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arXiv 1402.3103astro-ph.EP
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灶神星的分异:浅岩浆洋的意义

Differentiation of Vesta: Implications for a shallow magma ocean

Wladimir Neumann, Doris Breuer, Tilman Spohn

摘要:

黎明号任务证实了小行星4灶神星具有富铁核心,硅酸盐地幔和玄武岩地壳的预测,并确认灶神星是HED陨石的母体。为了更好地了解它的早期演化,我们采用黎明任务获得的新数据,如小行星的质量,体积密度和大小,对热化学演化进行了数值计算。我们扩展了Neumann等人(2012)的模型,该模型包括吸积,压实,熔化和相关的材料性质变化和26 Al的分配,平流热传输和多孔流的分异,以包括岩浆海洋中的对流和有效冷却。根据熔体分数,熔体偏析的热传输是由多孔流动或对流和热通量的岩浆海洋具有高的有效导热系数建模。我们发现,分配的26铝和硅酸盐熔体的运输是至关重要的岩浆海洋的形成。由于26 Al在次表层的聚集(形成时间t0<1.5Ma),形成了厚度为1至几十km(取决于硅酸盐熔体粘度)的浅岩浆海洋。浅岩浆洋的寿命为O(10^4)-O(10^6)年,该层的对流伴随着26 Al在表面的挤出。内部与外部向内的地幔是亏损的26铝和核心的形成完成约0.3 Ma。下地幔经历熔融低于45%,表明方辉橄榄岩到纯橄榄岩组成。研究结果支持了早期部分熔融挤压形成的真长英晶岩和闪长英晶岩可能来自于结晶的浅岩浆海。硅酸盐熔体是目前高达150马,和核心对流约100马,支持早期磁场的想法。

英文摘要:

The Dawn mission confirms predictions that the asteroid 4 Vesta is differentiated with an iron-rich core, a silicate mantle and a basaltic crust, and confirms Vesta as the parent body of the HED meteorites. To better understand its early evolution, we perform numerical calculations of the thermo-chemical evolution adopting new data obtained by the Dawn mission such as mass, bulk density and size of the asteroid. We have expanded the model of Neumann et al. (2012) that includes accretion, compaction, melting and associated changes of material properties and partitioning of 26Al, advective heat transport, and differentiation by porous flow, to include convection and effective cooling in a magma ocean. Depending on the melt fraction, the heat transport by melt segregation is modelled either by porous flow or by convection and heat flux of a magma ocean with a high effective thermal conductivity. We show that partitioning of 26Al and its transport with the silicate melt is crucial for the formation of a magma ocean. Due to the accumulation of 26Al in the sub-surface (for formation times t0<1.5 Ma), a shallow magma ocean with a thickness of 1 to a few tens of km (depending on the silicate melt viscosity) forms. The lifetime of the shallow magma ocean is O(10^4)-O(10^6) years and convection in this layer is accompanied by the extrusion of 26Al at the surface. The interior differentiates from the outside inward with a mantle that is depleted in 26Al and core formation is completed within ~0.3 Ma. The lower mantle experiences melting below 45% suggesting a harzburgitic to dunitic composition. Our results support the formation of eucrites by the extrusion of early partial melt and cumulative eucrites and diogenites may form from the crystallizing shallow magma ocean. Silicate melt is present for up to 150 Ma, and core convects for ~100 Ma, supporting the idea of an early magnetic field.

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