Hf-W thermochronometry: Closure temperature and constraints on the accretion and cooling history of the H chondrite parent body

We obtained Hf-W metal-silicate isochrons for several H chondrites of petrologic types 4, 5, and 6 to constrain the accretion and high-temperature thermal history of the H chondrite parent body. The silicate fractions have (180)Hf/(184)W ratios up to similar to 51 and (182)W/(184)W ratios up to similar to 33 epsilon units higher than the whole-rock. These high (180)Hf/(184)W and radiogenic W isotope ratios result in highly precise Hf-W ages. The Hf-W ages of the H chondrites become younger with increasing metamorphic grade and range from Delta t(CAI) = 1.7 +/- 0.7 Ma for the H4 chondrite Ste. Marguerite to Delta t(CAI)=9.6 +/- 1.0 Ma for the H6 chondrites Kernouve and Estacado. Closure temperatures for the Hf-W system in H chondrites were estimated from numerical simulations of W diffusion in high-Ca pyroxene, the major host of radiogenic (182)W in H chondrites, and range from 800 +/- 50 degrees C for H4 chondrites to 875 +/- 75 degrees C for H6 chondrites. Owing to these high closure temperatures, the Hf-W system closed early and dates processes associated with the earliest evolution of the H chondrite parent body. Consequently, the high-temperature interval of similar to 8 Ma as defined by the Hf-W ages is much shorter than intervals obtained from Rb-Sr and Pb-Pb dating. For H4 chondrites, heating on the parent body probably was insufficient to cause W diffusion in high-Ca pyroxene, such that the Hf-W age of Delta t(CAI) = 1.7 +/- 0.7 Ma for Ste. Marguerite was not reset and most likely dates chondrule formation. This is consistent with Al-Mg ages of similar to 2 Ma for Land LL chondrules and indicates that chondrules from all ordinary chondrites formed contemporaneously. The Hf-Wages for H5 and H6 chondrites Delta tt(CAI) = 5.9 +/- 0.9 Ma and Delta t(CAI) = 9.6 +/- 1.0 Ma correspond closely to the time of the thermal peak within the H chondrite parent body. Combined with previously published chronological data the Hf-W ages reveal an inverse correlation of cooling rate and metamorphic grade: shortly after their thermal peak H6 chondrites cooled at similar to 10 degrees C/Ma, H5 chondrites at similar to 30 degrees C/Ma and H4 chondrites at similar to 55 degrees C/Ma. These Hf-W age constraints are most consistent with an onion-shell structure of the H chondrite parent body that was heated internally by energy released from (26)Al decay. Parent body accretion started after chondrule formation at 1.7 +/- 0.7 Ma and probably ended before 5.9 +/- 0.9 Ma, when parts of the H chondrite parent body already had cooled from their thermal peak. The well-preserved cooling curves for the H chondrites studied here indicate that these samples derive from a part of the H chondrite parent body that remained largely unaffected by impact disruption and reassembly but such processes might have been important in other areas. The H chondrite parent body has a (180)Hf/(184)W ratio of 0.63 +/- 0.20, distinctly lower than the (180)Hf/(184)W = 1.21 +/- 0.06 of carbonaceous chondrite parent bodies. This difference reflects Hf-W fractionation within the first similar to 2 Ma of the solar system, presumably related to processes in the solar nebula. (C) 2008 Elsevier B.V. All rights reserved.