THE ORIGIN AND EVOLUTION OF LUNAR HIGH-TI BASALTS - PERIODIC MELTING OF A SINGLE-SOURCE AT MARE TRANQUILLITATIS

Five groups of basalts (A, B1, B2, B3, D) with three principal ages exist at the Apollo 11 site. These range from the low-K, low rare-earth element (REE) Groups B1, B2, and B3 to the low-K, high-REE Group D basalts, to the high-K, high-REE Group A basalts. The Group A basalts are the only high-K (>0.2 wt% K2O) basalts, and youngest, with an age of 3.59 +/- .02 Ga; Groups B3 and B1 are 3.71 +/- .02 and 3.67 +/- .02 Ga, respectively; Group B2 basalts are the oldest, at 3.85 +/-.02 Ga. Group D basalts have not been dated. Fractionation modelling for major and trace elements indicates that the B1 basalts could have formed from a B3-like parent liquid. The B2 and D basalts can also be related to liquids similar to the B3-B1 composition through the presence of varying amounts of modal whitlockite. Thus, the entirety of low-K high-Ti basalts at Apollo 11 may have formed through melting of the same source region. The Group A basalt compositions are consistent with formation from a different parent liquid, with the composition of Apollo 11 orange glass. Modelling of major- and trace-elements in the Apollo 11 orange glass, indicate that the composition of the Group A basalts is consistent with fractionation of this glass, coupled with some assimilation (approximately 7.5-15%) of an evolved KREEP-like material. However, this component is much younger than primitive KREEP (4.4 Ga). This ''neuKREEP'' component, similar to the composition of quartz monzodiorites described from the Apollo 15 site, probably represents evolved material formed during plutonism prior to the formation of the Apollo 11 orange glass liquid.