H2O-rich melt inclusions in fayalitic olivine from Hekla volcano: Implications for phase relationships in silicic systems and driving forces of explosive volcanism on Iceland
H2O-rich melt inclusions in fayalitic olivine from Hekla volcano: Implications for phase relationships in silicic systems and driving forces of explosive volcanism on Iceland
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DOI：
10.1016/j.epsl.2012.09.047
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发表时间：
2012-12
影响因子：
5.3
通讯作者：
M. Portnyagin;K. Hoernle;S. Storm;N. Mironov;C. V. D. Bogaard;R. Botcharnikov
中科院分区：
文献类型：
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作者：
M. Portnyagin;K. Hoernle;S. Storm;N. Mironov;C. V. D. Bogaard;R. Botcharnikov
人们普遍认为硅质冰岛岩浆在脱气之前含有低至中等的H2O含量，其高爆炸性主要是由于岩浆与冰或大气水的相互作用造成的。在这里，我们报告了来自冰岛赫克拉火山最大的全新世喷发（H3和H4）的富铁橄榄石（Fo2-42）中的玻璃夹杂物（SiO2=57–72wt%，K2O=1.3–2.6wt%）的成分，这些橄榄石保存了具有非常高的一次H2O含量（3.3–6.2wt%）的淬火熔体。硅质 Hekla 熔体主要源自贫 H2O (~0.6wt%) 玄武岩的广泛 (~90%) 晶体分异，并且代表陆相岩浆系统学中的终端成员，因为它们起源于低 fO2 (ΔQFM ~-0.1 至 -0.4)，并且具有与氧化程度更高的岛弧岩浆 (ΔQFM≥1) 一样高的 H2O 含量。这表明，在不同构造背景的硅质岩浆中，H2O 和 ΔQFM 并不相关，并且 fO2（而不是 H2O 含量）显示了硅质洋岛（例如冰岛）和岛弧岩浆之间的主要差异。对现有实验数据的分析表明，高 H2O 活性和低 fO2 扩大了硅熔体中橄榄石稳定性的范围。低fO2和低MgO含量也可以抑制角闪石的结晶。基于这些结果，我们提出，演化火山和斯卡加德型侵入岩中含有富铁橄榄石的无水矿物组合并不意味着脱气前岩浆中的 H2O 含量较低，而是与普遍持有的观点相反，它是在低 fO2 下结晶的富含 H2O 的硅质母岩浆的指标。最后，岩浆中的高 H2O 含量是赫克拉火山最大规模爆发性喷发的主要驱动力，对于驱动冰岛硅质爆发性火山活动来说，其重要性至少与岩浆-冰相互作用一样重要。
Silicic Icelandic magmas are widely believed to contain low to moderate H2O content prior to degassing, and that their high explosivity mostly results from the interaction of the magmas with ice or meteoric water. Here we report the compositions of glass inclusions (SiO2=57–72wt%, K2O=1.3–2.6wt%) in Fe-rich olivines (Fo2–42) from the largest Holocene eruptions of Hekla volcano (H3 and H4) on Iceland, which preserved quenched melts with very high primary H2O contents (3.3–6.2wt%). The silicic Hekla melts originate primarily by extensive (∼90%) crystal fractionation of H2O-poor (∼0.6wt%) basalts and represent an end member in the systematics of terrestrial magmas because they originate at low fO2(ΔQFM ∼−0.1 to −0.4) and have as high H2O contents as significantly more oxidized island-arc magmas (ΔQFM≥1). This demonstrates that H2O and ΔQFM do not correlate in silicic magmas from different tectonic settings, and that fO2, not H2O content, shows a major difference between silicic ocean-island (e.g., Icelandic) and island-arc magmas. Analysis of available experimental data suggests that high H2O activity and low fO2expand the field of olivine stability in silicic melts. Low fO2and low MgO content could also suppress crystallization of amphibole. On the basis of these results we propose that an anhydrous mineral assemblage bearing Fe-rich olivine in evolved volcanic and Skaergaard-type intrusive rocks does not imply low H2O in magmas prior to degassing but, in contrast to the commonly held view, is an indicator of H2O-rich silicic parental magmas crystallized at low fO2. Finally, the high H2O content in magma was a major driving force of the largest explosive eruptions of Hekla volcano and must be at least as important for driving silicic explosive volcanism on Iceland as magma–ice interaction.