Mineralogy and petrology of lunar meteorite Northwest Africa 2977 consisting of olivine cumulate gabbro including inverted pigeonite

Abstract

Lunar meteorite Northwest Africa (NWA) 2977 is identified as an olivine cumulate gabbro (OC), consisting of coarse cumulate olivine crystals up to 1 mm with low-Ca and high-Ca pyroxenes, plagioclase, and interstitial incompatible element-rich pockets of K-feldspar, Ca-phosphates, ilmenite, and troilite. These minerals and textures are similar to those of the OC clasts of the NWA 773 clan of meteorites. NWA 2977 contains a variety of pyroxene textures and compositions including augite, pigeonite, and rare orthopyroxene, all having exsolution lamellae. Some of the orthopyroxene has abundant augite lamellae with compositions indicating formation by inversion of pigeonite. This pigeonite was inverted at 1140°C according to the pigeonite eutectoid reaction (PER) temperatures. Inverted pigeonite has not been found previously in the NWA 773 clan of meteorites. The presence of inverted pigeonite indicates that NWA 2977 cooled more slowly than most other OC clasts of the NWA 773 clan. The relatively slow cooling of NWA 2977 can be explained by formation in a deeper level of the original igneous body of the NWA 773 clan OC lithology.

Formation of ferroan dacite by lunar silicic volcanism recorded in a meteorite from the Moon

“…Silica minerals are not found in the OG clast. These petrological features are similar to other OG clasts in the NWA 773 clan (Fagan et al 2003;Jolliff et al 2003;Zhang et al 2011;Nagaoka et al 2015;Valencia et al 2019).…”

“…The gabbroic lithologies were roughly divided into three types (Fagan et al 2003(Fagan et al , 2014Jolliff et al 2003;Zhang et al 2011;Valencia et al 2019): olivine gabbro (OG, an olivine-pyroxeneplagioclase-rich rock); anorthositic gabbro (AG, a plagioclase-rich rock with some pyroxene); and ferroan gabbro (FG, a group of lithologies all having high Fe# = molar Fe/[Fe + Mg]; see below). OG clasts in NWA 773 clan have been well characterized by previous studies (Fagan et al 2003;Jolliff et al 2003;Zhang et al 2011;Nagaoka et al 2015;Valencia et al 2019). Isotopic Sm-Nd isochron ages of the OG lithology have been analyzed by Borg et al (2009), who reported an age of 2.99 ± 0.03 Ga, and by Nyquist et al (2009), reporting an age of 3.10 ± 0.05 Ga. Isotopic analyses of baddeleyite crystals in the OG lithology and in other types of clasts yield Pb-Pb ages near 3.1 Ga (Zhang et al 2011;Shaulis et al 2017).…”

“…6a Al versus Ti and b Fe# versus Ti# in pyroxenes from the silicic clast (this work). Compositions of ARFe clasts in NWA 773(Fagan et al 2014), NWA 2977 OG(Nagaoka et al 2015), and 15405 QMD (56, 145)(Fagan et al 2014) are also shown Nagaoka et al Progress in Earth and Planetary Science (2020) 7:12…”

Petrography, relationships, and petrogenesis of the gabbroic lithologies in Northwest Africa 773 clan members Northwest Africa 773, 2727, 3160, 3170, 7007, and 10656

“…Inverted pigeonite grains in the NWA 2977 OG, found by Nagaoka et al. (), support an origin for the OG as a shallow intrusive magma chamber, as the inversion of pigeonite occurs during slow cooling of plutonic lunar rocks (Papike and Bence ). The size of the chamber need not have been very large and it could have taken the form of a thick sill.…”

Petrography and chronology of lunar meteorite Northwest Africa 6950

“…It is inferred to be paired with other lunar meteorites called the NWA 773 clan [4,5], which is a group of paired or petrogenetically related stones that include NWA 773, NWA 2977, NWA 2700, NWA 2727, NWA 3160, NWA 7007, and etc. [6][7][8][9][10][11][12]. In recent years, NWA 773 and its paired meteorites have received extensive attention from planetary chemists because they are significantly different from Apollo and Luna samples and other lunar meteorites [13].…”