Other
Scientific paper
Mar 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996lpi....27.1113r&link_type=abstract
Lunar and Planetary Science, volume 27, page 1113
Other
2
Apollo 15: Kreep, Granite: Lunar, Immiscibility, Quartz Monzodiorite (Qmd)
Scientific paper
A number of recent studies have been directed at determining the petrogenetic relationship of KREEP-rich lunar lithologies, such as lunar granites and quartz monzodiorite (QMD), to one another and to some original "ur-"KREEP. It has been proposed that KREEP or urKREEP is the residuum formed by extreme fractionation of the lunar magma ocean, but pristine KREEP basalts such as 15386 have an mg* which is apparently much too high for such a residual melt. Emphasizing this point, Warren has suggested that the more evolved QMD may be used to represent urKREEP. However, the petrogenesis of QMD is not well known; previous work indicated that fractionation of a KREEP basalt composition at 1 atm produces a QMD residual melt composition just before becoming immiscible, but it is not known if this same process occurred at depth. It is generally accepted that immiscibility was involved in the genesis of lunar granites in the near-surface environment; it has not been established whether immiscibility is stable at the base of the lunar crust (3 kbar). Two recent discoveries bear on the general problem: 1) the ages of lunar granites have been shown to extend from 4.4 to 3.9 Ga, and 2) modeling indicates that crystal cumulates of the lunar magma ocean would have overturned on a short time scale tending to mitigate against the strong decrease in the mg* of the late stage cumulates [8]. These developments give additional potential importance to magmas such as the A15 KREEP basalt, 15386. Accordingly, experiments have been done at 3 kbar in order to determine what melt compositions are produced by fractionation A15 KREEP at the base of the lunar crust in contrast to those produced at the surface . Are QMD-like compositions produced by fractional crystallization at the base of the crust, and can immiscibility occur at this depth?
Holmberg Beth B.
Rutherford Malcolm J.
Tonks B.
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