Magmatic processes revealed by noble gas signatures: the case of Unzen Volcano, Japan
Magmatic processes revealed by noble gas signatures: the case of Unzen Volcano, Japan
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惰性气体特征揭示的岩浆过程：日本云仙火山的案例
DOI：
10.2343/geochemj.31.395
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发表时间：
1997
影响因子：
0.8
通讯作者：
I. Kaneoka
中科院分区：
文献类型：
--
作者：
T. Hanyu;I. Kaneoka
惰性气体同位素系统学对了解火山系统的有用性在日本云仙火山的案例中得到了证明。测定了两种英安质火山岩中斜长石、角闪石和黑云母的稀有气体浓度和Ar同位素比值。岩浆中共存的不同矿物具有不同的~（40）Ar/~（36）Ar值，且~（40）Ar/~（36）Ar值与斑晶粒度相关，表明它们是在岩浆演化体系中结晶形成的。样品间40 Ar/36 Ar的变化可以用岩浆Ar和大气Ar的混合来解释。为了解释~（40）Ar/~（36）Ar的变化，我们提出了一种岩浆模型，即岩浆中的~（40）Ar/~（36）Ar由于大气来源的Ar的加入而逐渐降低，在同一时间序列中，斑晶随着岩浆的冷却而相继结晶。地下水是最有可能的候选人作为载体的大气来源的Ar，但地壳污染是不能完全排除的。
The usefulness of noble gas isotope systematics for understanding the volcanic systems is demonstrated in the case of Unzen Volcano, Japan. Noble gas concentrations and Ar isotope ratios were determined for coexisting plagioclase, hornblende and biotite in two dacitic volcanic rocks. Different species of minerals coexisting in a magma show different 40Ar/36Ar, and the 40Ar/36Ar is correlated to the grain sizes of the phenocrysts, indicating that they had crystallized in an evolving system of the magma. The variation of 40Ar/36Ar observed among samples can be explained by a two-component mixing: magmatic Ar and atmosphere-derived Ar. In order to explain the variation of 40Ar/36Ar, we propose a magmatic model in which the 40Ar/36Ar of magma had gradually decreased due to addition of atmosphere-derived Ar and, in the same sequence of time, phenocrysts had crystallized one after another as cooling of the magma had proceeded. Groundwater is the most probable candidate as carrier of atmosphere-derived Ar although crustal contamination is not completely excluded.