Geochemistry and Sm‐Nd chronology of a Stannern‐group eucrite, Northwest Africa 7188

Abstract

AbstractWe report the results of a detailed study of the basaltic eucrite Northwest Africa (NWA) 7188, including its mineralogical and bulk geochemical characteristics, oxygen isotopic composition, and147,146Sm‐143,142Nd mineral isochron ages. The texture and chemical composition of pyroxene and plagioclase demonstrate thatNWA7188 is a monomict eucrite with a metamorphic grade of type 4. The oxygen isotopic composition and the Fe/Mn ratios of pyroxene confirmed thatNWA7188 belongs to the howardite–eucrite–diogenite meteorite suite, generally considered to originate from asteroid 4 Vesta. Whole‐rock TiO2, La, and Hf concentrations and aCIchondrite‐normalized rare earth element pattern are in good agreement with those of representative Stannern‐group eucrites. The147,146Sm‐143,142Nd isochrons forNWA7188 yielded ages of 4582 ± 190 and 4554 +17/−19 Ma, respectively. The closure temperature of the Sm‐Nd system for different fractions ofNWA7188 was estimated to be >865 °C, suggesting that the Sm‐Nd decay system has either been resistant to reheating at ~800 °C during the global metamorphism or only partially reset. Therefore, the146Sm‐142Nd age ofNWA7188 corresponds to the period of initial crystallization of basaltic magmas and/or global metamorphism on the parent body, and is unlikely to reflect Sm‐Nd disturbance by late reheating and impact events. In either case,NWA7188 is a rare Stannern‐group eucrite that preserves the chronological information regarding the initial crustal evolution of Vesta.