In situ observation of crystal growth in a basalt melt and the development of crystal size distribution in igneous rocks
In situ observation of crystal growth in a basalt melt and the development of crystal size distribution in igneous rocks
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DOI：
10.1007/s00410-014-1003-9
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发表时间：
2014-04
影响因子：
3.5
通讯作者：
H. Ni;H. Keppler;N. Walte;F. Schiavi;Yang Chen;M. Masotta;Zhenjiang Li
中科院分区：
文献类型：
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作者：
H. Ni;H. Keppler;N. Walte;F. Schiavi;Yang Chen;M. Masotta;Zhenjiang Li
为了了解天然岩浆的凝固过程和火成岩的结构演化，我们对莫桑石晶胞中从约 1,240 °C 冷却的高 K 玄武岩熔体的结晶进行了现场观察。在一系列不同热历史的实验中，橄榄石或单斜辉石（cpx）在斜长石形成之前作为液相出现。在 100 °C/h 的冷却过程中，橄榄石和 cpx 的形态从板状转变为料斗状。对于一阶近似，晶体生长速率（橄榄石为 2 × 10−9 至 7 × 10−9m/s，cpx 为 6 × 10−9 至 17 × 10−9m/s），可能受到化学扩散的限制，与晶体尺寸成正比。在一项以橄榄石结晶为主的实验中，良好的图像质量允许分析长时间内的纹理演变。橄榄石的成核仅发生在液相线以下的狭窄温度和时间间隔内。基于长度和面积的二维晶体尺寸分布 (CSD) 显示围绕 8 μm 和 100 μm2 轴逆时针旋转，这与成比例的晶体生长一致。 CSD 和直接观察都显示小晶体的溶解和奥斯特瓦尔德熟化。这些数据表明，对火成岩晶体尺寸分布的常规分析可能是错误的——CSD的斜率不能用均匀的生长速率来解释，并且与垂直轴的截距并不对应于成核密度。
To understand the solidification processes of natural magma and the texture evolution of igneous rocks, we have carried out in situ observation of the crystallization of a high-K basaltic melt cooling from ~1,240 °C in a moissanite cell. In a series of experiments with different thermal history, olivine or clinopyroxene (cpx) appeared as the liquidus phase before the formation of plagioclase. During cooling at 100 °C/h, the morphology of olivine and cpx transited from tabular to hopper habit. To first order approximation, crystal grow rate (2 × 10−9to 7 × 10−9m/s for olivine and 6 × 10−9to 17 × 10−9m/s for cpx), probably limited by chemical diffusion, is proportional to crystal size. In one experiment dominated by olivine crystallization, the good image quality allows the analysis of texture evolution over an extended period. Nucleation of olivine occurred only in a narrow temperature and time interval below the liquidus. Two-dimensional length- and area-based crystal size distributions (CSDs) show counterclockwise rotation around axes of 8 μm and 100 μm2, which is consistent with the proportionate crystal growth. Both CSDs and direct observation show the dissolution of small crystals and Ostwald ripening. These data suggest that conventional analyses of crystal size distributions of igneous rocks may be in error—the slope of the CSD cannot be interpreted in terms of a uniform growth rate, and the intercept with the vertical axis does not correspond to a nucleation density.