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首页> 外文期刊>Contributions to Mineralogy and Petrology >An experimental study of Ostwald ripening of olivine and plagioclase in silicate melts: implications for the growth and size of crystals in magmas
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An experimental study of Ostwald ripening of olivine and plagioclase in silicate melts: implications for the growth and size of crystals in magmas

机译：硅酸盐熔体中橄榄石和斜长石的Ostwald成熟的实验研究：对岩浆中晶体的生长和大小的影响

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摘要

We carried out an experimental study to characterize the kinetics of Ostwald ripening in the forsterite-basalt system and in the plagioclase (An65)-andesite system. Eight experiments were done in each system to monitor the evolution of mean grain size and crystal size distribution (CSD) with time t; the experiments were performed in a 1-atmosphere quench furnace, at 1,250°C for plagioclase and 1,300°C for olivine. Very contrasted coarsening kinetics were observed in the two series. In the plagioclase series, the mean grain size increased as log(t), from ≈3 μm to only 8.7 μm in 336 h. The kinetic law in log(t) means that Ostwald ripening was rate-limited by surface nucleation at plagioclase-liquid interfaces. In the olivine series, the mean grain size increased as t 1/3, from ≈3 μm to 23.2 μm in 496 h. A kinetic law in t 1/3 is expected when Ostwald ripening is rate-limited either by diffusion in the liquid or by grain growth/dissolution controlled by a screw dislocation mechanism. The shape of olivine CSDs, in particular their positive skewness, indicates that grain coarsening in the olivine experiments was controlled by a screw dislocation mechanism, not by diffusion. As the degrees of undercooling ΔT (or supersaturation) involved in Ostwald ripening are essentially <1°C, the mechanisms of crystal growth identified in our experiments are expected to be those prevailing during the slow crystallisation of large magma chambers. We extrapolated our experimental data to geological time scales to estimate the effect of Ostwald ripening on the size of crystals in magmas. In the case of plagioclase, Ostwald ripening is only efficient for mean grain sizes of a few microns to 20 μm, even for a time scale of 105 years. It can, however, result in a significant decrease of the number of small crystals per unit volume, and contribute to the development of convex upwards CSDs. For olivine, the mean grain size increases from 2–3 μm to ≈70 μm in 1 year and 700 μm in 103 years; a mean grain size of 3 mm is reached in 105 years. Accordingly, the rate of grain size-dependent processes, such as compaction of olivine-rich cumulates or melt extraction from partially molten peridotites, may significantly be enhanced by textural coarsening.
机译：我们进行了一项实验研究，以表征镁橄榄石-玄武岩系统和斜长石（An65 ）-安山岩系统中Ostwald熟化的动力学。在每个系统中进行了八次实验，以监测平均晶粒尺寸和晶体尺寸分布（CSD）随时间t的变化。实验是在1个大气压淬火炉中进行的，斜长石在1,250°C，橄榄石在1,300°C。在两个系列中观察到非常相反的粗化动力学。在斜长石系列中，平均晶粒尺寸随log（t）的增加而增加，在336小时内从约3μm增加到仅8.7μm。 log（t）中的动力学定律意味着，Ostwald成熟受斜长石-液体界面表面成核的速率限制。在橄榄石系列中，平均晶粒度增加了t 1 / 3sup，在496 h内从≈3μm增加到23.2μm。当奥斯特瓦尔德熟化受液体中的扩散或受螺杆位错机制控制的晶粒长大/溶解的速率限制时，可以预期其动力学规律为t 1/3 。橄榄石CSD的形状，特别是它们的正偏度，表明在橄榄石实验中晶粒的粗化是通过螺钉位错机制而不是通过扩散来控制的。由于与奥斯特瓦尔德（Ostwald）熟化有关的过冷度ΔT（或过饱和）基本上<1°C，因此在我们的实验中确定的晶体生长机理有望是大型岩浆室缓慢结晶过程中普遍存在的机理。我们将实验数据外推至地质时标，以估算奥斯特瓦尔德熟化对岩浆中晶体大小的影响。对于斜长石而言，奥斯特瓦尔德熟化仅对于几微米至20μm的平均晶粒尺寸有效，即使时间尺度为105 。但是，这可能导致每单位体积的小晶体数量显着减少，并有助于形成向上凸起的CSD。对于橄榄石，平均晶粒尺寸在1年内从2-3μm增加到≈70μm，在103年内增加到700μm。在105年内平均晶粒尺寸达到3毫米。因此，通过晶粒粗化可以显着提高晶粒尺寸依赖性过程的速率，例如富含橄榄石的堆积物的压实或从部分熔融橄榄岩的熔融提取。

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  • 来源
    《Contributions to Mineralogy and Petrology》 |2005年第1期|37-53|共17页
  • 作者

    Hugues Cabane; Didier Laporte; Ariel Provost;

  • 作者单位

    Laboratoire Magmas et Volcans CNRS/Université Blaise Pascal/IRD OPGCSaint-Gobain Cristaux et Détecteurs BP 521;

    Laboratoire Magmas et Volcans CNRS/Université Blaise Pascal/IRD OPGC;

    Laboratoire Magmas et Volcans CNRS/Université Blaise Pascal/IRD OPGC;

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