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Crystal growth of clinopyroxene in mafic alkaline magmas
Earth and Planetary Science Letters ( IF 5.3 ) Pub Date : 2021-06-04 , DOI: 10.1016/j.epsl.2021.117005
Jin-Sheng Zhou , Qiang Wang , Chang-Ming Xing , Lin Ma , Lu-Lu Hao , Qi-Wei Li , Zi-Long Wang , Tong-Yu Huang

Recent experimental and natural studies have shown that igneous crystals can start to crystallize through skeletal or dendritic growth, and then backfill progressively to form polyhedral crystal morphologies, in contrast to the traditional tree-ring model of crystal growth. Identifying the growth mechanism of crystal is fundamental to understand crystal growth and associated magmatic processes. Clinopyroxene is an ubiquitous phase in magmatic systems. However, crystallographically controlled zoning patterns (e.g., sector zoning) observed in different studies are diverse or even show opposite trends, which make it challenging to understand the crystal growth mechanisms of clinopyroxene. This study presents a new finding of the contemporary occurrence of both clinopyroxenes with Al-rich and -poor {hk0} sectors in the Sailipu alkaline volcanic field in southern Tibet. Based on detailed textural and compositional analyses of a large number of clinopyroxene grains, we suggest a new crystal growth mechanism for clinopyroxene in mafic alkaline magmas. At low degrees of undercooling, all crystal faces grow concurrently, producing equilibrium Si–Mg-rich {111} sectors and disequilibrium Al–Ti-rich {hk0} sectors; at high degrees of undercooling, the equilibrium sectors {hk0} initially form, which is followed by backfill of disequilibrium Al-rich {111} sectors. Al2O3 is a useful indicator of equilibrium, and Al2O3-poor sectors, rather than specific crystal faces, are closer to equilibrium. The behavior of other cations is complex and the distribution of their oxide components is variable. Textural observations and the modeling results for P transfer across the crystal–melt diffusive boundary layer show that apatite inclusions in clinopyroxene can indicate rapid crystal growth. These results may contribute to a better understanding of magma storage, ascent and eruption on the basis of clinopyroxene.



中文翻译:

镁铁质碱性岩浆中单斜辉石的晶体生长

最近的实验和自然研究表明,火成岩晶体可以通过骨架或树枝状生长开始结晶,然后逐渐回填以形成多面体晶体形态,这与晶体生长的传统树轮模型形成对比。确定晶体的生长机制是理解晶体生长和相关岩浆过程的基础。斜辉石是岩浆系统中普遍存在的相。然而,在不同研究中观察到的晶体学控制的分区模式(例如扇形分区)是多种多样的,甚至显示出相反的趋势,这使得理解单斜辉石的晶体生长机制具有挑战性。这项研究提出了同时存在富铝和贫铝的单斜辉石的新发现。{H0}藏南赛里铺碱性火山区的板块。基于对大量单斜辉石颗粒的详细结构和成分分析,我们提出了一种新的单斜辉石在基性碱性岩浆中的晶体生长机制。在低过冷度下,所有晶面同时生长,产生平衡的富硅镁{-111} 部门和不平衡 富铝钛 {H0}部门;在高度过冷度下,平衡扇区{H0} 最初形成,然后回填不平衡的富铝 {-111}部门。Al 2 O 3是平衡的有用指标,并且Al 2 O 3贫乏的扇区,而不是特定的晶面,更接近于平衡。其他阳离子的行为很复杂,它们的氧化物组分的分布是可变的。晶体-熔体扩散边界层上 P 转移的纹理观察和建模结果表明,斜辉石中的磷灰石包裹体可以表明晶体快速生长。这些结果可能有助于更好地了解基于单斜辉石的岩浆储存、上升和喷发。

更新日期:2021-06-05
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