sup 40 Ar/ sup 39 Ar ages of six Apollo 15 impact melt rocks by laser step heating

Journal Article · Sat Jun 01 04:00:00 EDT 1991 · Geophysical Research Letters (American Geophysical Union); (United States)

DOI:https://doi.org/10.1029/91GL01143· OSTI ID:5190880

Dalrymple, G B ^[1]; Ryder, G ^[2]

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1. Geological Survey, Menlo Park, CA (USA)
2. Lunar and Planetary Inst., Houston, TX (USA)

The authors have obtained 15 high resolution (21-51 step) {sup 40}Ar/{sup 39}Ar age spectra on six Apollo 15 impact melt rocks of different compositions using a continuous laser system on submilligram subsamples and on single crystal plagioclase clasts. Four of the six samples gave reproducible age spectra with well-defined intermediate temperature plateaus over 48% or more of the {sup 39}AR released; the plateaus are interpreted as crystallization ages. Samples 15304,7,69, 15294,6,21, and 15314,26,156 gave virtually identical plateau ages whose weighted mean is 3,870 {plus minus} 6 Ma. These three melt rocks differ in composition and likely formed in three separate impact events. Sample 15356,9 gave replicate plateau ages that average 3,836 {plus minus} 12 Ma and date a fourth and younger impact event. The age spectra for samples 15308,9 and 15414,3,36 increase with increasing increment temperature and may have been formed in or affected by impacts at about 2,700 Ma and 3,870 Ma, respectively. So far there continues to be no convincing evidence in the lunar record for impact melts older than about 3.9 Ga.

• OSTI ID: 5190880
• Journal Information: Geophysical Research Letters (American Geophysical Union); (United States), Journal Name: Geophysical Research Letters (American Geophysical Union); (United States) Vol. 18:6; ISSN 0094-8276; ISSN GPRLA
• Country of Publication: United States
• Language: English

Related Subjects

640107* -- Astrophysics & Cosmology-- Planetary Phenomena
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
AGE ESTIMATION
APOLLO PROJECT
CHEMICAL COMPOSITION
COLLISIONS
CRYSTALLIZATION
DATA
IMPACT SHOCK
INFORMATION
ISOTOPE DATING
LASERS
LUNAR MATERIALS
MATERIALS
MOON
ORIGIN
PHASE TRANSFORMATIONS
PLANETARY EVOLUTION
RESOLUTION
SATELLITES
SOLAR SYSTEM EVOLUTION
SPECTRA
SURFACES