玄武质陨石NWA 8545的岩石地球化学特征以及对其母体岩浆过程的启示

Petrological and geochemical characteristics of the basaltic achondrite NWA 8545and implications for the magmatic process on its parent body

NWA 8545是一块玄武质无球粒陨石,它与碳质陨石(CC)NWA 011成对。CC被认为是来自于外太阳系的一类物质,由于同位素异常,它们区别于来自内太阳系的非碳质陨石(NC)。NWA 011及其成对陨石作为CC中稀有的玄武质无球粒陨石,其记录的岩浆过程可以被用来研究外太阳系早期行星母体的岩浆活动。本文利用扫描电镜、电子探针和激光剥蚀电感耦合等离子质谱仪(LA-ICP-MS)对NWA 8545中的辉石、斜长石和陨磷钙钠石进行岩相学以及原位主微量元素的分析,并根据矿物模式丰度计算全岩稀土元素含量。电子探针结果显示NWA 8545与HED族陨石Eucrite(钙长辉长无球粒陨石)具有相似的主量元素特征,同时其岩相学与5型Eucrite类似。激光微量数据表明辉石、斜长石和陨磷钙钠石的稀土元素配分都表现出略微的Ce异常,但其辉石的Ba、Sr等元素并未出现明显的富集现象,即该陨石受地球风化作用影响不明显。利用辉石和斜长石的稀土元素含量,计算平衡熔体的成分,显示其平衡熔体的成分都与全岩的成分比较接近,可以认为两者是在封闭的体系下接近同时结晶。结合变质过程和母体岩浆的成分,本文认为NWA 8545是由其母体岩浆在经历分离结晶过程后喷发到母体行星表面冷却形成的。

The NWA 8545 is a basaltic achondrite paired with the carbonaceous achondrite NWA 011. Carbonaceous meteorites(CCs) are considered to have formed in the outer solar system and are distinguished from the Non-carbonaceous meteorites(NCs) that come from the inner solar system using isotopic anomalies. The NWA 8545 and its paired meteorites are the only groups of basaltic achondrites that might record the magmatic process on their parent body. Here we report the petrology and mineralogy of the NWA 8545 obtained using scanning electron microscope(SEM), electron-probe microanalysis(EMPA), and laser ablation-inductively coupled plasma-mass spectrometer(LA-ICP-MS). Based on mineral modal abundances, this work calculated the whole-rock REE concentrations. The result demonstrated that the NWA 8545 and Eucrites have similar major elemental and REE distribution characteristics. All minerals(pyroxene, plagioclase, and merrillite) exhibit positive Ce anomaly. However, Ba and Sr are not enriched in pyroxene, which means that the meteorite suffered only limited terrestrial weathering. The thermal metamorphism degree of the NWA 8545 is similar to Type 5 Eucrite. The REE contents of the equilibrium magma of pyroxene and plagioclase are similar to the bulk REE concentrations of the NWA 8545, which implies that pyroxene and plagioclase may crystallize simultaneously in a closed system. Combining the metamorphism process and the magmacontent, this work implies that the NWA 8545 may have erupted from its parent body after fractional crystallization and crystallized rapidly near the surface.