Chips Off an Old Lava Flow
• Lava flows with compositions like those of mare basalts began to form before those that occupy the visible maria
• Now hidden beneath the ejecta from impact basins, these ancient lavas are called cryptomaria
• Evidence for their existence comes from analysis of samples (Apollo and meteorites) and remote sensing studies www.psrd.hawaii.edu/Dec07/cryptomareSample.html

Chips Off an Old Lava Flow
• Cryptomaria are revealed when craters punch through overlying crater ejecta to make dark-haloed craters
• Ancient ages of mare basalts in highland breccias also show that mare-like volcanism operated long before 3.8 billion years ago
• Rock on right is 4.23 billion years old www.psrd.hawaii.edu/Dec07/cryptomareSample.html

Chips Off an Old Lava Flow
Isotopic measurements by Kentaro Terada et al ., using an ion microprobe on phosphate grains in a basalt fragment in
Kalahari 009, were used to date the time of crystallization • Phosphates contain considerable uranium, so U-Pb dating is feasible
• Three ways of plotting the data are shown
• Diagram on right is a projection from the
3-dimensional space containing the other two and gives the best estimate of the age of the clast, 4.35 ± 0.15 billion years www.psrd.hawaii.edu/Dec07/cryptomareSample.html

Chips Off an Old Lava Flow
• Crystallization ages of the ferroan anorthosite suite of lunar rocks indicate formation in a magma ocean 4.46 billion years ago
• Formation of the other main rock type in the highlands, the Mg-suite, began after formation of the anorthosite crust and continued until about
3.9 billion years ago
• Sample studies like that done by Terada et al . show that basalt formation overlapped that of Mg-suite magmatism, beginning at least 4.35 billion years ago
• Remote sensing observations indicate basalt formation ceased about a billion years ago, but peaked during the 3.3-3.8 billion years time interval www.psrd.hawaii.edu/Dec07/cryptomareSample.html