Age and origin of IIE iron meteorites inferred from Hf-W chronology. (1st October 2019)

• Record Type: Journal Article
• Authors: Kruijer, Thomas S.; Kleine, Thorsten
• Is Part Of: Geochimica et cosmochimica acta. Volume 262(2019)
• Page Start: 92
• Page End: 103
• Publication Date: 2019-10-01
• Subjects: Hf-W chronology -- Metal-silicate separation -- Iron meteorites -- Planetesimals -- Collisions -- Planetary differentiation; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2019.07.039
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Non-magmatic iron meteorites, including the IIE group, can provide important insights into the history of metal-silicate differentiation and collisions on planetesimals. To better constrain the evolution of metal segregation and impacts on the IIE parent body, W isotopic data are reported for 10 IIE iron meteorites and a metal vein from the Portales Valley H6 chondrite. In addition, Pt isotopic data were obtained to quantify cosmic ray-induced neutron capture effects on W isotopes. After correction for these effects, the IIE iron meteorites exhibit variable pre-exposure 182 W compositions, translating into Hf-W model age clusters of ∼4–5 million years (Ma), ∼10 Ma, ∼15 Ma, and ∼27 Ma after CAI formation. These distinct 182 W clusters likely represent samples from several discrete metallic melt pools on the IIE parent asteroid. The earliest metal segregation event at ∼4–5 Ma was likely facilitated by 26 Al decay as an internal heat source. By contrast, the younger Hf-W model ages may not be chronologically meaningful, and probably reflect the effects of secondary mixing and re-equilibration of metal and silicates, likely facilitated by impacts on the IIE parent body. Thus, contrary to prior work, the Hf-W systematics of IIE iron meteorites do not require a protracted history of metal-silicate separation on the IIE parent body. Instead the results of this study are fully consistent with a single partial metal-silicate differentiation event driven by endogenic heating at ∼4–5 Ma, followed by one or multiple impact events causing mixing and re-equilibration of metal and silicates at a later stage. The exact timing of these impact event(s) remains poorly constrained, but they most likely occurred in the first few tens of Ma of Solar System history.