Other
Scientific paper
Mar 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59.1185s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 6, pp.1185-1203
Other
19
Scientific paper
Alkalic rocks from the highlands of the Moon, though relatively minor in volume, yield important information in understanding the later development of the lunar crust. However, until recently little information has been available on the crystallization ages of samples from this diverse suite of rocks. Previous workers suggested a link between pristine KREEP basalts and lunar quartz monzodiorites and granites. Through mineral and chemical modelling of all known pristine highlands alkali suite (HAS) rocks, and radiogenic isotopic analyses of rather large HAS clasts from the Apollo 14 landing site, we explore further this potential link. Two clasts of alkalic affinity from the western highlands of the Moon have been analyzed for their neodymium and strontium isotopic compositions. An alkali anorthosite (14304,267) yields a Sm---Nd mineral isochron of 4108 ± 53 Ma (MSWD = 0.06) and an Nd ( T ) of -1.0 ± 0.2. Two splits of the same alkali norite clast (14304,270 and 14304,272) yield similar Nd values when calculated for this age. Rb---Sr systematics for alkali anorthosite 14304,267 scatter about a line which yields an "age" of 4336 ± 81 Ma. However, the large mean square of weighted deviates (MSWD) for this line (12.3) indicates that the age is suspect and that Rb---Sr sytematics may have been disturbed. The Sm---Nd age for 14304,267, in conjunction with U---Pb zircon ages for two other highlands alkali suite (HAS) rocks from the Apollo 14 landing site and one from the Apollo 16 landing site indicate production of HAS rocks over an extended time period spanning at least 300 million years from 4.34 to 4.02 Ga. Since the last dregs of the lunar magma ocean (LMO) likely crystallized prior to 4.3 Ma, all rocks from this alkalic period cannot represent direct remnants of the late LMO and may include material resulting from the remelting of evolved portions of the Moon by upward-moving basaltic melts from the deep lunar interior. These "contaminated" melts may be represented by pristine KREEP basalts from the Apollo 15 landing site. These KREEP basalt melts could have crystallized within the crust to form cumulate gabbros, norites, anorthosites, monzodiorites, and possibly granites, with the proportion of trapped KREEPy residual liquid determining the large-ion lithophile element enrichment of the rock. Generally, the proportion of trapped KREEPy liquid is small (2-15%). The broad age range for the lunar HAS indicates that parental KREEP basalt magmatism was not a unique event, but was an important process possibly repeated several times throughout the first 600 to 700 million years of lunar history.
Halliday Alex N.
Snyder Gregory A.
Taylor Lawrence A.
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