摘要
橄榄陨铁作为太阳系早期分异小行星的碎片,记录了母体深部核幔分异过程,其成因研究对重建太阳系行星分异历史具有重要的指示意义.五龙陨石大约于1992年在内蒙古被发现,但直到2020年才被学界关注.本文对该陨石进行了详细的岩相学、矿物化学和同位素地球化学研究.五龙陨石的矿物模式丰度为:橄榄石(～72.7 vol.％)、金属(～26.2 vol.％)、陨硫铁(～0.6 vol.％)、陨磷铁镍石(～0.4 vol.％)、铬铁矿(～0.1 vol.％)等.橄榄石(Fa12.5±0.2)和铬铁矿(Al#～15.2±0.9、Fe#～67.9±1.5)颗粒成分均一,未发现成分环带.Fe-Ni金属成分:Ni-7.71 wt.％、Ga-17.8 μg/g、Ge-26.2 μg/g.橄榄石δ18O(δ表示样品中的同位素比值与标样中的同位素比值的相对偏差)为3.04±0.01‰、△17O(△代表同位素分馏值)为-0.18±0.01‰.综合五龙陨石的岩相结构特征、矿物化学成分、Fe-Ni金属亲铁元素含量及橄榄石氧同位素组成,确定其为新发现的主群橄榄陨铁.
Abstract
As the fragments of differentiated asteroids in the early solar system,pallasites record deep-mantle and core processes.As such,studying the origin of pallasites could help us understand the formation and differentiation processes governing the evolution of planets.Wulong pallasite was found in Inner Mongolia,China around 1992,but didn't get much attention until 2020.In this study,we reported the detailed petrography and mineralogy,oxygen isotopic composition and metal phase chemistry of Wulong pallasite.Wulong pallasite is mainly composed of olivine(～72.7 vol.％),Fe-Ni metal(～26.2 vol.％),troilite(～0.6 vol.％),schreibersite(～0.4 vol.％),chromite(～0.1 vol.％).Olivine(Fa12.5±0.2)and chromite(Al#～15.2±0.9、Fe#～67.9±1.5)composition are homogeneous and no chemical zoning is observed.Siderophile element contents of a metal phase are Ni,7.71 wt.％;Ga,17.8 μg/g;Ge,26.2 μg/g.The meanδ18O value is 3.04±0.01‰,Δ17O value is-0.18±0.01‰.In terms of the Fa value of olivine,metal phase chemistry and oxygen isotope ratios,Wulong could be a new member of main group pallasite.
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