A nanolite record of eruption style transition
A nanolite record of eruption style transition
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喷发方式转变的纳米岩记录
DOI：
10.1130/g35553.1
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发表时间：
2014
期刊：
影响因子：
5.8
通讯作者：
Mayumi Mujin & Michihiko Nakamura
中科院分区：
文献类型：
--
作者：
Mayumi Mujin & Michihiko Nakamura
微晶特征已成功地用作岩浆减压速率的指标。然而，微晶没有记录的爆炸-喷出的转变，在次普林尼喷发，当这种转变是由浅管道中的出溶挥发压力的减少，而不是由岩浆上升速率。为了克服这一局限性，我们研究了2011年Shinmoedake（日本）爆发的淬火产物中的纳米粒子。微晶石是基质矿物的纳米级组分，并且比微晶石具有更陡的晶体尺寸分布斜率。2011年火山活动经历了从次普林尼期喷发到火山爆发和间歇性喷流的转变。研究发现，虽然不同喷发活动形成的产物具有相似的微晶特征，但通过其微晶矿物组合可以清楚地区分。亚普林尼期喷发和火山爆发的浮石样品和致密的熔岩幼年碎片（按爆炸性降序排列）分别含有低钙辉石、低钙辉石+斜长石和低钙辉石+斜长石+铁钛氧化物的纳米晶。当岩浆的过冷主要是由于脱水在地表附近迅速增加时，纳米晶结晶。填隙玻璃的含水量表明，淬火深度并没有显着不同的喷发风格。因此，不同的nanolite组合的每一个喷发风格被认为是由于在地表附近的岩浆停留时间的差异。因此，我们建议，Nanoparticle有潜力指示岩浆的物理化学条件的喷发风格的过渡点。
Microlite characteristics have been successfully used as indicators of magma decompression rate. However, microlites do not record the explosive-effusive transitions in sub-Plinian eruptions, when such transitions are governed by the decrease of exsolved volatile pressure in the shallow conduit rather than by the magma ascent rate. To overcome this limitation, we studied the nanolites in the quenched products of the 2011 Shinmoedake (Japan) eruption. Nanolites are nanometer-scale components of the groundmass minerals and exhibit a steeper slope of crystal size distribution than that of microlites. In the 2011 eruption, the style of activity had undergone transformation from sub-Plinian eruption to Vulcanian explosion and intermittent effusion of lava. We found that, although the products formed by different eruptive activities have similar microlite characteristics, such products can be distinguished clearly by their mineral assemblage of nanolites. The samples of pumice of sub-Plinian eruptions and Vulcanian explosions and the dense juvenile fragments of lava (in descending order of explosivity) contained, respectively, nanolites of low-Ca pyroxene, low-Ca pyroxene + plagioclase, and low-Ca pyroxene + plagioclase + Fe-Ti oxides. Nanolites crystallize when undercooling of the magma due primarily to dehydration increases rapidly near the surface. The water contents of the interstitial glass indicate that the quenched depths did not differ greatly between eruption styles. Hence, the different nanolite assemblages of each eruption style are assumed to have resulted from differences in magma residence time near the surface. Therefore, we propose that nanolites have the potential to indicate the physicochemical conditions of magma at the transition points of eruption styles.