The Ar-Ar age and petrology of Miller Range 05029: Evidence for a large impact in the very early solar system

J. R. Weirich, A. Wittmann, C. E. Isachsen, D. Rumble, Timothy Swindle, D. A. Kring

Research output: Contribution to journalArticle

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Abstract

Miller Range (MIL) 05029 is a slowly cooled melt rock with metal/sulfide depletion and an Ar-Ar age of 4517 ± 11 Ma. Oxygen isotopes and mineral composition indicate that it is an L chondrite impact melt, and a well-equilibrated igneous rock texture with a lack of clasts favors a melt pool over a melt dike as its probable depositional setting. A metallographic cooling rate of approximately 14 °C Ma-1 indicates that the impact occurred at least approximately 20 Ma before the Ar-Ar closure age of 4517 Ma, possibly even shortly after accretion of its parent body. A metal grain with a Widmanstätten-like pattern further substantiates slow cooling. The formation age of MIL 05029 is at least as old as the Ar-Ar age of unshocked L and H chondrites, indicating that endogenous metamorphism on the parent asteroid was still ongoing at the time of impact. Its metallographic cooling rate of approximately 14 °C Ma-1 is similar to that typical for L6 chondrites, suggesting a collisional event on the L chondrite asteroid that produced impact melt at a minimum depth of 5-12 km. The inferred minimum crater diameter of 25-60 km may have shattered the 100-200 km diameter L chondrite asteroid. Therefore, MIL 05029 could record the timing and petrogenetic setting for the observed lack of correlation of cooling rates with metamorphic grades in many L chondrites.

Original languageEnglish (US)
Pages (from-to)1868-1888
Number of pages21
JournalMeteoritics and Planetary Science
Volume45
Issue number12
DOIs
StatePublished - Dec 2010

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petrology
chondrites
chondrite
solar system
melt
asteroids
asteroid
impact melts
cooling
igneous rocks
rock intrusions
metal
oxygen isotopes
parent body
craters
clast
metals
igneous rock
closures
crater

ASJC Scopus subject areas

  • Geophysics
  • Space and Planetary Science

Cite this

The Ar-Ar age and petrology of Miller Range 05029 : Evidence for a large impact in the very early solar system. / Weirich, J. R.; Wittmann, A.; Isachsen, C. E.; Rumble, D.; Swindle, Timothy; Kring, D. A.

In: Meteoritics and Planetary Science, Vol. 45, No. 12, 12.2010, p. 1868-1888.

Research output: Contribution to journalArticle

Weirich, J. R. ; Wittmann, A. ; Isachsen, C. E. ; Rumble, D. ; Swindle, Timothy ; Kring, D. A. / The Ar-Ar age and petrology of Miller Range 05029 : Evidence for a large impact in the very early solar system. In: Meteoritics and Planetary Science. 2010 ; Vol. 45, No. 12. pp. 1868-1888.
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