Melt Diffusion-Moderated Crystal Growth and its Effect on Euhedral Crystal Shapes
Melt Diffusion-Moderated Crystal Growth and its Effect on Euhedral Crystal Shapes
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熔体扩散慢化晶体生长及其对自形晶体形状的影响
DOI：
10.1093/petrology/egad054
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发表时间：
2023
影响因子：
3.9
通讯作者：
Mangler M
中科院分区：
文献类型：
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作者：
Mangler M
晶体生长通常被描述为界面控制或扩散控制。在这里，我们研究了晶体生长的中间场景，其中晶体-熔体界面的反应速率类似于离子通过熔体向前进的晶体表面扩散传输的速率。为此，我们实验研究了干燥的镁铁质(玄武岩)和水合硅质(单长岩质)熔体中的正面体斜长石晶体形态。在镁铁质熔体和硅质熔体中，3d短晶(S)和中晶(I)的长径比和相对生长速率差异很大，玄武岩中的δS：δI= 1：6~1：20，水合单安岩中的1：2.5~1：8。硅质熔体中生长的斜长石的长宽比较低，与镁铁质熔体中的熔体扩散速率低10~100倍相吻合。利用各向异性生长模型，我们发现熔体扩散系数的这种差异可以解释斜长石长宽比的差异：如果界面反应和熔体扩散速率相似，则界面反应速率较高的晶面的生长可能受到熔体扩散的限制，而界面反应速率较低的同一晶体的另一面可能不受熔体扩散系数的限制。熔体扩散对晶体生长速度的选择性控制导致随着扩散系数的降低，逐渐产生更均匀的晶体形状，这与我们的实验观察一致。重要的是，在这种扩散控制的中间生长区域中形成的晶体可能不会表现出任何经典的扩散控制生长特征。所提出的模型适用于斜长石微岩，但应适用于火山岩中所有各向异性的微岩生长。
Crystal growth is often described as either interface-controlled or diffusion-controlled. Here, we study crystal growth in an intermediate scenario where reaction rates at the crystal-melt interface are similar to the rates of diffusive transport of ions through the melt to the advancing crystal surface. To this end, we experimentally investigated euhedral plagioclase crystal shapes in dry mafic (basaltic) and hydrous silicic (haplodacitic) melts. Aspect ratios and inferred relative growth rates of the 3D short (S) and intermediate (I) crystal dimensions vary significantly between mafic and silicic melts, withδS:δI= 1:6–1:20 in basalt and 1:2.5–1:8 in hydrous haplodacite. The lower aspect ratios of plagioclase grown in the silicic melt coincide with 10 to 100× lower melt diffusion rates than in the mafic melt. Using an anisotropic growth model, we show that such differences in melt diffusivity can explain the discrepancy in plagioclase aspect ratios: if interface reaction and melt diffusion rates are of similar magnitude, then the growth of a crystal facet with high interfacial reaction rates may be limited by melt diffusion, while another facet of the same crystal with lower interfacial reaction rates may grow uninhibited by melt diffusivity. This selective control of melt diffusion on crystal growth rates results in progressively more equant crystal shapes as diffusivity decreases, consistent with our experimental observations. Importantly, crystals formed in this diffusion-moderated, intermediate growth regime may not show any classical diffusion-controlled growth features. The proposed model was developed for plagioclase microlites but should be generalisable to all anisotropic microlite growth in volcanic rocks.