NWA 11562: A Unique Ureilite with Extreme Mg-rich Constituents

Abstract

A comprehensive study of an ungrouped achondrite meteorite, North West Africa (NWA) 11562, was conducted, involving petrology, mineralogy, and mass-independent chromium isotopic composition. NWA 11562 comprises 34.9 vol% olivine, 56.1 vol% pyroxenes, 6.7 vol% Fe–Ni metal and oxides, 2.2 vol% carbon, and 0.2 vol% anhydrite. The oxygen isotopic composition (δ 18O = 6.24‰ ± 0.13‰ and Δ17O = −1.81‰ ± 0.03‰; Meteoritical Bulletin Database) and chromium isotopic compositions (ε 54Cr = −0.82 ± 0.10, 2SE) are consistent with NWA 11562 being a ureilite, and petrographic observations show features similar to those of the common ureilite group meteorites. Olivine (core: Fo99.0 ± 0.1; rim: Fo98.9 ± 0.1) and pyroxene (orthopyroxene: Mg# 99.0 ± 0.2; clinopyroxene: Mg# 99.1 ± 0.7) core compositions are more magnesian than in any previously known ureilite and lack the characteristic reduction rims of ureilites. Rounded small olivine grains within NWA 11562 indicate that the meteorite experienced impact and associated melting. Combined with the characteristic Fe/Mn ratio (3.84 ± 0.16) and Mg# (99.0 ± 0.1) of olivine cores, we suggest that NWA 11562 represents a more Mg-rich ureilite than any previously reported. NWA 11562 has a high 55Mn/52Cr ratio, and when combined with literature data, it plots on a well-defined 53Mn–53Cr isochron, providing a more accurate age, i.e., 4566.7 ± 0.8 Ma, overlapping previous work. This age may represent the result of early partial melting of the ureilite parent body.