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Geochemistry and Sm‐Nd chronology of a Stannern‐group eucrite, Northwest Africa 7188
Author(s) -
Kagami Saya,
Haba Makiko K.,
Yokoyama Tetsuya,
Usui Tomohiro,
Greenwood Richard C.
Publication year - 2019
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.13382
Subject(s) - geochemistry , geology , isochron , pyroxene , isochron dating , partial melting , meteorite , ordinary chondrite , basalt , plagioclase , metamorphism , closure temperature , chondrite , olivine , astrobiology , quartz , paleontology , physics
We 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, and 147,146 Sm‐ 143,142 Nd mineral isochron ages. The texture and chemical composition of pyroxene and plagioclase demonstrate that NWA 7188 is a monomict eucrite with a metamorphic grade of type 4. The oxygen isotopic composition and 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 TiO 2 , La, and Hf concentrations and a CI chondrite‐normalized rare earth element pattern are in good agreement with those of representative Stannern‐group eucrites. The 147,146 Sm‐ 143,142 Nd isochrons for NWA 7188 yielded ages of 4582 ± 190 and 4554 +17/−19 Ma, respectively. The closure temperature of the Sm‐Nd system for different fractions of NWA 7188 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, the 146 Sm‐ 142 Nd age of NWA 7188 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, NWA 7188 is a rare Stannern‐group eucrite that preserves the chronological information regarding the initial crustal evolution of Vesta.