Formation and exposure history of non-magmatic iron meteorites and winonaites: Clues from Sm and W isotopes

Abstract

New W and Sm isotope measurements for metals and silicates of non-magmatic iron meteorite groups and winonaites are presented and compiled with literature data to assess their exposure history and parent body formation. We report high-precision 182W data for eight IAB metals supplemented by literature data and introduce a method to calculate their zero-exposure values. Our estimate reveals a common radiogenic 182W signature of −2.83±0.03 ε-units for the IAB iron meteorite complex. This suggests metal separation at 5.06 +0.42/−0.41Ma after solar system formation. The refined age estimate for core formation agrees remarkably well with previously published 182Hf–182W ages for silicate melting (4.6 +0.7/−0.6Ma; Schulz et al., 2009) and the formation of winonaites (4.8 +3.1/−2.6Ma; Schulz et al., 2010), which are assumed to be derived from the same parent body. If interpreted in favour of an asteroid-wide (and therefore most likely internal) heat source, these ages correspond to an accretion age for the IAB/winonaite parent body of ∼2Ma after solar system formation. However, metals from ungrouped IAB specimen segregated at significantly different times. Separation of Mundrabilla metals at ∼13Ma after solar system formation can best be explained via impact triggered melt pool formation, a process that could also be responsible for metamorphism of IAB silicates and winonaites between ∼11 and ∼14Ma (Schulz et al., 2009, 2010). 149Sm and 150Sm compositions, indicative of cosmic ray effects, for five IAB silicates reveal a correlation with exposure ages obtained from metal phases and, together with data on three winonaites, provide no compelling evidence for exposure of silicates within near surface regions of the IAB/winonaite parent asteroid.Tungsten isotope compositions of metals from six IIE iron meteorites, measured in this study and reported in the literature, reveal three consecutive metal segregation events at ∼3, ∼13 and ∼28Ma after formation of the solar system. Whereas the oldest event could potentially be explained by internal heat sources, impacts provide the only viable explanation for the latter. A prolonged time-span for thermal events, as deduced from IIE metals, is supported by 182Hf–182W data for two silicate inclusions from Watson (Snyder et al., 2001) and Miles (this study). Samarium isotope data for a silicate inclusion from the Miles meteorite provides no evidence for 2π-exposure.