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

1Saya Kagami,1Makiko K. Haba,1Tetsuya Yokoyama,2,3Tomohiro Usui,4Richard C. Greenwood
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13382]
1Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama, Tokyo 152-8551, Japan2Earth-Life Science Institute, Tokyo
2Institute of Technology, Ookayama, Tokyo 152-8550, Japan
3Department of Solar System Sciences, ISAS, JAXA, Sagamihara, Kanagawa 252-5210, Japan
4Planetary and Space Sciences, School of Physical Sciences, The Open University, Walton Hall, UK
Published by arrangement with John Wiley & Sons

We report the results of a detailed study of the basaltic eucrite Northwest Africa(NWA) 7188, including its mineralogical and bulk geochemical characteristics, oxygenisotopic composition, and147,146Sm-143,142Nd mineral isochron ages. The texture andchemical composition of pyroxene and plagioclase demonstrate that NWA 7188 is amonomict eucrite with a metamorphic grade of type 4. The oxygen isotopic compositionand the Fe/Mn ratios of pyroxene confirmed that NWA 7188 belongs to the howardite–eucrite–diogenite meteorite suite, generally considered to originate from asteroid 4 Vesta.Whole-rock TiO2, La, and Hf concentrations and a CI chondrite-normalized rare earthelement pattern are in good agreement with those of representative Stannern-group eucrites.The147,146Sm-143,142Nd isochrons for NWA 7188 yielded ages of 4582190 and 4554+17/19 Ma, respectively. The closure temperature of the Sm-Nd system for different fractionsof NWA 7188 was estimated to be>865°C, suggesting that the Sm-Nd decay system haseither been resistant to reheating at~800°C during the global metamorphism or onlypartially reset. Therefore, the146Sm-142Nd age of NWA 7188 corresponds to the period ofinitial 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 eithercase, NWA 7188 is a rare Stannern-group eucrite that preserves the chronologicalinformation regarding the initial crustal evolution of Vesta.

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