Uranium–lead systematics of low-Ti basaltic meteorite Dhofar 287A: Affinity to Apollo 15 green glasses

Dhofar 287 is a lunar meteorite found in Oman in 2001, which consists of a major portion (95%) of low-Ti mare basalt (Dho 287A) and a minor attached part ( 5%) of regolith breccia (Dho 287B). Here, we report the U–Pb systematics of Dho 287A using data collected with a Sensitive High Resolution Ion Microprobe (SHRIMP). In-situ analyses of five merrillite and three apatite grains, which are resistant to secondary petrologic events, resulted in a total Pb/U isochron age of 3.34 ± 0.20 Ga, in 238U/206Pb–207Pb/206Pb–204Pb/206Pb 3-D space (95% confidence level). The observed Pb–Pb isochron of these eight phosphates coupled with four plagioclase grains also yielded a 207Pb/206Pb age of 3.35 ± 0.13 Ga. This formation age, when considered as the crystallization age of Dho 287A, is similar to crystallization ages of Apollo 15 low-Ti olivine-normative basalts (ONB; 3.3 ± 0.1 Ga). However, the estimated μ-value (238U/204Pb ratio) of Dho 287A is 18, which is very different from the reported μ-values of 300 for mare basalts from the Apollo collections, including the Apollo 15 ONBs. These μ-values are still significantly lower than those of Apollo KREEP basalt (500 to 1000), although a possible assimilation with KREEP has been previously proposed for Dho 287A using geochemical criteria. Our U–Pb study of Dho 287A, instead, indicates a closer affinity to Apollo 15 green glasses (207Pb/206Pb age of 3.41 Ga with μ-value of 19 to 55), which are considered to be the most primitive products of lunar volcanism. Combining our U–Pb data with the previously reported Sm–Nd systematics (negative εNd) of Dho 287A clearly distinguishes this meteorite from those of the Yamato 793169 and Asuka 88175 group which have extremely low μ-value of 10–22, old crystallization ages of 3.9 Ga, and high positive εNd, suggesting that Dho 287A may be a representative of an entirely new group of mare basalt derived from previously unsampled source region on the Moon