No effect of H2O degassing on the oxidation state of magmatic liquids
No effect of H2O degassing on the oxidation state of magmatic liquids
复制标题
H2O脱气对岩浆液氧化态没有影响
DOI：
10.1016/j.epsl.2016.04.030
复制
发表时间：
2016
影响因子：
5.3
通讯作者：
R. Lange
中科院分区：
文献类型：
--
作者：
L. Waters;R. Lange
为什么俯冲带的岩浆比大洋中脊形成的岩浆氧化程度更高，其根本原因是一个激烈争论的话题。它是氧化洋壳向地幔俯冲过程中继承下来的一个主要特征，也是由于H2O脱气和/或岩浆分异作用而形成的一个次要特征。低的总铁含量和高的熔体H2 O含量使Rhythmic对H2 O脱气对铁-铁比率的任何影响都很敏感。在这里，喷发前的岩浆Fe 2+浓度，测量使用铁-钛氧化物，共结晶与硅酸盐斑晶在含水条件下，与Fe 2+喷发后的浓度在10个晶体穷人，完全脱气的黑曜石样品进行了比较，五是微晶免费。此外，Fe-Ti氧化物的数据，从这项研究和文献表明，电弧岩浆系统的氧化比玄武岩和含水的玄武岩熔体从冰岛和黄石国家公园广泛的脱气之前。也没有任何证据表明，分化（即，晶体分馏，地壳同化）是较高的氧化还原状态的弧岩浆相对于冰岛/黄石流纹岩的原因。相反，证据表明，俯冲的氧化地壳和释放的H2 O丰富的流体和/或熔体具有高的氧逸度（f O 2），这在H2 O通量熔融的地幔在创建玄武岩，相对氧化的过程中发挥了作用。
The underlying cause for why subduction-zone magmas are systematically more oxidized than those formed at mid-ocean spreading ridges is a topic of vigorous debate. It is either a primary feature inherited from the subduction of oxidized oceanic crust into the mantle or a secondary feature that develops because of H 2 O degassing and/or magma differentiation. Low total iron contents and high melt H 2 O contents render rhyolites sensitive to any effect of H 2 O degassing on ferric–ferrous ratios. Here, pre-eruptive magmatic Fe 2+ concentrations, measured using Fe–Ti oxides that co-crystallized with silicate phenocrysts under hydrous conditions, are compared with Fe 2+ post-eruptive concentrations in ten crystal-poor, fully-degassed obsidian samples; five are microlite free. No effect of H 2 O degassing on the ferric–ferrous ratio is found. In addition, Fe–Ti oxide data from this study and the literature show that arc magmas are systematically more oxidized than both basalts and hydrous silicic melts from Iceland and Yellowstone prior to extensive degassing. Nor is there any evidence that differentiation (ie, crystal fractionation, crustal assimilation) is the cause of the higher redox state of arc magmas relative to those of Iceland/Yellowstone rhyolites. Instead, the evidence points to subduction of oxidized crust and the release of an H 2 O-rich fluid and/or melt with a high oxygen fugacity (f O 2), which plays a role during H 2 O-flux melting of the mantle in creating basalts that are relatively oxidized.