Evidence for late impact‐induced metasomatism on the brachinite parent body recorded by a phosphate assemblage in NWA 7828

Brachinites are a group of ultramafic achondritic meteorites thought to sample a planetesimal from the early inner solar system. They yield predominately ancient crystallization ages within 4 Ma of CAI formation, and while the formation mechanism for these samples is debated, they are widely thought to be partial melt residues from a differentiated planetesimal(s). Here, we conduct a correlated microstructural (electron backscatter diffraction; EBSD), trace element, and U–Pb age (laser ablation inductively coupled plasma mass spectrometry; LA-ICP-MS) study of a unique, large phosphate mineral assemblage in brachinite Northwest Africa (NWA) 7828 to constrain the origin and evolution of this sample and its parent body. Oxygen isotope analysis of NWA 7828 yields values in agreement with other brachinites and supportive of origin from the brachinite parent body. The phosphate assemblage is >90% chlorapatite, with merrillite occurring around grain boundaries and within fractures that crosscut the larger crystal. All calcium phosphate grains are highly crystalline, with domains of chlorapatite displaying <16° of internal misorientation, with merrillite displaying a range of unique orientations. When all concordant apatite and merrillite U-Th-Pb analyses are considered together, they yield a precise weighted average 207Pb-206Pb date of 4431 ± 5 Ma suggestive of a single population recording their crystallization age. Textural, chemical, and isotopic measurements of NWA 7828 are hard to reconcile with the formation of the phosphate assemblage in an igneous environment, instead supporting a metasomatic origin. The relatively younger age of the assemblage (4431 Ma) places it outside the estimated prolonged heating period on the brachinite parent body, instead requiring a later source of energy such as through impact-induced heating. This event coincides with the timing of impacts recorded by other brachinite (and brachinite-like) meteorites, as well as impact ages recorded by some Apollo melt breccias, and suggests a widespread, significant bombardment event around 4430 Ma.