星子中的岩浆上升：受粒度控制
Magma ascent in planetesimals: control by grain size
摘要：
早期太阳系中的岩石状微行星由于Al-26的辐射加热而内部熔化并进行化学演化。在这里，我们量化的参数控制岩浆成因和运输使用一个耦合的岩石学和流体力学模型的反应两相流。我们发现硅酸盐矿物的平均粒度是岩浆上升的关键控制。对于大于$\approx$ 1 mm的晶粒尺寸，熔体偏析产生明显的径向结构和化学分层。这种分层是最明显的机构形成后，形成约1万年的钙，铝丰富的夹杂物。这些发现表明，星子形成的时间和轨道位置与其随后的结构和化学演化之间存在联系。根据我们的模型，部分熔融的星子内部的演化分为两类。在岩浆海洋情景中，整个星子内部经历几乎完全的熔融，这将导致湍流对流和核幔分异的降雨机制。在岩浆岩床的情况下，分离熔体逐渐耗尽的放射性热源的内部深处。在这种情况下，岩浆可能会在一个凉爽而稳定的盖子下形成富含熔体的层，而核心的形成将通过渗滤进行。我们的研究结果表明，晶粒尺寸普遍在内部加热阶段的岩浆上升的星子。无论是否朝着岩浆海洋或岩浆岩床结构的演化进展，我们的模型预测，由于Al-26的快速再分布的温度反演仅限于早于$\approx$ 1百万年后CAIs形成的机构。我们发现，如果晶粒尺寸小于$\approx$ 1毫米，在峰值内部熔化，只有升高的固体熔体密度对比（如顽火辉石球粒陨石组合物）将允许大量的熔体偏析发生。
英文摘要：
Rocky planetesimals in the early solar system melted internally and evolved chemically due to radiogenic heating from Al-26. Here we quantify the parametric controls on magma genesis and transport using a coupled petrological and fluid mechanical model of reactive two-phase flow. We find the mean grain size of silicate minerals to be a key control on magma ascent. For grain sizes larger than $\approx$ 1 mm, melt segregation produces distinct radial structure and chemical stratification. This stratification is most pronounced for bodies formed at around 1 Myr after formation of Ca,Al-rich inclusions. These findings suggest a link between the time and orbital location of planetesimal formation and their subsequent structural and chemical evolution. According to our models, the evolution of partially molten planetesimal interiors falls into two categories. In the magma ocean scenario, the whole interior of a planetesimal experiences nearly complete melting, which would result in turbulent convection and core-mantle differentiation by the rainfall mechanism. In the magma sill scenario, segregating melts gradually deplete the deep interior of the radiogenic heat source. In this case, magma may form melt-rich layers beneath a cool and stable lid, while core formation would proceed by percolation. Our findings suggest that grain sizes prevalent during the internal heating stage governed magma ascent in planetesimals. Regardless of whether evolution progresses toward a magma ocean or magma sill structure, our models predict that temperature inversions due to rapid Al-26 redistribution are limited to bodies formed earlier than $\approx$ 1 Myr after CAIs. We find that if grain size was smaller than $\approx$ 1 mm during peak internal melting, only elevated solid-melt density contrasts (such as found for enstatite chondrite compositions) would allow substantial melt segregation to occur.
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