Understanding of the diversity and petrogenesis of achondrites is critical for deciphering magmatic processes and the early evolution of planets and asteroids. Here, we report the detailed petrologic, mineralogical, geochemical, and chronological features of the unbrecciated Vestan meteorite Northwest Africa (NWA) 8326. We found that NWA 8326 is composed of coarse-grained orthopyroxene (∼74 vol%), plagioclase (∼19 vol%), fine-grained augite (∼5 vol%), and many accessory minerals such as chromite, ilmenite, Fe-sulfide, silica phases, K-feldspar, Ca-phosphate phases, zircon, baddeleyite, rutile, and primary Si,Al,K-rich glass, differing from typical howardite-eucrite-diogenite meteorites. Based on textural feature and compositional calculation of pyroxene, we suggest that the coarse-grained orthopyroxene was inverted from primary pigeonite and NWA 8326 should be classified as a pigeonite cumulate eucrite. The oxygen and chromium isotope data (Δ17O = − 0.254 ± 0.009 ‰; ε54Cr = − 0.60 ± 0.06) support this classification. A few zircon aggregates are observed in NWA 8326 and the grains therein show a core-mantle zoned texture in cathodoluminescence (CL) images, with the cores being dark and Al-rich while the mantles being bright and Al-poor. We interpret that the CL-dark cores are xenocrystic zircon grains derived from eucrites, whose presence indicates that NWA 8326 should have formed through partial melting of the Vestan mantle, with assimilation of eucritic material. The presence of xenocrystic zircon and primary Si,Al,K-rich glass and the large compositional variation of plagioclase indicate that NWA 8326 is an unequilibrated cumulate eucrite and hence the zircon 207Pb/206Pb age of 4559.2 ± 5.2 (2σ) Ma represents the crystallization of NWA 8326. Reconciling the cumulative texture with the presence of the chemically evolved glass, NWA 8326 would be excavated during the late stage of its crystallization and escaped the prevalent crustal thermal metamorphism of the eucrite parent body. The Mg isotopic composition of NWA 8326 is higher than most diogenites, which suggests that the parent magma of such a pigeonite cumulate eucrite was derived from a source region with heavier magnesium isotopic composition (μ25Mg: −90 to − 96 ppm).

中文翻译：

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未角砾岩累积真晶石的岩石成因 西北非洲 8326：弥合真晶石和闪长岩之间的差距


了解软骨岩的多样性和岩石成因对于破译岩浆过程以及行星和小行星的早期演化至关重要。在这里，我们报告了未角砾碎的维斯坦陨石西北非洲 （NWA） 8326 的详细岩石学、矿物学、地球化学和年代学特征。我们发现 NWA 8326 由粗粒斜辉石 （∼74 vol%）、斜长石 （∼19 vol%） 、细粒奥金石 （∼5 vol%） 和许多辅助矿物组成，如铬铁矿、钛铁矿、硫化铁、二氧化硅相、K-长石、Ca-磷酸盐相、锆石、白铁矿、金红石和原生 Si，Al，K 玻璃，与典型的霍华德石-钙铁矿-二生石陨石不同。根据辉石的织构特征和成分计算，我们建议粗粒斜辉石是从原生鸽子石倒置而来的，NWA 8326 应归类为鸽子石累积绿铁矿。氧和铬同位素数据 （Δ17O = − 0.254 ± 0.009 ‰;ε54Cr = − 0.60 ± 0.06）支持这种分类。在 NWA 8326 中观察到一些锆石聚集体，其中的晶粒在阴极发光 （CL） 图像中显示出核-地幔分区纹理，核心是深色且富含铝的，而地幔是明亮的且贫铝的。我们解释说 CL 暗核是源自真晶石的异晶锆石晶粒，其存在表明 NWA 8326 应该是通过 Vestan 地幔的部分熔化形成的，并同化了 eucritic 材料。异晶锆石和原生富硅、铝、钾玻璃的存在以及斜长石的巨大成分变化表明 NWA 8326 是一种不平衡的累积钙镁铁矿，因此锆石 207Pb/206Pb 年龄为 4559.2 ± 5.2 （2σ） 马代表 NWA 8326 的结晶。 将累积的纹理与化学析出的玻璃的存在相协调，NWA 8326 将在其结晶的后期被挖掘出来，并逃脱了真铜矿母体普遍存在的地壳热变质作用。NWA 8326 的 Mg 同位素组成高于大多数闪长岩，这表明这种鸽子石累积真方石的母体岩浆来自镁同位素组成较重的源区（μ25Mg：-90 至 -96 ppm）。