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首页> 外文期刊>Contributions to Mineralogy and Petrology >Influence of magma dynamics on melt structure: spectroscopic studies on volcanic glasses from the Cretaio Tephra of Ischia (Italy)
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Influence of magma dynamics on melt structure: spectroscopic studies on volcanic glasses from the Cretaio Tephra of Ischia (Italy)

机译：岩浆动力学对熔体结构的影响：来自伊斯基亚（意大利）的克里特奥·特菲拉火山玻璃的光谱研究

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摘要

29Si nuclear magnetic resonance (NMR), electron spin resonance (ESR) and Mössbauer spectroscopy reveal some influences of magma dynamics on the structure of natural volcanic glasses. Our study concerns three carefully selected pumice samples collected from the Cretaio Tephra, deposited during a small-volume eruption that occurred in Roman times at the Island of Ischia, in the Phlegraean Volcanic District. We analysed glass separated from fragments of these pumice samples, which represent three different stratigraphic members of the Cretaio pyroclastic succession. Erupted during distinct phases of the eruption, these sampled members formed by either magmatic fragmentation or phreatomagmatic processes. Previous geochemical studies showed a homogeneous trachytic composition throughout the Cretaio sequence and a slight variation of the Sr-isotope ratio. The results of microprobe analyses confirm the homogeneous trachytic composition of the glass at the 20 µm scale. The curve-fittings of the 29Si MAS NMR spectra show variable relative proportions of Q4 and Q3 species in the glass structure. The glass/melt extruded during the first phase of the eruption contains a larger amount of Q4 than glass from the later phase, and it has negligible quantities of Q2 species. This finding could indicate that disproportionation reactions such as 2Q3⇔Q2+Q4 might not be applied to describe the Q-speciation in highly polymerized natural trachytic melts, and the first erupted glass with the highest amount of Q4 is interpreted as representative of a more polymerized melt, even if chemically indistinguishable from the others. Also Mössbauer spectra show that this glass differs from those extruded during the later phases of the eruption because it contains Fe3+ in network-forming tetrahedral coordination. 29Si cross-polarization–magic angle spinning (CP–MAS) experiments do not reveal a significant contribution of water speciation to silicon depolymerization in glass from surge deposits indicating that the magma probably interacted with water after the melt–glass transition temperature interval was crossed. We suggest that the detected structural variations reflect variable time–temperature paths during rise of different melt batches possibly caused by ascension in a segmented dyke prior to or during the Cretaio eruption.
机译：29 Si核磁共振，电子自旋共振和Mössbauer光谱揭示了岩浆动力学对天然火山玻璃结构的一些影响。我们的研究涉及从Cretaio Tephra收集的三个精心挑选的浮石样品，这些样品是在罗马爆发时在Phlegraean火山区的伊斯基亚岛发生的小批量喷发中沉积的。我们分析了从这些浮石样品的碎片中分离出的玻璃，这些碎片代表了Cretaio火山碎屑演替的三个不同的地层成员。这些喷出的成员在喷发的不同阶段被喷发，它们是由岩浆破裂或吞噬岩浆过程形成的。先前的地球化学研究表明，整个Cretaio序列中的均质特拉西奇成分以及Sr-同位素比的细微变化。显微探针分析的结果证实了20 µm尺寸的玻璃具有均匀的特拉克成分。 29 MAS NMR光谱的曲线拟合显示了玻璃结构中Q4 和Q3 种类的相对比例可变。在喷发的第一阶段挤出的玻璃/熔体包含的Q4 量大于第二阶段的玻璃，其Q2种类数量可忽略不计。这一发现可能表明歧义反应如2Q 3 ⇔Q 2 + Q 4 可能不被用于描述高Q值聚合的天然Trachytic熔体，即使化学上无法区分，第一个喷出的Q4量最高的玻璃也代表了聚合程度更高的熔体。莫斯鲍尔光谱还表明，这种玻璃与喷发后期的挤压玻璃不同，因为它在网络形成的四面体配位中含有Fe3 + 。 29 Si交叉极化-魔角旋转（CP-MAS）实验没有揭示水的形态对涌入沉积物中玻璃中硅解聚的显着贡献，表明熔岩-玻璃化转变后岩浆可能与水相互作用超过温度间隔。我们建议，在Cretaio喷发之前或期间，检测到的结构变化反映出不同熔体批次上升期间的可变时温路径，这可能是由分段堤坝上扬引起的。

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    《Contributions to Mineralogy and Petrology》 |2001年第5期|532-542|共11页
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    Dipartimento di Scienze della Terra Università di Trieste Via E. Weiss 8 34127 Trieste Italy;

    Istituto di Chimica Quantistica ed Energetica Molecolare CNR Via Alfieri 1 56010 Ghezzano Pisa Italy;

    Osservatorio Vesuviano Via Diocleziano 328 80124 Napoli Italy;

    Osservatorio Vesuviano Via Diocleziano 328 80124 Napoli Italy;

    Istituto di Chimica Quantistica ed Energetica Molecolare CNR Via Alfieri 1 56010 Ghezzano Pisa Italy;

    Istituto di Scienze Fisiche ed Unità INFM di Parma Università di Parma Via Volturno 39 43100 Parma Italy;

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