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Pre-nucleation geochemical heterogeneity within glassy anatectic inclusions and the role of water in glass preservation
Contributions to Mineralogy and Petrology ( IF 3.5 ) Pub Date : 2021-08-21 , DOI: 10.1007/s00410-021-01826-0
Tommaso Tacchetto 1, 2 , Steven M. Reddy 1, 2 , Denis Fougerouse 1, 2 , Chris Clark 1 , William D. A. Rickard 2 , David W. Saxey 2 , Omar Bartoli 3 , Zakaria Quadir 4
Affiliation  

Glassy melt inclusions are unique geological repositories that preserve evidence of the formation and evolution of mantle and crustal-derived magmas. However, the mechanisms responsible for their preservation in slowly cooled crustal rocks remain contentious, in some part due to their small size (commonly < 10 µm) and the technical difficulty in quantifying composition and microstructures. In this work, time-of-flight secondary ion mass spectrometry, transmission electron microscopy and atom probe tomography are used to characterize glassy melt inclusions found in peritectic garnets of a migmatite from the Spanish Betic Cordillera. The glassy melt inclusions coexist in a close spatial relationship with partially to totally crystallized melt inclusions (nanogranitoids). Analyses of the glassy inclusions show a heterogeneous, patchy distribution of Na and K within the glass and along inclusion walls. Nanoscale spherical domains of Al, Fe, K, Na, Cl and Li are also found systematically distributed at inclusion edges, and are interpreted to represent pre-nucleation clusters. The location and compositional similarity of these clusters with micas and feldspars in nanogranitoids indicate that the glassy inclusions represent former nanogranitoids “captured” at an earlier stage of crystallization, suggesting a likely common origin for both the glassy inclusions and nanogranitoids. A comparison between the composition of melt inclusions with previously published data reveals that preserved glassy inclusions contain significant less H2O (av. 2.72 wt%) than nanogranitoids (average of 6.91 wt%). This suggests the low-H2O content representing a further impediment to crystallization, along with the very small volume of these cavities, favouring the coexistence of glassy inclusions and nanogranitoids. In contrast, crystal nucleation is enhanced in more hydrous melts, where H2O reduces melt viscosity and promotes diffusion.



中文翻译:

玻璃深熔包裹体中的预成核地球化学异质性和水在玻璃保存中的作用

玻璃状熔体包裹体是独特的地质储藏库,保存了地幔和地壳衍生岩浆形成和演化的证据。然而,在缓慢冷却的地壳岩石中保存它们的机制仍然存在争议,部分原因是它们的尺寸小(通常 < 10 µm)以及量化成分和微观结构的技术难度。在这项工作中,飞行时间二次离子质谱、透射电子显微镜和原子探针断层扫描被用来表征在西班牙贝蒂科迪勒拉混合岩的包晶石榴石中发现的玻璃状熔体包裹体。玻璃状熔融包裹体与部分至完全结晶的熔融包裹体(纳米花岗岩)以密切的空间关系共存。玻璃状内含物的分析表明,Na 和 K 在玻璃内和沿着夹杂物壁呈片状分布。还发现 Al、Fe、K、Na、Cl 和 Li 的纳米级球形域系统地分布在夹杂物边缘,并被解释为代表预成核簇。这些簇与纳米花岗岩中的云母和长石的位置和组成相似性表明,玻璃状包裹体代表了在早期结晶阶段“捕获”的前纳米花岗岩,这表明玻璃状包裹体和纳米花岗岩可能具有共同的起源。将熔体夹杂物的组成与先前公布的数据进行比较表明,保存下来的玻璃状夹杂物含有显着较少的 H 还发现 Cl 和 Li 系统地分布在夹杂物边缘,并被解释为代表预成核簇。这些簇与纳米花岗岩中的云母和长石的位置和组成相似性表明,玻璃状包裹体代表了在早期结晶阶段“捕获”的前纳米花岗岩,这表明玻璃状包裹体和纳米花岗岩可能具有共同的起源。将熔体夹杂物的组成与先前公布的数据进行比较表明,保存下来的玻璃状夹杂物含有显着较少的 H 还发现 Cl 和 Li 系统地分布在夹杂物边缘,并被解释为代表预成核簇。这些簇与纳米花岗岩中的云母和长石的位置和组成相似性表明,玻璃状包裹体代表了在早期结晶阶段“捕获”的前纳米花岗岩,这表明玻璃状包裹体和纳米花岗岩可能具有共同的起源。将熔体夹杂物的组成与先前公布的数据进行比较表明,保存下来的玻璃状夹杂物含有显着较少的 H 这些簇与纳米花岗岩中的云母和长石的位置和组成相似性表明,玻璃状包裹体代表了在早期结晶阶段“捕获”的前纳米花岗岩,这表明玻璃状包裹体和纳米花岗岩可能具有共同的起源。将熔体夹杂物的组成与先前公布的数据进行比较表明,保存下来的玻璃状夹杂物含有显着较少的 H 这些簇与纳米花岗岩中的云母和长石的位置和组成相似性表明,玻璃状包裹体代表了在早期结晶阶段“捕获”的前纳米花岗岩,这表明玻璃状包裹体和纳米花岗岩可能具有共同的起源。将熔体夹杂物的组成与先前公布的数据进行比较表明,保存下来的玻璃状夹杂物含有显着较少的 H2 O(平均 2.72 重量%)比纳米花岗岩(平均 6.91 重量%)。这表明低 H 2 O 含量代表进一步阻碍结晶,以及这些空腔的体积非常小,有利于玻璃状内含物和纳米花岗岩的共存。相比之下,晶体成核在更含水的熔体中得到增强,其中 H 2 O 降低熔体粘度并促进扩散。

更新日期:2021-08-23
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