Mathematics – Logic
Scientific paper
Jun 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008e%26psl.270..106k&link_type=abstract
Earth and Planetary Science Letters, Volume 270, Issue 1-2, p. 106-118.
Mathematics
Logic
24
Scientific paper
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 ˜ 33 ɛ 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 ± 0.7 Ma for the H4 chondrite Ste. Marguerite to ΔtCAI = 9.6 ± 1.0 Ma 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 ˜ 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 ΔtCAI = 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 ˜ 2 Ma 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 ± 0.9 Ma and ΔtCAI = 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 ˜ 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 ± 0.7 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 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 ˜ 2 Ma of the solar system, presumably related to processes in the solar nebula.
Bourdon Bernard
Halliday Alex N.
Kleine Thorsten
Maden Colin
Mezger Klaus
No associations
LandOfFree
Hf W thermochronometry: Closure temperature and constraints on the accretion and cooling history of the H chondrite parent body does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Hf W thermochronometry: Closure temperature and constraints on the accretion and cooling history of the H chondrite parent body, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Hf W thermochronometry: Closure temperature and constraints on the accretion and cooling history of the H chondrite parent body will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1246853