Pb-Pb ages and initial Pb isotopic composition of lunar meteorites: NWA 773 clan, NWA 4734, and Dhofar 287.

Abstract

Constraining the duration of magmatic activity on the Moon is essential to understand how the lunar mantle evolved chemically through time. Determining age and initial isotopic compositions of mafic lunar meteorites is a critical step in defining the periods of magmatic activity that occurred during the history of the Moon and to constrain the chemical characteristics of mantle components involved in the sources of the magmas. We have used the in situ Pb-Pb SIMS technique to investigate eight lunar gabbros and basalts, including six meteorites from the Northwest Africa (NWA) 773 clan (NWA 2727, NWA 2700, NWA 3333, NWA 2977, NWA 773, and NWA 3170), NWA 4734, and Dhofar 287A. These samples have been selected as there is no clear agreement on their age and they are all from the dominant low titanium chemical group. We have obtained ages of 2981 ± 12 Ma for NWA 4734 and 3208 ± 22 Ma for Dhofar 287. For the NWA 773 clan, four samples (the fine-grained basalt NWA 2727 and the three gabbros NWA 773, NWA 2977, NWA 3170) out of six yielded isochron-calculated ages that are identical within uncertainties and yielding an average age of 3086 ± 5 Ma. The age obtained for the fine-grained basalt NWA 2700 is not precise enough for comparison with the other samples. The gabbroic sample NWA 3333 yielded an age of 3038 ± 20 Ma suggesting that two distinct magmatic events may be recorded in the meteorites of the NWA 773 clan. The present study aims to identify and assess all potential issues that are associated with different ways to date lunar rocks using U-Pb–based methods. To achieve this, we have compared the new ages with the previously published data set. The entire age data set from lunar mafic meteorites was also screened to identify data showing analytical issues and evidence of resetting and terrestrial contamination. The data set combining the ages of mafic lunar meteorites and Apollo rocks suggests pulses of magmatic activity with two distinct phases between 3950 and 3575 Ma and between 3375 and 3075 Ma with the two phases separated by a gap of approximately 200 Ma. The evolution of the Pb initial ratios of the low-Ti mare basalts between approximately 3400 and 3100 Ma suggests that these rocks were progressively contaminated by a KREEP-like component.