Thermal shock fragmentation of Mg silicates within scoriaceous micrometeorites reveal hydrated asteroidal sources
Thermal shock fragmentation of Mg silicates within scoriaceous micrometeorites reveal hydrated asteroidal sources
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矿渣微陨石中硅酸镁的热冲击破碎揭示了水合小行星的来源
DOI：
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发表时间：
2017
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通讯作者：
M. Ginneken
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作者：
M. Genge;M. Suttle;M. Ginneken
蝎子状微陨石是高度气泡的外星尘埃颗粒，在进入大气时经历了部分融化。我们报告了在这些颗粒中出现的无面体残余镁橄榄石晶体簇，这证明了原位碎裂。未熔化的微陨石中没有类似的星团，这表明碎裂发生在进入大气期间，而不是母体激波再处理过程中。破碎的镁橄榄石晶体簇被证明是由于入口加热过程中产生的热应力而破裂的结果，需要>；200Kµm-1的温度梯度才能使差热膨胀超过橄榄石的临界剪切强度。这种量级的温度梯度大大超过了热传导产生的温度梯度，需要层状硅酸盐的吸热分解。因此，碎屑状微陨石中残留的镁橄榄岩表明，其前体颗粒类似于CI和CM2球粒陨石，并在进入大气之前保留了层状硅酸盐，因此没有受到冲击或热变质作用在母小行星上的脱水。因此，含水小行星在碰撞过程中的爆炸性碎裂并不会显著影响行星际尘埃的数量。细粒MMS上的火成岩边缘支撑着类似于皂石和蛇纹石。通过与Llana-Fúnez等人(2007)的数据进行拟合，得到了使用521kJ-1a频率因子A4.3×10 24的An活化能Ea实验确定的反应速率。利用蛇纹石在每个时间步骤的浓度变化来确定脱水所消耗的能量，使用414kJ kg-1至3的反应热，其中C是，并且是反应级数。实验研究表明，和之间的顺序不同(Llana-Fúnez等人，2007年)。在目前的研究中假定了一个值。模型采用隐式数值积分方法，时间步长为1×10−9，单元长度为0.0 1µm，不考虑气体或熔体的迁移换热，但不考虑导热的影响。
Scoriaceous micrometeorites are highly vesicular extraterrestrial dust particles that have experienced partial melting during atmospheric entry. We report the occurrence of clusters of anhedral relict forsterite crystals within these particles that testify to in situ fragmentation. The absence of similar clusters within unmelted micrometeorites suggests that fragmentation occurs during atmospheric entry rather than by parent body shock reprocessing. Clusters of broken forsterite crystals are shown to form as a result of fracturing owing to thermal stress developed during entry heating and require thermal gradients of >200 K µm –1 in order for differential thermal expansion to exceed the critical shear strength of olivine. Thermal gradients of this magnitude significantly exceed those resulting from thermal conduction and require the endothermic decomposition of phyllosilicates. Fragmented relict forsterite within scoriaceous micrometeorites, therefore, indicate that the precursor grains were similar to CI and CM2 chondrites and retained phyllosilicate prior to atmospheric entry and thus were not dehydrated on the parent asteroid by shock or thermal metamorphism. Explosive fragmentation of hydrous asteroids during collisions, therefore, does not significantly bias the interplanetary dust population. of igneous rims on fine-grained MMs supports similar of saponite and serpentine. Experimentally determined reaction rates using the an activation energy E a of 521 kJ –1 a frequency factor A 4.3 10 24 were obtained by fitting to the data of Llana-Fúnez et al., (2007). The change in concentration of serpentine at each time step was used to determine the energy dissipated by dehydration using an enthalpy of reaction of 414 kJ kg –1 via 3 where C is the and is the order of reaction. Experimental studies suggest that the order of varies between and (Llana-Fúnez et al., 2007). A value was assumed in the current study. The transient heat flow was simulated using an implicit numerical integration with a time step of 1 × 10 −9 s and an element of 0.01 µm. Heat transfer by gas or melt migration is not considered in the model but is likely to be less important than conduction.