Advances in Crystal Growth: Experimental and Microscopic Study of Olivine Phosphorus Zoning

橄榄石是地球上最常见的矿物之一。它直接从岩浆中结晶，因此可以在地幔、岩浆室、熔岩流、爆发的火山产物和陨石中找到。最近的研究表明，来自所有这些环境的橄榄石晶体都含有一个富含磷(P)的树突核，而磷是橄榄石中通常不丰富的元素。这一发现挑战了对地幔和火山动力学以及陨石形成的传统理解。由于晶体的形状和化学成分对结晶条件（如温度、压力、岩浆的化学成分）很敏感，因此橄榄石中树突状富磷核的存在代表了这种矿物的一个重要但尚未完全了解的特征。通过实验室实验，包括晶体生长的实时观测，以及对橄榄石组成和结构的微纳米级分析，本提案中的工作努力推进对地球内部和太空中岩浆如何冷却和结晶的科学理解。以往的动态结晶实验表明，当岩浆快速冷却时，树状富p橄榄石可以快速生长。在这个项目中，实验范围扩展到在缓慢冷却速率下运行的岩浆过程，这对于深度环境更现实。这些实验的建立将通过(1)热梯度（模拟岩浆房和陨石的冷边缘）和(2)岩浆过热和成核延迟（例如绝热减压或闪热）促进橄榄石的快速生长。实验将在1-atm、气体混合炉和加热台显微镜下进行，并使用光学显微镜、场发射枪电子探针和透射电镜对实验产物进行分析。实验产品中P的x射线分布图将确定形成P带的最小驱动力（最小的热梯度和最小的过热），如在世界范围内的橄榄石和其他行星体中所见。透射电镜研究将记录橄榄石P带的微观结构，并为P和其他不相容元素在晶格中的结合和容纳提供新的见解。加热阶段的实验将允许直接观察热梯度和熔体过热对橄榄石生长的影响。它们还将使在海洋岛屿玄武岩熔体中进行新的增长率测量成为可能。对所有实验产品中枝晶臂间距的测量将允许构建参考生长图，以根据其晶体形态或富p区的质地估计天然晶体的生长速率。这些实验将完成已有的数据集，并将解决熔体结晶动力学和橄榄石晶体生长前沿的基本问题。研究结果将与火山、深部和地外系统有关。该项目连接了矿物学和岩石学领域，需要先进的材料表征，并且与火成岩岩石学密切相关。

Olivine is one of Earth's most common minerals. It crystallizes directly from magma, and hence can be found in Earth's mantle, magma chambers, lava flows, explosively-erupted volcanic products, and meteorites. Recent studies have shown that olivine crystals from all of these settings contain a dendritic core enriched in phosphorus (P), an element typically not abundant in olivine. This finding challenges the conventional understanding of mantle and volcano dynamics, and meteorite formation. Because the shape and chemical composition of crystals are sensitive to the conditions of crystallization (e.g., temperature, pressure, chemical composition of the magma), the presence of a dendritic, P-rich core in olivine represents an important but not yet fully understood feature of the mineral. Through laboratory experiments, including real-time observations of crystal growth, and micro-to-nano-scale analyses of composition and structure of olivine, the work in this proposal strives to advance the scientific understanding of how magmas are cooled and crystallize, both in Earth's interior and in space.Previous dynamic crystallization experiments have shown that dendritic P-rich olivine can be obtained through rapid growth when a magma is cooled rapidly. In this project, the range of experiments is extended to magmatic processes operating at slow cooling rates, which are more realistic for environments at depth. The set up of these experiments will promote rapid olivine growth through (1) a thermal gradient (mimicking the cold margins of magma chambers and meteorites) and (2) magma superheating and nucleation delay (e.g. by adiabatic decompression or flash heating). Experiments will be carried on in 1-atm, gas-mixing furnace and heating stage microscope, and the experimental products will be analyzed using optical microscopy, field-emission gun electron microprobe and transmission electron microscope. X-ray distribution mapping of P in the experimental products will determine the minimal driving forces (smallest thermal gradient and smallest superheating) for the formation of P zoning as seen in olivine worldwide and in other planetary bodies. The TEM investigations will document the microstructure of olivine P zoning and bring new insights on the incorporation and accommodation of P and other incompatible elements in the lattice. Heating stage experiments will allow direct observation of the effect of thermal gradients and melt superheating on the growth of olivine. They will also enable new growth rate measurements in a mafic ocean island basalt melt. Measurements of the dendrite arm spacing in all the experimental products will allow construction of a reference growth chart to estimate the growth rates of natural crystals from their crystal morphology or the texture of their P-rich zones. These experiments will complete pre-existing datasets and will address topics fundamental to crystallization kinetics in melts and frontiers in olivine crystal growth. The results will be relevant to volcanic, plutonic and extraterrestrial systems. The project bridges the fields of mineralogy and petrology, requires advanced materials characterization, and is strongly relevant to igneous petrology.