Lunare Mare Basalt Meteorite NEA003-A: Chronology, Chemical and Petrological Composition

Lunar mare basalt Northeast Africa (NEA) 003-A is an unbrecciated, coarse-grained, low-Ti, low-Al, low-K olivine- rich basalt similar to olivine basalts from Apollo 12 and olivine-normative basalts from Apollo 15. This lunar basalt is chemically and petrographically distinct from the previously described lunar mare basalt meteorites Asuka 881757, Yamato 793169 LaPaz (LAP) 02205 clan, Northwest Africa (NWA) 032/479, Dhofar 287A, Miller Range (MIL) 05035 and NWA 773clan. NEA003-A has a coarse-grained magmatic texture consisting mainly of olivine, pyroxene and plagioclase grains. It presents the second highest modal abundance of olivine and the lowest modal abundance of plagioclase (all converted to maskelynite) compared to other unbrecciated mare basalt meteorite samples and is similar to Apollo 12 and Apollo 15 basalts. The distribution of silicate minerals in NEA 003-A is relatively homogeneous. Only small areas of the sample represent the late-stage mineral association composed mainly of elongated plagioclase, Fe-rich pyroxene and ilmenite, troilite and rare SiO2 phase and K-rich glass. Other minerals typical of the mesostasis, described for lunar basalt samples, (e.g. fayalite, fluorapatite, whitlockite, etc.) are not present in this sample. The conspicuous feature of olivine and pyroxene grains is the presence of numerous cracks and fractures that are likely due to shock. Some of the larger fractures near the surface of the meteorite are filled by the products of terrestrial weathering (mainly secondary Ca-carbonate). Shock event(s) seems to have converted all plagioclase into maskelynite. The Sm-Nd isotope systematics of four mineral fractions composed of different proportions of pyroxene and feldspar have been analysed. All four data points show a good collinear array. If pooled together in one single regression calculation, the result is t = 3.089±0.064 Ga, Ndi = 0.508600±0.000095 (corresponding to ɛNd3089 = -0.4±0.3), and MSWD = 1.04 (n = 4). The K-Ar systematics are disturbed and suggest an Ar-Ar age of 1.762±0.054 Ga (2σ) for pyroxene separates and of 2.315±0.040 Ga (2σ) and 2.293±0.042 Ga for bulk and maskelynite separates respectively. The incongruent Ar-Ar and Sm-Nd ages suggest prolonged post-shock temperatures (<900°C) that permitted the total to partial loss of the initial radiogenic 40Ar.