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首页> 外文期刊>Contributions to Mineralogy and Petrology >Magnetite scavenging and the buoyancy of bubbles in magmas. Part 1: Discovery of a pre-eruptive bubble in Bishop rhyolite
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Magnetite scavenging and the buoyancy of bubbles in magmas. Part 1: Discovery of a pre-eruptive bubble in Bishop rhyolite

机译：磁铁矿清除和岩浆中气泡的浮力。第1部分：在Bishop流纹岩中发现喷发前的气泡

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摘要

Little is known about the presence, distribution and size of bubbles in rhyolitic magmas prior to eruption. Using X-ray tomography to study pumice from early-erupted Bishop rhyolite, we discovered a large vesicle with abundant magnetite crystals attached to its walls. Attachment of magnetite crystals to bubble walls under pre-eruptive conditions can explain the cluster of magnetite crystals as a result of bubbles rising and collecting magnetite crystals. Alternatively, bubbles may have nucleated on magnetite crystals and then coalesced to form one large bubble with multiple magnetite crystals attached to it. We argue that the clusters of magnetite crystals could not have formed during or after eruptive decompression, and conclude that this vesicle corresponds to a bubble present prior to eruptive decompression. The inferred presence of pre-eruptive bubbles in the Bishop magma confirms the interpretation that the magma was volatile-saturated prior to eruption. The pre-eruptive size of this bubble is estimated based on three independent approaches: (1) the current size of the vesicle, (2) the total cross-sectional area of the magnetite crystals, and (3) the bubble size required for the aggregate to be neutrally buoyant. These approaches suggest a pre-eruptive bubble 300–850 μm in diameter, with a preferred value of 600–750 μm.
机译：人们对喷发前流纹岩浆中气泡的存在，分布和大小知之甚少。使用X射线断层扫描研究早期萌发的Bishop流纹岩中的浮石，我们发现了一个大囊泡，其壁上附着了丰富的磁铁矿晶体。在喷发前的条件下，磁铁矿晶体附着在气泡壁上可以解释由于气泡上升并聚集磁铁矿晶体而导致的磁铁矿晶体簇。或者，气泡可能在磁铁矿晶体上成核，然后聚结形成一个大气泡，并附有多个磁铁矿晶体。我们认为在喷发减压期间或之后不可能形成磁铁矿晶体簇，并得出结论，该囊泡对应于喷发减压之前存在的气泡。推测Bishop岩浆中存在喷发前泡沫，这证实了岩浆在喷发之前是挥发性饱和的。根据三种独立的方法估算气泡的喷发前大小：（1）囊泡的当前大小；（2）磁铁矿晶体的总横截面积；（3）气泡所需的气泡大小聚集起来呈中性浮力。这些方法表明，喷发前的气泡直径为300–850μm，首选值为600–750μm。

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