Decoupled convection cells from mixing experiments with alkaline melts from Phlegrean Fields
Decoupled convection cells from mixing experiments with alkaline melts from Phlegrean Fields
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DOI：
10.1016/j.chemgeo.2004.08.045
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发表时间：
2004-12-15
期刊：
影响因子：
3.9
通讯作者：
Fehr, KT
中科院分区：
文献类型：
--
作者：
De Campos, CP;Dingwell, DB;Fehr, KT
在Phlegrean领域（Campi Flegrei）火山系统的喷发前岩浆混合的可能性的地球化学证据促使我们研究的物理混合类似成分的岩浆的效率。我们进行了粗面岩到响岩粗面岩成分的混合实验。为了加强混合，最初的岩浆含有部分溶解的晶体。实验采用时间序列的形式，温度为1300 ℃，搅拌速度为每分钟0.5转。在16和25小时后，在恒定搅拌下，Al-Ti-Fe尖晶石晶体仍然存在，并且局部追踪流动方向。两个单独的对流细胞起源：一个较低的一个与平行的坩埚的底部和侧壁的主流，和一个较高的，与主流大致成直角的较低cell.Electron微探针分析的产品表明，一个复杂的分层的细胞界定的明确的差距，氧化物的比例和包含成分梯度。在细胞界面处，所有分析的氧化物都显示出一个水平，其中氧化物含量超过2%（SiO2和Na 2 O）。在这些实验中，非常低的雷诺数（10（-7）到10（-9））表明在层流条件下混合。在没有显著的温度梯度的情况下，在我们的实验中的对流主要由所施加的强制对流与沿样品沿着导致类似于双扩散系统的密度分布的局部成分梯度（扩散）的效应相结合来驱动。
Geochemical evidence for the possibility of pre-eruptive magma mixing in the Phlegrean Fields (Campi Flegrei) volcanic system has prompted us to study the efficiency of the physical mixing of magmas of similar compositions. We have performed mixing experiments of trachytic to phonolitic trachytic compositions. To enhance mixing, the initial magmas contained partially dissolved crystals. The experiments took the form of a time series at 1300 degreesC and stirring at 0.5 rotations per minute.The two starting compositions were loaded as cylinders and convection was forced via rotation of a spindle. After 16 and 25 h, under constant stirring, Al-Ti-Fe spinel crystals were still present and locally traced the flow directions. Two separate convection cells originated: a lower one with a primary flow parallel to the bottom and side walls of the crucible, and an upper one, with a primary flow approximately at right angle to that of the lower cell.Electron microprobe analyses of the products indicate a complex layering of cells bounded by clear gaps in oxide ratios and containing compositional gradients. At the cell interface, all analyzed oxides exhibit a horizon, where oxides contents range over 2% (SiO2 and Na2O).The very low Reynolds numbers (10(-7) to 10(-9)) in these experiments point towards mixing under laminar flow conditions. In the absence of a significant temperature gradient, convection in our experiments was driven primarily by the applied forced convection combined with the effect of local compositional gradients (diffusion) along the sample leading to a density distribution similar to a double diffusive system.