Unique achondrite Northwest Africa 11042: Exploring the melting and breakup of the L chondrite parent body

Abstract

Northwest Africa (NWA) 11042 is a heavily shocked achondrite with mediumgrained cumulate textures. Its olivine and pyroxene compositions, oxygen isotopic composition, and chromium isotopic composition are consistent with L chondrites. Sm-Nd dating of its primary phases shows a crystallization age of 4100 AE 160 Ma. Ar-Ar dating of its shocked mineral maskelynite reveals an age of 484.0 AE 1.5 Ma. This age coincides roughly with the breakup event of the L chondrite parent body evident in the shock ages of many L chondrites and the terrestrial record of fossil L chondritic chromite. NWA 11042 shows large depletions in siderophile elements (<0.019CI) suggestive of a complex igneous history involving extraction of a Fe-Ni-S liquid on the L chondrite parent body. Due to its relatively young crystallization age, the heat source for such an igneous process is most likely impact. Because its mineralogy, petrology, and O isotopes are similar to the ungrouped achondrite NWA 4284 (this work), the two meteorites are likely paired and derived from the same parent body. Sample

Northwest Africa 6486: Record of large impact events and fluid alteration on the L chondrite asteroid

“…The type 2 melt pockets are much larger, much more abundant, and have a different texture and composition than the type 1 and 3 melt pockets. The texture of the type 2 melt pockets is similar to those observed in shocked chondrites and shocked chondritic melt rocks (Acosta-Maeda et al, 2013;Feng et al, 2011;Pitarello et al, 2015;Vaci et al, 2020;Wu & Hsu, 2019). The heterogeneous texture and composition of the olivine inclusions in the melt and the olivine grains of the host meteorite are similar to those cited above and are related to transformation of olivine to its highpressure polymorphs.…”

“…L chondrite melt rocks make up only 0.1% of L chondrites (Meteoritical Bulletin Database, 2021). Most probably, they represent the fragments of impact melt dikes or sheets and generally are similar to the melts occurring in shock-melted chondrites (Fujiwara & Nakamura, 1993;Herd et al, 2013;Hutson et al, 2013;Kuehner et al, 2017;Metzler et al, 2011;Mittlefehldt & Lindstrom, 2001;Vaci et al, 2020) and in ordinary chondrites as igneous-textured, unfractionated inclusions (Armstrong & Ruzicka, 2015;Corrigan & Lunning, 2013;Ruzicka et al, 1998Ruzicka et al, , 2000Ruzicka et al, , 2019. However, some L chondrite melt rocks and igneoustextured inclusions also have features of chemical fractionation (Goodrich & Kring, 2016;Migdisova et al, 1992Migdisova et al, , 1994Ruzicka et al, 2000).…”

“…The NWA 6486 metal and troilite contents (3.26 and 3.31 wt%) are less than those in L chondrites (8.39 AE 0.95 and 6.12 AE 1.33 wt%, respectively; Dunn et al, 2010;McSween et al, 1991), indicating segregation of these phases from the source chondrite melt of the NWA 6486. Metal and troilite depletion was observed as a specific feature of some melt rocks related to L chondrites (e.g., Benedix et al, 2008;Fujiwara & Nakamura, 1993;Harvey, 1993;Kuehner et al, 2017;Vaci et al, 2020) and as a feature of the larger melt rock inclusions in ordinary chondrites (Armstrong & Ruzicka, 2013Ruzicka et al, 1998Ruzicka et al, , 2000Ruzicka et al, , 2019.…”

“…L chondrite melt rocks typically have fine‐ to medium‐grained porphyritic or poikilitic textures, as well as chemical compositions of silicates and bulk oxygen isotopic compositions similar to those of L chondrites. Some L chondrite melt rocks are shocked (Mittlefehldt & Lindstrom, 2001; Vaci et al., 2020), indicating their formation during different shock events of the parent body history.…”

“…L chondrites most frequently show disturbed radiometric clocks. The 39 Ar‐ 40 Ar chronometer of L chondritic meteorites indicates a single asteroid breakup event at 470 ± 6 Ma (e.g., Korochantseva et al., 2007; Vaci et al., 2020; Weirich et al., 2012). Radioisotopic dating techniques such as Rb‐Sr and U‐Pb are more refractory and commonly show older ages than the K‐Ar system.…”