Chips Off an Old Lava Flow Lava flows

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Chips Off an Old Lava Flow • Lava flows with compositions like those of

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/Dec 07/cryptomare. Sample. html

Chips Off an Old Lava Flow • • • Cryptomaria are revealed when craters

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/Dec 07/cryptomare. Sample. html

Chips Off an Old Lava Flow Isotopic measurements by Kentaro Terada et al. ,

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/Dec 07/cryptomare. Sample. html

Chips Off an Old Lava Flow • Crystallization ages of the ferroan anorthosite suite

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/Dec 07/cryptomare. Sample. html