Magnetite-bubble aggregates at mixing interfaces in andesite magma bodies
Magnetite-bubble aggregates at mixing interfaces in andesite magma bodies
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DOI：
10.1144/sp410.7
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发表时间：
2015-01-01
期刊：
影响因子：
--
通讯作者：
Woods, A.
中科院分区：
文献类型：
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作者：
Edmonds, M.;Brett, A.;Woods, A.
磁铁矿是岩浆中异相气泡成核的一个特别有利的场所，但很少有证据表明，由于无数的过程，这些过程会叠加这种关联。岩浆中气泡磁铁矿聚集体的可能性对岩浆体的流体力学和负责安山岩形成的岩浆混合过程具有有趣的意义。我们使用图像分析和统计的方法来说明磁铁矿和泡沫之间的空间关联在镁铁质包体。包体中磁铁矿含量变化较大（高达7.5%），这与包体的孔隙度有关，反映了镁铁质岩浆对磁铁矿的富集机制。在安山岩中，气泡与磁铁矿之间没有空间组合，安山岩中磁铁矿含量很小。我们提出了一种机制，即在蒸汽饱和的岩浆，气泡成核磁铁矿包体的形成。当侵入安山岩岩浆体的底部时，这些气泡磁铁矿聚集体上升并“席卷”其他磁铁矿，导致聚集体在岩浆界面处的积聚。不稳定性导致包体的浮选，其特征在于磁铁矿和气泡的富集。
Magnetite is a particularly favourable site for heterogeneous bubble nucleation in magma and yet only very rarely is evidence for this preserved due to the myriad of processes that act to overprint such an association. The possibility of bubble-magnetite aggregates in magmas carries with it interesting implications for the fluid mechanics of magma bodies and for the magma mixing process responsible for the formation of andesites. We use image analysis and statistical methods to illustrate a spatial association between magnetite and bubbles in mafic enclaves. There is a large range in magnetite contents in the enclaves (up to 7.5%) which is related to the porosity of the enclaves, indicating a mechanism of enrichment of the mafic magma in magnetite. In the andesite there is no spatial association between bubbles and magnetite and the magnetite content of the andesite is small. We suggest a mechanism for enclave formation whereby in vapour-saturated magma, bubbles nucleate on magnetite. Upon intrusion into the base of an andesite magma body, these bubble-magnetite aggregates rise and 'sweep up' other magnetites, resulting in the accumulation of aggregates at the magma interface. Instabilities lead to the flotation of enclaves, characterized by enrichment in magnetite and bubbles.