Timescales of convection in magma chambers below the Mid-Atlantic ridge from melt inclusions investigations
Timescales of convection in magma chambers below the Mid-Atlantic ridge from melt inclusions investigations
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通过熔体包裹体研究得出大西洋中脊下方岩浆房中对流的时间尺度
DOI：
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发表时间：
2012
影响因子：
3.5
通讯作者：
P. Burnard
中科院分区：
文献类型：
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作者：
A. Colin;F. Faure;P. Burnard
过去在大洋中脊玄武岩和陆上拉斑火山中都描述了橄榄石微晶的封闭漏斗和复杂的燕尾形貌，并表明了岩浆过冷的波动。我们描述了大西洋中脊枕形玄武岩（样品 RD87DR10）中的类似形态，此外，我们还估计了产生这些纹理所需的温度波动的持续时间，如下所示：（1）成对的熔体包裹体对称排列在漏斗晶体的中心周围，每对代表一个加热-冷却循环。利用文献中与观察到的形态相关的橄榄石生长速率，并测量两个连续内含物之间的距离，我们估计了一个对流周期内经过的最短时间。 (2) 发现熔体包裹体的主要元素组成（通过电子探针分析）位于橄榄石周围玻璃中测量的边界层范围内，无论其大小如何。几个主要元素表明，该边界层是海底快速淬火的结果，而不是深部晶体生长的结果，这意味着包裹体在被密封时与周围的岩浆具有相同的成分。利用慢速扩散元素（例如 Al2O3）的扩散率，我们估算了夹杂物形成所需的最短时间。这两种独立的方法给出了一致的结果：每个冷却-加热循环持续几分钟到最少 1 小时。因此，这些晶体可能记录了喷发前最后一小时或几小时内小型岩浆体（岩脉或浅岩浆室）中的热对流。
Closed hopper and complex swallowtail morphologies of olivine microcrysts have been described in the past in both mid-oceanic ridge basalts and subaerial tholeitic volcanoes and indicate fluctuations in magma undercooling. We describe similar morphologies in a Mid-Atlantic ridge pillow basalt (sample RD87DR10), and in addition we estimate the duration of temperature fluctuations required to produce these textures as follows: (1) Pairs of melt inclusions are arranged symmetrically around the centre of hopper crystals and each pair represents a heating–cooling cycle. Using the literature olivine growth rates relevant to the observed morphologies, and measuring the distance between two successive inclusions, we estimate the minimum time elapsed during one convection cycle. (2) The major element composition of melt inclusions (analysed by electron microprobe) was found to be in the range of the boundary layer measured in the glass surrounding the olivines, irrespective of their size. Several major elements demonstrate that this boundary layer results from rapid quenching on the seafloor, and not from crystal growth at depth, implying the inclusions had the same composition as the surrounding magma when they were sealed. Using diffusivity of slow diffusing elements such as Al2O3, we estimate the minimum time required for inclusion formation. These two independent approaches give concordant results: each cooling–heating cycle lasted between a few minutes and 1 h minimum. Thus, these crystals probably recorded thermal convection in small magmatic bodies (a dyke or shallow magma chamber) during the last hour or hours before eruption.