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

Abstract

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 26Al 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 Δ t CAI = 5.2 ± 0.9 Ma and Δ 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 °C and 1025 ± 50 °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 ~ 1.5–2 Ma after CAI formation, was internally heated by 26Al decay, and reached its thermal peak ~ 3 Ma after CAI formation. Cooling rates for acapulcoites decreased from ~ 120 °C/Ma just below the thermal peak to ~ 50 °C/Ma at ~ 600 °C. Over the same temperature interval the cooling rate for lodranites decreased from ~ 100 °C/Ma to ~ 40 °C/Ma. These thermal histories may reflect cooling in the uppermost ~ 10 km of a parent body with a radius of ~ 35–100 km. Acapulcoites and lodranites evolved with a 180Hf/ 184W ratio of ~ 0.64, which is indistinguishable from that of H chondrites but significantly lower than 180Hf/ 184W~ 1.23 for carbonaceous chondrites. The low 180Hf/ 184W ratios of acapulcoites–lodranites were established before ~ 2 Ma and, hence, prior to partial melting in the parent body at ~ 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 Δ t CAI < 1 Ma for differentiation of the parent bodies of magmatic iron meteorites and an Hf–W age of Δ t CAI ~ 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 26Al abundance, which is determined by their accretion time.