Formation mechanisms of macroscopic globules in andesitic glasses from the Izu–Bonin–Mariana forearc (IODP Expedition 352)
Formation mechanisms of macroscopic globules in andesitic glasses from the Izu–Bonin–Mariana forearc (IODP Expedition 352)
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DOI：
10.1007/s00410-020-01756-3
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发表时间：
2020-12
影响因子：
3.5
通讯作者：
R. Fonseca;L. Michely;M. Kirchenbaur;J. Prytulak;J. Ryan;Kerstin Hauke;F. Leitzke;R. Almeev;C. Marien;A. Gerdes;R. Schellhorn
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作者：
R. Fonseca;L. Michely;M. Kirchenbaur;J. Prytulak;J. Ryan;Kerstin Hauke;F. Leitzke;R. Almeev;C. Marien;A. Gerdes;R. Schellhorn
伊豆-小笠原-马里亚纳火山弧位于会聚的板块边缘，俯冲开始触发了MORB样弧前玄武岩的形成，作为减压熔融和近海沟扩张的结果。国际海洋发现计划（IODP）第352次考察队在弧前玄武岩地层中回收了样本，这些样本包含安山质玻璃中的不寻常宏观球状纹理（6号单元，1440 B孔）。目前还不清楚这些安山岩是如何形成的，它们在以弧前玄武岩为主的地层序列中是独一无二的，其中的球状结构也可能是如此。在这里，我们提出了详细的结构证据，主要和微量元素分析，以及B和Sr同位素组成，调查这些球状安山岩的成因。样品由富含玄武岩的球状体组成，镶嵌在安山岩组成的玻璃状基质中。在这两个结构域之间，可能发生析晶玻璃的水合界面，根据结构证据，这似乎与小球的形成有遗传联系。安山质基质富含Cl（约），而小球和界面是贫Cl的（约）。流体流动的微量元素的浓度似乎也被分馏的小球，并显示富集在B，K，Rb，Cs，和Tl，但不是在Ba和W相对于安山质基块，而界面显示在后者的耗尽，但在前者富集。有趣的是，球状体和安山质基块在分析不确定性（的）内具有相同的Sr同位素组成，这表明它们可能来自同一来源。然而，球状体显示高B（约。+ 7$$\permille$$），而其宿主安山岩的同位素较轻（约。- 1 $$\permille$$），这可能表明，无论是什么过程导致它们的形成，要么引入更重的B同位素的球，或诱导稳定的同位素分馏之间的球和它们的基底B。根据大量的纹理信息和地球化学数据从这些样品中获得的，我们得出结论，这些安山岩可能形成的同化的结果浅蚀变洋壳（AOC）在弧前玄武岩岩浆活动。同化作用可能将放射成因Sr和较重的B同位素引入相对非放射成因和低弧前玄武岩母岩浆中（平均值为0.703284）。此外，球状纹理与它们的形成是一致的，这是流体-熔体不混溶的结果，这可能是由同化的AOC中的水快速释放引起的，AOC的逸出可能形成了界面。如果这些样品中存在的球状结构确实是流体-熔体不相容性的结果，那么这个过程导致了显着的微量元素和稳定同位素分馏。球的纹理和化学成分突出了未来的实验研究的需要，旨在调查的出溶过程中潜在的微量元素和同位素分馏弧岩浆，也许以前没有考虑。
The Izu–Bonin–Mariana volcanic arc is situated at a convergent plate margin where subduction initiation triggered the formation of MORB-like forearc basalts as a result of decompression melting and near-trench spreading. International Ocean Discovery Program (IODP) Expedition 352 recovered samples within the forearc basalt stratigraphy that contained unusual macroscopic globular textures hosted in andesitic glass (Unit 6, Hole 1440B). It is unclear how these andesites, which are unique in a stratigraphic sequence dominated by forearc basalts, and the globular textures therein may have formed. Here, we present detailed textural evidence, major and trace element analysis, as well as B and Sr isotope compositions, to investigate the genesis of these globular andesites. Samples consist of-rich basaltic globules set in a glassy groundmass of andesitic composition. Between these two textural domains a likely hydrated interface of devitrified glass occurs, which, based on textural evidence, seems to be genetically linked to the formation of the globules. The andesitic groundmass is Cl rich (ca.), whereas globules and the interface are Cl poor (ca.). Concentrations of fluid-mobile trace elements also appear to be fractionated in that globules and show enrichments in B, K, Rb, Cs, and Tl, but not in Ba and W relative to the andesitic groundmass, whereas the interface shows depletions in the latter, but is enriched in the former. Interestingly, globules and andesitic groundmass have identical Sr isotopic composition within analytical uncertainty (of), indicating that they likely formed from the same source. However, globules show highB (ca. + 7$$\permille$$), whereas their host andesites are isotopically lighter (ca. – 1 $$\permille$$), potentially indicating that whatever process led to their formation either introduced heavier B isotopes to the globules, or induced stable isotope fractionation of B between globules and their groundmass. Based on the bulk of the textural information and geochemical data obtained from these samples, we conclude that these andesites likely formed as a result of the assimilation of shallowly altered oceanic crust (AOC) during forearc basaltic magmatism. Assimilation likely introduced radiogenic Sr, as well as heavier B isotopes to comparatively unradiogenic and lowforearc basalt parental magmas (averageof 0.703284). Moreover, the globular textures are consistent with their formation being the result of fluid-melt immiscibility that was potentially induced by the rapid release of water from assimilated AOC whose escape likely formed the interface. If the globular textures present in these samples are indeed the result of fluid-melt immiscibility, then this process led to significant trace element and stable isotope fractionation. The textures and chemical compositions of the globules highlight the need for future experimental studies aimed at investigating the exsolution process with respect to potential trace element and isotopic fractionation in arc magmas that have perhaps not been previously considered.