Rapid accretion and differentiation of iron meteorite parent bodies inferred from 182Hf-182W chronometry and thermal modeling

Abstract

New high-precision W isotope measurements are presented for 33 iron meteorites from 8 magmatic groups (IC, IIAB, IID, IIIAB, IIIE, IIIF, I VA and IVB), 2 non-magmatic groups (IAB-IIICD and IIE), and one ungrouped iron (Deep Springs). All magmatic irons have ε¹⁸²W values that are, within errors, equal to, or less radiogenic than, the Solar System initial of - 3.47 ± 0.20. A method was developed to correct the measured ε¹⁸²W values of magmatic iron meteorites for the presence of cosmogenic effects produced during space exposure to galactic cosmic rays. The corrected data provide new constraints on the timing of metal-silicate differentiation in iron meteorite parent bodies, which must have taken place within a few million years (< 2 to 6 My) of condensation of calcium-aluminum-rich inclusions (CAIs). Metal-silicate differentiation ages (from ¹⁸²Hf-¹⁸²W systematics) were combined with parent body sizes (from metallographic cooling rates) into a model of planetesimal heating by ²⁶Al-decay, to constrain the accretion timescale of iron meteorite parent bodies. Accretion of iron meteorite parent bodies most likely occurred within 1.5 My of the formation of CAIs. The fast accretion times of iron meteorite parent bodies are consistent with dynamical models indicating that these objects may have originated in the terrestrial planet-forming region, where the accretion rates were high. Our W isotopic data for non-magmatic IAB-IIICD and IIE irons provide new constraints for their formation mechanisms. In particular, they support formation of IAB-IIICD iron meteorites by melting during a single collision event dated at 4-7 My after formation of the Solar System. © 2008 Elsevier B.V. All rights reserved.