Lee et al (Science 1997 278:1098) report a study of the age and
origin of the Moon with the hafnium-tungsten chronometric method.
The tungsten isotopic compositions of 21 lunar samples were found
to range from chondritic to slightly radiogenic. The authors
suggest this heterogeneity is probably the result of late
radioactive decay within the Moon itself, and that the Moon
formed 4.52 to 4.50 billion years ago and its mantle has since
remained poorly mixed.

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