The bombardment history of the Moon as recorded by super(40)Ar- super(39)Ar chronology

New petrography and super(40)Ar- super(39)Ar ages have been obtained for 1-3 mm sized rock fragments from Apollo 16 Station 13 soil 63503 (North Ray crater ejecta) and chips from three rocks collected by Apollo 16 and Apollo 17 missions. Selection of these samples was aimed at the old super(40)Ar- s...

Full description

Saved in:
Bibliographic Details
Published in:Meteoritics & planetary science 2013-02, Vol.48 (2), p.241-269
Main Authors: Fernandes, V A, Fritz, J, Weiss, B P, Garrick-Bethell, I, Shuster, D L
Format: Article
Language:English
Subjects:
Age
Basins
Flux
Fragmentation
Fragments
Impact melts
Rock
Soil (material)
Online Access:Get full text
Tags: Add Tag
No Tags, Be the first to tag this record!
Description
Summary:New petrography and super(40)Ar- super(39)Ar ages have been obtained for 1-3 mm sized rock fragments from Apollo 16 Station 13 soil 63503 (North Ray crater ejecta) and chips from three rocks collected by Apollo 16 and Apollo 17 missions. Selection of these samples was aimed at the old super(40)Ar- super(39)Ar ages to understand the early history of the lunar magnetic field and impact flux. Fifteen samples were studied including crustal material, polymict feldspathic fragmental breccias, and impact melts. The impact ages obtained range between approximately 3.3 and 4.3 billion years (Ga). Polymict fragmental breccia 63503,1 exhibits the lowest signs of recrystallization observed and a probable old relic age of 4.547 plus or minus 0.027. The plateau age of 4.293 plus or minus 0.044 Ga obtained for impact melt rock 63503,13 represents the oldest known age for such a lithology. Possibly, this age represents the minimum age for the South Pole-Aitken (SPA) Basin. In agreement with literature data, these results show that impact ages >3.9 Ga are found in lunar rocks, especially within soil 63503. Impact exhumation of deep-seated warm crustal material onto the lunar surface is considered to explain the common 4.2 Ga ages obtained for weakly shocked samples from soil 63503 and Apollo 17. This would directly imply that one or more basin-forming events occurred at that time. Some rock fragments showing none to limited petrologic features indicate thermal annealing. These rocks may have lost Ar while resident within the hot-ejecta of a large basin. Concurrent with previous studies, these results lead us to advocate for a complex impact flux in the inner solar system during the initial approximately 1.3 Ga.
ISSN:1086-9379
1945-5100
DOI:10.1111/maps.12054