Magma evolution recorded in plagioclase zoning in 1991 Pinatubo eruption products
Magma evolution recorded in plagioclase zoning in 1991 Pinatubo eruption products
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DOI：
10.2138/am-1996-7-820
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发表时间：
1996-08
影响因子：
3.1
通讯作者：
K. Hattori;Hiroaki Sato
中科院分区：
文献类型：
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作者：
K. Hattori;Hiroaki Sato
摘要斜长石是皮纳图博火山最丰富的斑晶，在1991年喷发的产物中显示出不同的结构和成分。在6月7-14日的穹顶状安山岩中，斜长石斑晶显示出明显的边缘，其镁(0.04-0.06wt%)、Fe2O3T(0.6-0.8wt%)和K2O均高于内部。边缘的成分与微石的成分相同，微石在基质玻璃中含量丰富。白色英安岩浮石是6月15日喷发的最大体积的产物，含有部分受侵蚀的斜长石斑晶，但没有突出的边缘，也没有微石。英安质浮岩和穹顶安山岩的斑晶内部在结构和成分上非常相似。它们具有振荡环带(主要是An35-60)，低的MgO(-lt；0.02wt%)和Fe2O3T(0.10-0.30wt%)，相似的K2O。这种相似性表明这两种斜长石斑晶形成于同一流纹岩熔体中。这种振荡分带可能是由对流岩浆的温度波动和外部流体的并入和脱气形成的。长英质岩浆的一部分(约800°C)与镁铁质熔体(约1000-1100°C)混合，形成安山岩浆，挤出形成6月7日至14日的穹顶。安山岩中的所有斜长斑晶均来自长英质岩浆。混合导致斑晶的脱稳，形成筛状结构，尘土飞扬的区域和部分吸收。混合岩浆的挤压作用导致了一次吸收斑晶的过度生长和基质玻璃中斜长石微石的成核。在未混合的、残留的长英质岩浆部分，一些斜长石发生了部分吸收，但没有发育过度生长。6月15日英安质浮岩的研磨玻璃中没有过度生长和微石，表明喷发期间岩浆迅速上升，从镁铁质熔体注入到灾难性喷发之间的时间跨度很短，支持两者之间的联系。
Abstract Plagioclase, the most abundant phenocryst at Mount Pinatubo, displays varying textures and compositions within the 1991 eruption products. In June 7-14 dome-forming andesite, plagioclase phenocrysts show prominent rims with higher MgO (0.04-0.06 wt%), Fe2O3T (0.6-0.8 wt%), and K2O at given An than the interiors. The compositions of the rims are identical to those of microlites, which are abundant in the groundmass glass. White dacitic pumice, the most voluminous product of the June 15 eruption, contains partly corroded plagioclase phenocrysts but no prominent rims and no microlites. The interiors of phenocrysts from the dacitic pumice and the dome-forming andesite are remarkably similar in terms of textures and compositions. They show oscillatory zoning (mostly An35-60), low MgO (<0.02 wt%) and Fe2O3T (0.10-0.30 wt%), and similar K2O at given An. This similarity indicates that the two types of plagioclase phenocrysts formed in the same rhyolitic melt. The oscillatory zoning likely formed by temperature fluctuations in the convecting magma and incorporation and degassing of external fluids. A portion of the felsic magma (~800 °C) mixed with a mafic melt (~ 1000-1100 °C) to become an andesitic magma that extruded to form the June 7-14 dome. All plagioclase phenocrysts in the andesite were derived from the felsic magma. The mixing caused de stabilization of phenocrysts, forming sieve textures, dusty zones, and partial resorption. Extrusion of the mixed magma resulted in overgrowths on once-resorbed phenocrysts and the nucleation of plagioclase microlites in the groundmass glass. In the unmixed, remaining portion of the felsic magma, some plagioclase underwent partial resorption but did not develop overgrowths. This lack of overgrowths and the absence of microlites in the ground- mass glass of the June 15 dacitic pumice indicate rapid magma ascent during the eruption and a short time span between the injection of a mafic melt and the cataclysmic eruption, supporting the linkage between the two.