Special Collection: Rates and Depths of Magma Ascent on Earth: Degassing of Hydrous Trachytic Campi Flegrei and Phonolitic Vesuvius Melts: Experimental Limitations and Chances to Study Homogeneous Bubble Nucleation
Special Collection: Rates and Depths of Magma Ascent on Earth: Degassing of Hydrous Trachytic Campi Flegrei and Phonolitic Vesuvius Melts: Experimental Limitations and Chances to Study Homogeneous Bubble Nucleation
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DOI：
10.2138/am-2016-5480
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发表时间：
2016
影响因子：
3.1
通讯作者：
O. Preuss;H. Marxer;Sarah Ulmer;Wolf Johannes;M. Nowak
中科院分区：
文献类型：
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作者：
O. Preuss;H. Marxer;Sarah Ulmer;Wolf Johannes;M. Nowak
摘要气泡形核和长大的熔体脱气是岩浆上升的驱动机制。因此，采用含水硅酸盐熔体的减压实验来研究H2O脱气的开始和动力学。以Campi Flegrei火山岩系岩浆成分为代表的名义上为H2O的不饱和粗面岩Campi Flegrei熔体和发音橄榄质Vesuvius熔体在1050C的超液相线温度下，用0.024和0.17MPa.S的连续减压速率，将初始压力从200兆帕减至100%、75兆帕和60兆帕。玻璃粉可以用来缩短减压前的平衡时间(TEQ)，以溶解熔体中的H2O。对减压后的样品进行淬火，并比较气泡数密度(NV)、孔隙率和熔体中残留H2O的含量。所有玻璃圆柱体样品的减压均一化成核，其NV高达105 mm-3。粗面岩的过饱和压力为-lt；76 Mpa，声晶熔体的过饱和压力为-lt；70 Mpa。与玻璃圆筒相比，减压前平衡使用24小时的玻璃粉可以防止减压过程中均匀的气泡形核。我们认为，在减压前，粉末孔隙空间中的滞留空气导致了整个样品中微小的H2O-N2气泡的形成。这些玻璃粉末样品的脱气是由这些预先存在的气泡的扩散生长促进的，因此不需要熔体明显的H2O过饱和。与玻璃圆柱体样品相比，气孔率相应较高时，NV和残余H2O含量较低几个数量级就证明了这一点。然而，在玻璃粉实验中，TEQ显著延长至96h，导致脱气结果与玻璃圆筒样品相当。这种效应可能是由于Ostwald成熟、合并以及在减压前的扩展TEQ期间先前存在的气泡上升所致。用玻璃瓶样品的NV来检验Toramaru(2006)提供的泡化模型的适用性。对于实际应用的减压速率，实验NV比模型预测值高出5个数量级。这主要归因于模型中用宏观表面张力和H2O总扩散系数来描述气泡成核的分子过程。采用降低的表面张力和较低的网络形成物的扩散系数作为形成气泡核的限制参数，可以显著提高模拟的NV。这项研究表明，研究均匀气泡成核需要一个优化的实验方案。我们强烈建议使用巨大的玻璃瓶作为起始材料进行实验。减压的时间刻度是一个限制参数，必须足够短，以最大限度地减少气泡合并降低NV的机会。考虑到我们相对较高的NV，以前许多用于校准均匀气泡成核模型的实验研究的样本可能会受到显著的NV降低的影响。从优化实验中获得的新数据将要求改进岩浆上升过程中均匀气泡成核的模型。
Abstract Melt degassing by bubble nucleation and growth is a driving mechanism of magma ascent. Therefore, decompression experiments with hydrous silicate melts were used to investigate the onset and the dynamics of H2O degassing. Nominally H2O undersaturated trachytic Campi Flegrei and phonolitic Vesuvius melts representative for the magma compositions of the Campi Flegrei volcanic system were decompressed at a super-liquidus temperature of 1050 °C from 200 MPa to final pressures (Pfinal) of 100, 75, and 60 MPa using continuous decompression rates of 0.024 and 0.17 MPa/s. Experiments started from either massive glass cylinders or glass powder to demonstrate the infl of the starting material on melt degassing. Glass powder can be used to shorten the equilibration time (teq) prior to decompression for dissolution of H2O in the melt. The decompressed samples were quenched and compared in terms of bubble number density (NV), porosity, and residual H2O content in the melt. Decompression of all glass cylinder samples led to homogeneous bubble nucleation with high NV of ~105 mm–3. The supersaturation pressures for homogeneous bubble nucleation were estimated to be <76 MPa for the trachytic and <70 MPa for the phonolitic melt. In contrast to glass cylinders, the usage of glass powder equilibrated for 24 h before decompression prevented homogeneous bubble nucleation during decompression. We suggest that trapped air in the powder pore space resulted in the formation of tiny H2O-N2 bubbles throughout the samples prior to decompression. Degassing of these glass powder samples was facilitated by diffusive growth of these pre-existing bubbles and thus did not require signifi H2O supersaturation of the melt. This is evidenced by several orders of magnitude lower NV and lower residual H2O contents at correspondingly higher porosities compared to the glass cylinder samples. However, a signifi extension of teq to 96 h in the glass powder experiments led to degassing results comparable to the glass cylinder samples. This effect is probably due to Ostwald ripening, coalescence, and the ascent of the pre-existing bubbles during the extended teq prior to decompression. The NV of the glass cylinder samples were used to test the applicability of the vesiculation model provided by Toramaru (2006). For the applied decompression rates, the experimental NV are up to 5 orders of magnitude higher than the values predicted by the model. This may be mainly attributed to the usage of the macroscopic surface tension and the total H2O diffusivity in the model to describe the molecular process of bubble nucleation. A signifi increase in modeled NV can be achieved by application of a reduced surface tension in combination with the lower diffusivity of network formers as a limiting parameter for the formation of a bubble nucleus. This study demonstrates that the investigation of homogeneous bubble nucleation necessitates an optimized experimental protocol. We strongly recommend performing experiments with massive glass cylinders as starting material. The timescale of decompression is a limiting parameter and must be short enough to minimize the opportunity for a reduction of NV by bubble coalescence. Considering our comparably high NV, the samples of many previous experimental studies that were used to calibrate models for homogeneous bubble nucleation were probably subject to signifi NV reduction. Newly derived data from optimized experiments will require improved models for homogeneous bubble nucleation during magma ascent.