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 ɛ-units for the IAB iron meteorite complex. This suggests metal separation at 5.06 +0.42/-0.41 Ma 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/-0.6 Ma; Schulz et al., 2009) and the formation of winonaites (4.8 +3.1/-2.6 Ma; 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 ∼2 Ma after solar system formation. However, metals from ungrouped IAB specimen segregated at significantly different times. Separation of Mundrabilla metals at ∼13 Ma 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 ∼14 Ma (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 ∼28 Ma 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.
- Publication:
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Geochimica et Cosmochimica Acta
- Pub Date:
- May 2012
- DOI:
- 10.1016/j.gca.2012.02.012
- Bibcode:
- 2012GeCoA..85..200S