Noble gas variation during partial crustal melting and magma ascent processes
Noble gas variation during partial crustal melting and magma ascent processes
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DOI：
10.1016/j.chemgeo.2021.120635
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发表时间：
2021-12-04
期刊：
影响因子：
3.9
通讯作者：
Rodriguez, J. A. Lozano
中科院分区：
文献类型：
--
作者：
Alvarez-Valero, A. M.;Sumino, H.;Rodriguez, J. A. Lozano
稀有气体同位素虽然存在于痕量中，但通常比主要的挥发性物质更可靠，更明确地记录其来源。特别是在火山环境中，这种优势来自于它们的化学惰性，因为惰性气体同位素和元素分馏与它们的来源密切相关，并且只在岩浆上升和喷发期间通过物理过程进行修改。新第三纪火山El Hoyazo（Betic Cordillera，西班牙东南部）是一个非常有利的天然实验室，可以研究深部地壳部分熔融过程与地表喷发产物之间的联系，因为部分熔融的地壳捕虏体保存在熔岩中。比较捕虏体和熔岩的稀有气体同位素组成，有可能对地壳中难以接近的深处熔融过程中的挥发性行为产生新的认识，在El Hoyazo，熔岩玻璃中捕获的稀有气体以及捕虏体和斑晶中的流体/熔体包裹体提供了以下方面的新信息：（i）它们对地壳熔融过程的反应，包括岩浆混合等机制（和地壳同化）两个端元：即从变绿壳中提取的长英质熔体和来自该地区底侵作用的基性-中基性岩浆。结果表明，岩浆惰性气体与部分熔融地壳的相互作用，显着的修改;（ii）惰性气体的变化，在脱气和岩浆上升，表现出较高的大气影响的熔岩样品从较浅的深度比在较深的熔岩和矿物;和（iii）岩浆对深熔岩中石榴石晶体的影响高于两种浅熔岩中的角闪石晶体，和石榴石晶体。此外，我们发现熔体包裹体中的稀有气体也可能在其收缩气泡中积累，而不仅仅是溶解在熔体中。
Noble gas isotopes, although present in trace amounts, are generally more reliable and less ambiguous recorders of their source than the major volatile species. In volcanic settings in particular, this advantage derives from their chemical inertness, as noble gas isotopic and elemental fractionations are strongly coupled to their source and modified only by physical processes during magma ascent and eruption. The Neogene volcano El Hoyazo (Betic Cordillera, SE Spain) is a highly favourable natural laboratory to study the links between partial crustal melting processes occurring at depth and the eruptive products at the surface, because partially melted crustal xenoliths are preserved in silicic lavas. Comparing the noble gas isotopic compositions of xenoliths and lavas has the potential to yield new insights into volatile behaviour during melting processes at inaccessible depths in the crust.At El Hoyazo, noble gases trapped in lava glasses, and the fluid/melt inclusions within xeno- and phenocrysts, provide novel information on: (i) their response to the crustal melting process including mechanisms such as magma mixing (and crustal assimilation) of two endmembers: i.e. the extracted felsic melt from the country metapelitic crust, and the basic-intermediate magma from the underplating in the region. The results reveal significant modification of magmatic noble gases by the interaction with the partially melted crust; (ii) noble gas variations during degassing and magma ascent, showing higher atmospheric influence in the lava samples from shallower depths than in the deeper lavas and minerals; and (iii) higher magmatic influence in crystals of garnet from deeper lava than in both shallower crystals of amphibole, and garnet crystals within the crustal xenoliths. In addition, we find that noble gases in melt inclusions are also likely accumulating in their shrinkage bubbles, and not only remaining dissolved in the melt.