochron gives an initial 182Hf/180Hf ratio of ð1:00 ^ 0:08Þ £ 1024 and an initial 182W/183W ratio corre- ¼ ^ planet formation from Hf–W sponding to 1w 23:45 0:25 (1w is defined in Table 1). The lowest initial 1w we have obtained based on metals from chondrites chronometry of meteorites and iron meteorites is about 23.5 ^ 0.5 (Table 1 and unpublished 182 180 24 data). Thus, the Hf/ Hf ratio of 1 £ 10 and the initial 1w of Qingzhu Yin*, S. B. Jacobsen*, K. Yamashita*†, J. Blichert-Toft‡, 23.5 must be very close to their respective initial solar values. This 182 180 P. Te´louk‡ & F. Albare`de‡ initial Hf/ Hf value is significantly lower than, and clearly inconsistent with, the commonly cited value of 2.75 £ 1024 (ref. * Department of Earth and Planetary Sciences, Harvard University, 18). In addition to obtaining a lower initial solar 182Hf abundance 20 Oxford Street, Cambridge, Massachusetts 02138, USA compared to the earlier work, one of the features of Fig. 1 is that ‡ Laboratoire des Sciences de la Terre, Ecole Normale Supe´rieure de Lyon, 46, three repeat measurements of Allende whole rock and the Dalgety Alle´e d’Italie, 69364 Lyon Cedex 7, France Downs whole rock all plot consistently ,2 1 units below the ............................................................................................................................................................................. Hf/W ¼ Determining the chronology for the assembly of planetary bodies terrestrial standard and have a chondritic Hf/W ratio (f 0; f Hf/W is defined in Table 1). This is inconsistent with previously in the early Solar System is essential for a complete under- 7,8 standing of star- and planet-formation processes. Various radio- measured whole-rock samples of Allende and Murchison that nuclide chronometers (applied to meteorites) have been used to yielded 1w < 0 (discrepancies with published chondrite data are determine that basaltic lava flows on the surface of the asteroid further discussed in Fig. 1 legend). Both ordinary chondrite iso- chrons are consistent with a chondritic uniform reservoir (CHUR) Vesta formed within 3 million years (3 Myr) of the origin of the Hf/W 1–3 value of 1w ¼ 21:9 ^ 0:2 and f ¼ 0. This means that the BSE Solar System . Such rapid formation is broadly consistent with 182 183 astronomical observations of young stellar objects, which suggest with its high Hf/W ratio has a radiogenic W/ W signature that formation of planetary systems occurs within a few million about 2 1 units higher than the chondritic value. A comparison of years after star formation4,5. Some hafnium–tungsten isotope results based on the old and the new data are given in Table 2. data, however, require that Vesta formed later6 (,16 Myr after Our Juvinas (eucrite) measurement plots on the eucrite isochron 182 180 25 the formation of the Solar System) and that the formation of the of ref. 6 with Hf= Hf ¼ 7:96 £ 10 : The eucrites post-date our 7–10 14504 , terrestrial planets took a much longer time (62214 Myr). ordinary chondrites by only 3 Myr. In contrast, the Hf–Wdata for Here we report measurements of tungsten isotope compositions eucrites6, when compared to the currently accepted high initial solar 182 180 and hafnium–tungsten ratios of several meteorites. Our Hf/ Hf value7,8,18,19, indicate a late parent body (Vesta) differ- measurements indicate that, contrary to previous results7–10, entiation at ,16 Myr after the formation of the Solar System. This is the bulk of metal–silicate separation in the Solar System was completed within <30 Myr. These results are completely consist- ent with other evidence for rapid planetary formation1–5, and are also in agreement with dynamic accretion models11–13 that pre- Table 1 Hf–W isotope data for meteorites Hf/W ^ dict a relatively short time (,10 Myr) for the main growth stage Sample Hf Hf/W f 1W 2j (p.p.b.) of terrestrial planet formation. ............................................................................................................................................................................. It has been argued that the rate of terrestrial core formation was Allende, WR no. 1 187.5 1.096 20.0236 21.93 0.28 limited by accretion, that it started very early, and that it was largely Allende, WR no. 2 187.7 1.098 20.0226 22.03 0.82 completed within the first 10–20 Myr or less of Earth history14,15. Allende, WR no. 3 210.2 1.229 0.0946 22.02 0.36 182 182 Allende, CAI (A44A) 5,639 3.477 2.095 1.16 0.55 The decay of now extinct Hf (half-life, 9 Myr) to W is an ideal Murchison, WR 157.1 0.878 20.218 22.62 0.33 chronometer for tracing this process, because Hf is retained in the Dhurmsala, silicate 185.2 6.717 4.982 þ5.81 0.67 Dhurmsala, metal 8.25 0.0382 20.966 23.07 0.35 silicate mantle while W is largely partitioned into the core during Dhurmsala, WR 181.1 1.395 0.242 21.27 0.51 core segregation6–10,14–20. However, it is now widely accepted that the Dalgety Downs, silicate 177.0 2.494 1.221 þ0.18 0.69 reports of almost identical 182W/183W ratios of chondritic meteor- Dalgety Downs, metal 6.09 0.00946 20.992 23.50 0.23 Dalgety Downs, WR 159.7 1.125 0.001 21.85 0.46 ites and the bulk silicate Earth (BSE) imply that core segregation in Juvinas, WR 1,268 10.48 8.332 þ9.84 0.56 the Earth took place about 60 Myr after the beginning of the Solar Toluca, MC-ICPMS (Lyon), N ¼ 1 23.31 0.31 System7,8,10 that is, when 182Hf was essentially extinct. As core Toluca, MC-ICPMS (Harvard), N ¼ 8 23.16 0.15 — Toluca, N-TIMS (Harvard), N ¼ 7 23.07 0.39 segregation probably occurred in an early terrestrial magma ocean, ............................................................................................................................................................................. Data representation and analytical procedures. Here f Hf=W ¼ 180 183 180 183 180 183 the observed solar He and Ne components of the Earth’s mantle also ð Hf= WÞsample=ð Hf= WÞCHUR 2 1; where ( Hf/ W)CHUR ¼ 2.836 and the weight ratio 182 183 182 183 4 require that most of the planetary accretion occurred before the gas (Hf/W)CHUR ¼ 1.123 (refs 14, 15). Also 1w ¼½ð W= WÞsample=ð W= WÞstandard 2 1 £ 10 ; 182 183 ^ 21,22 using ( W/ W)standard ¼ 1.85130 4 (ref. 16) obtained for our terrestrial W standard (Harvard- of the solar nebula had dissipated . The timescale for this W1). WR, ‘whole rock’ sample. The samples were all ,1–2 g, except for the CAI (,0.2 g). In dissipation is ,3 Myr after the beginning of the Solar System4,23, selecting samples, we specifically avoided using un-equilibrated chondrites to construct internal isochrons, especially petrologic types between 3.0 and 3.9. This is because there is good evidence and highlights the problem of inconsistent chronologies for the that metamorphism resulted in a net transfer of W from oxidized phases to metal before the early Earth. complete consumption of reductants (carbides, organics, and so on), which may complicate the interpretation of the resulting Hf–W systematics. This process is considered complete for type 4 and Our new data (Table 1 and ref. 24) for ordinary chondrites higher29. Therefore, an L4 chondrite, and an LL6 chondrite were selected for our work. The LL6 Dalgety Downs and Dhurmsala yield a well defined ‘fossil isochron’ meteorite Dhurmsala was chosen for this study because its silicate fraction yielded a very high Hf/W ratio, which helps to define a precise isochron. Hf and W were separated at Harvard by anion array (Fig. 1). Four separate whole-rock samples of carbonaceous exchange separation chemistry and their concentrations were determined precisely and accurately chondrites (three from Allende and one from Murchison) and a (to within ^0.5%) using calibrated 180Hf and 184W isotopic tracers. Total procedural blanks for Hf and W are negligible (13–15 pg and ,0.19 ng, respectively). For the BCR-1 standard we obtained calcium-aluminium-rich inclusion (CAI) from the Allende meteor- Hf ¼ 4,995 ^ 24 p.p.b. and W ¼ 412 ^ 3 p.p.b., identical within error to accepted values for this ite plot on this isochron. Although the absolute ages of the two standard, demonstrating the accuracy of our Hf–W ratios (analysed by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) and N-TIMS at Harvard and MC-ICP-MS in ordinary chondrites are not well known independently, the fact that Lyon). The W isotope data were corrected for instrumental mass fractionation using an exponential 184 183 186 183 law with W/ W ¼ 2.14078 and/or W/ W ¼ 1.98613 ^ 4. The 1w values in this table were a CAI from the Allende meteorite plots within error of this isochron obtained by MC-ICP-MS in Lyon, except for most of the Toluca measurements. The accuracy of requires the Hf–Wage of these ordinary chondrites to be within ,1 our measurements of the 182W/183W ratio in meteorite samples relative to our terrestrial W standard (the 1 w value) was verified by comparing data for this meteorite (N ¼ number of repeat measurements) obtained using three different mass spectrometers and two different techniques † Present address: Department of Earth & Planetary Sciences, University of Kobe, 1-1 Rokkodai-Cho, (MC-ICP-MS vs N-TIMS): (1) MC-ICP-MS using both the Micromass IsoProbe at Harvard as well Nada-Ku, Kobe, 657-8501 Japan.
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