Northwest Africa 773: Lunar origin and iron‐enrichment trend

Abstract

Abstract-The meteorite Northwest Africa 773 (NWA 773) is a lunar sample with implications for the evolution of mafic magmas on the moon. A combination of key parameters including whole-rock oxygen isotopic composition, Fe/Mn ratios in mafic silicates, noble gas concentrations, a KREEP-like rare earth element pattern, and the presence of regolith agglutinate fragments indicate a lunar origin for NWA 773. Partial maskelynitization of feldspar and occasional twinning of pyroxene are attributed to shock deformation. Terrestrial weathering has caused fracturing and precipitation of Carich carbonates and sulfates in the fractures, but lunar minerals appear fresh and unoxidized. The meteorite is composed of two distinct lithologies: a two-pyroxene olivine gabbro with cumulate texture, and a polymict, fragmental regolith breccia. The olivine gabbro is dominated by cumulate olivine with pigeonite, augite, and interstitial plagioclase feldspar. The breccia consists of several types of clasts but is dominated by clasts from the gabbro and more FeO-rich derivatives. Variations in clast mineral assemblage and pyroxene Mg/(Mg + Fe) and Ti/(Ti + Cr) record an igneous Fe-enrichment trend that culminated in crystallization of fayalite + silica + hedenbergitebearing symplectites.The Fe-enrichment trend and cumulate textures observed in NWA 773 are similar to features of terrestrial ponded lava flows and shallow-level mafic intrusives, indicating that NWA 773 may be from a layered mafic intrusion or a thick, differentiated lava flow. NWA 773 and several other mafic lunar meteorites have LREE-enriched patters distinct from Apollo and Luna mare basalts, which tend to be LREE-depleted. This is somewhat surprising in light of remote sensing data that indicates that the Apollo and Luna missions sampled a portion of the moon that was enriched in incompatible heatproducing elements.

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

“…Our sample has less low-Ca pyroxene and more olivine and plagioclase than the NWA 2977 OC studied by Zhang et al (2011) and has more low-Ca pyroxene and less olivine than the NWA 773 OC studied by Fagan et al (2003). The differences between these modal abundances are likely attributed at least in part to a heterogeneous distribution of coarse minerals in the OC lithology, as discussed in Zhang et al (2011).…”

“…Previous studies, including our work, have used minerals and textures to conclude that the OC of NWA 2977 is part of the NWA 773 clan (Bunch et al 2006;Nagaoka et al 2010Nagaoka et al , 2011Zhang et al 2011). In addition to their petrologic affinities, NWA 2977 and NWA 773 both have bulk rock chemical compositions with similar KREEP-like REE patterns (Fagan et al 2003;Jolliff et al 2003;Nyquist et al 2009;Zhang et al 2011). The pairing of NWA 2977 with NWA 773 is supported further by similar isotopic ages that are relatively young for lunar rocks.…”

“…Lunar meteorite NWA 2977 is an OC that is texturally and mineralogically similar to the OC clasts found in the NWA 773 clan (e.g., Bunch et al 2006;Fagan et al 2003;Jolliff et al 2003;Zhang et al 2011).…”

“…The OC lithology has one of the youngest crystallization ages among lunar samples collected (e.g., Borg et al 2009;Nyquist et al 2009;Zhang et al 2011). Many of the breccia clasts are considered to be parts of a comagmatic crystallization sequence, with the OC representing an early stage of magmatic differentiation (Fagan et al 2003(Fagan et al , 2014Jolliff et al 2003).…”

Petrography, mineralogy, and geochemistry of lunar meteorite Sayh al Uhaymir 300

“…SaU 300 falls at the lower end of the trend. Data are from Koeberl et al (1996), Fagan et al (2003), Korotev et al (2003), Warren (2003), Cahill et al (2004), Cohen et al (2004).…”