Hf-W thermochronometry: II. Accretion and thermal history of the acapulcoite-lodranite parent body

Acapulcoites and lodranites are highly metamorphosed to partially molten meteorites with mineral and bulk compositions similar to those of ordinary chondrites. These properties place the acapulcoites and lodranites between the unmelted chondrites and the differentiated meteorites and as such acapulcoites-lodranites are of special interest for understanding the initial stages of asteroid differentiation as well as the role of Al-26 heating in the thermal history of asteroids. To constrain the accretion timescale and thermal history of the acapulcoite-lodranite parent body, and to compare these results to the thermal histories of other meteorite parent bodies, the Hf-W system was applied to several acapulcoites and lodranites. Acapulcoites Dhofar 125 and NWA 2775 and lodranite NWA 2627 have indistinguishable Hf-W ages of Delta t(CAI) = 5.2 +/- 0.9 Ma and Delta t(CAI) = 5.7 +/- 1.0 Ma, corresponding to absolute ages of 4563.1 +/- 0.8 Ma and 4562.6 +/- 0.9 Ma. Closure temperatures for the Hf-W system for acapulcoites and lodranites, estimated from numerical simulations of W diffusion in high-Ca pyroxene, are 975 +/- 50 degrees C and 1025 +/- 50 degrees C. respectively. Owing to these high closure temperatures, the Hf-W ages provide information on the earliest high-temperature evolution, and combined with thermal modeling indicate that the acapulcoite-lodranite parent body accreted similar to 1.5-2 Ma after CAI formation, was internally heated by Al-26 decay, and reached its thermal peak similar to 3 Ma after CAI formation. Cooling rates for acapulcoites decreased from similar to 120 degrees C/Ma just below the thermal peak to similar to 50 degrees C/Ma at similar to 600 degrees C. Over the same temperature interval the cooling rate for lodranites decreased from similar to 100 degrees C/Ma to similar to 40 degrees C/Ma. These thermal histories may reflect cooling in the uppermost similar to 10 km of a parent body with a radius of similar to 35-100 km. Acapulcoites and lodranites evolved with a Hf-180/W-184 ratio of similar to 0.64, which is indistinguishable from that of H chondrites but significantly lower than Hf-180/W-184 similar to 1.23 for carbonaceous chondrites. The low Hf-180/W-184 ratios of acapulcoites-lodranites were established before similar to 2 Ma and, hence, prior to partial melting in the parent body at similar to 3 Ma. Thus, they must reflect Hf-W fractionation of the precursor material by processes in the solar nebula. Combined with Hf-W ages of Delta t(CAI)<1 Ma for differentiation of the parent bodies of magmatic iron meteorites and an Hf-W age of Delta t(CAI)similar to 2.5 Ma for the accretion of the H chondrite parent body, the Hf-W results for acapulcoites and lodranites reveal an inverse correlation between accretion age of asteroids and peak temperature in their interiors. The different thermal histories of most meteorite parent bodies, therefore, primarily reflect variations in their initial Al-26 abundance, which is determined by their accretion time. (C) 2009 Elsevier B.V. All rights reserved.