Ilmenit lunar basalts

Internal 87Rb-87Sr isochrons were particularly useful in unraveling the history of the Moon and Mars. As an example, consider mare basalts from the Sea of Tranquility collected by Apollo 11. These lunar basalts consist primarily of calcium-rich (anorthitic) plagioclase and low-calcium pyroxene, along with ilmenite, crystobalite, and other minor phases. Strontium is concentrated in the calcium sites in plagioclase and is excluded from most other phases, resulting in a variation of a factor of several hundred in the strontium... 

Samarium-neodymium evolution diagram for lunar mare basalt 75075. Data points for the total rock, plagiodase, ilmenite, and pyroxene form a precise linear array, the slope of which gives a crystallization age of 3.70 0.07 Ga for this rock (7= 6.54 x 10 12 yr 1). The insert shows the deviations from the best-fit line in parts in 104. After Lugmair et al. (1975a). 

Altliough plagioclase, unlike calcium-rich pyroxene, ilmenite, or zircon, does not readily incorporate uranium or thorium, isotopic compositions of lead in plagioclase are of particular interest. Plagioclase is a dominant mineral phase in the lunar crust and a major constituent of mare basalts that can incorporate lead into cation sites normally occupied by potassium, sodium, or calcium and thereby play an important role in the U-Th-Pb isotopic systematics. 

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