太阳前灰云母的透射电镜研究
A transmission electron microscopy study of presolar hibonite
摘要：
我们报告的Krymka LL3.1普通球粒陨石的酸残留物中确定的五个presolar黑云母颗粒的同位素和显微结构数据。二次离子质谱法（SIMS）的同位素测量证实了这些颗粒的太阳系前拱星起源。通过用聚焦离子束扫描电子显微镜原位切片和剥离，能够对晶粒的晶体结构和化学进行透射电子显微镜（TEM）检查。同位素组成与模型的比较表明，五颗颗粒中有四颗形成于通过红巨星/渐近巨星分支演化的低质量恒星，而一颗颗粒形成于II型超新星的喷出物。选区电子衍射图表明，所有的晶粒是黑云母的单晶。一些晶粒包含堆垛层错和小的取向扩展，这可以归因于生长缺陷和通过晶粒-晶粒碰撞的机械加工的组合。超新星颗粒的结构与RGB/AGB星的结构相似，表明其形成条件相似。辐射损伤，如果存在的话，发生在我们的检测极限以下。在我们研究的五种颗粒中，只有一种具有CaAl 12 O 19的纯黑云母成分。所有其他的都含有少量的Mg、Si、Ti和Fe。微结构数据通常与理论预测一致，理论预测将拱星冷凝温度限制在1480 K至1743 K的范围内，假设相应的总气压在1 x 10-3和1 x 10-6 atm之间。TEM数据被用来开发一个校准SIMS测定氧化物颗粒中的Ti含量。与极端的18 O耗尽，表明深混合发生在其母星AGB恒星的颗粒，是轻微的钛富集相比，从没有深混合的恒星的颗粒，最有可能反映在颗粒凝聚条件的差异。
英文摘要：
We report isotopic and microstructural data on five presolar hibonite grains identified in an acid residue of the Krymka LL3.1 ordinary chondrite. Isotopic measurements by secondary ion mass spectrometry (SIMS) verified a presolar circumstellar origin for the grains. Transmission electron microscopy (TEM) examination of the crystal structure and chemistry of the grains was enabled by in situ sectioning and lift-out with a focused-ion-beam scanning-electron microscope. Comparisons of isotopic compositions with models indicate that four of the five grains formed in low-mass stars that evolved through the red-giant/asymptotic-giant branches, whereas one grain formed in the ejecta of a Type II supernova. Selected-area electron-diffraction patterns show that all grains are single crystals of hibonite. Some grains contain stacking faults and small spreads in orientation that can be attributed to a combination of growth defects and mechanical processing by grain-grain collisions. The similar structure of the supernova grain to those from RGB/AGB stars indicates a similarity in the formation conditions. Radiation damage, if present, occurs below our detection limit. Of the five grains we studied, only one has the pure hibonite composition of CaAl12O19. All others contain minor amounts of Mg, Si, Ti, and Fe. The microstructural data are generally consistent with theoretical predictions, which constrain the circumstellar condensation temperature to a range of 1480 K to 1743 K, assuming a corresponding total gas pressure between 1 x 10-3 and 1 x 10-6 atm. The TEM data were used to develop a calibration for SIMS determination of Ti contents in oxide grains. Grains with extreme 18O depletions, indicating deep mixing has occurred in their parent AGB stars, are slightly Ti-enriched compared to grains from stars without deep mixing, most likely reflecting differences in grain condensation conditions.
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