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 180Hf/184W ratios up to ∼ 51 and 182W/184W ratios up to ∼ 33ε units higher than the whole-rock. These high 180Hf/184W 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 ΔtCAI = 1.7 ± 0.7Ma for the H4 chondrite Ste. Marguerite to ΔtCAI = 9.6 ± 1.0Ma for the H6 chondrites Kernouvé 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 182W in H chondrites, and range from 800 ± 50°C for H4 chondrites to 875 ± 75°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 ∼ 8Ma 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 ΔtCAI = 1.7 ± 0.7Ma for Ste. Marguerite was not reset and most likely dates chondrule formation. This is consistent with Al–Mg ages of ∼ 2Ma for L and LL chondrules and indicates that chondrules from all ordinary chondrites formed contemporaneously. The Hf–W ages for H5 and H6 chondrites of ΔtCAI = 5.9 ± 0.9Ma and ΔtCAI = 9.6 ± 1.0Ma 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 ∼ 10°C/Ma, H5 chondrites at ∼ 30°C/Ma and H4 chondrites at ∼ 55°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 26Al decay. Parent body accretion started after chondrule formation at 1.7 ± 0.7Ma and probably ended before 5.9 ± 0.9Ma, 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 180Hf/184W ratio of 0.63 ± 0.20, distinctly lower than the 180Hf/184W = 1.21 ± 0.06 of carbonaceous chondrite parent bodies. This difference reflects Hf–W fractionation within the first ∼ 2Ma of the solar system, presumably related to processes in the solar nebula.
Thorsten Kleinea,,,et al. Hf–W thermochronometry: Closure temperature and constraints on the accretion and cooling history of the H chondrite parent body[J]. Earth and Planetary Science Letters,2008-01-01,Volume 270, Issues 1–2(Issues 1–2)