The effect of initial H2O concentration on decompression-induced phase separation and degassing of hydrous phonolitic melt
The effect of initial H2O concentration on decompression-induced phase separation and degassing of hydrous phonolitic melt
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初始H2O浓度对水合酚醛熔体减压诱导相分离和脱气的影响
DOI：
10.1007/s00410-020-1659-2
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发表时间：
2020
影响因子：
3.5
通讯作者：
Nowak M
中科院分区：
文献类型：
--
作者：
Allabar A;Salis Gross E;Nowak M
在岩浆上升过程中，H2O的过饱和导致熔体通过形成和生长小泡而排出气体，这可能为火山爆发提供动力。在这里，我们提供了实验来研究初始溶解H2O浓度(CH2O)对声晶熔体中小泡的形成、生长和合并的影响。将cH2O在3.3wt%~6.3wt%范围内的维苏威声质熔体以1.70和0.17兆帕·S−1的速率减压，在 ≥ 1323K下，从270兆帕和200兆帕开始减压到150-20兆帕的最终压力，样品在此等压淬火。光学显微镜和拉曼光谱测量证实，得到的玻璃没有微晶和氧化铁纳米颗粒，这意味着实验是超液相线，水合熔体的相分离是均匀的。初始形成的小泡的最小数目由归一化到无泡玻璃体积的数密度定义，在 ~ 5wt%cH2O时观察到 ~ 5的对数VnD(以mm−3为单位)。对数VND随H_2O_2浓度的升高而增大一个数量级。此外，在 ~ 5.6wt%cH_2O_1时，囊泡形成的演化发生了重要的转变。在较低的H2Oini下，初始的VND在进一步减压过程中被保留，直到融化30-50%的孔隙率。在高H_2O_i时，在进一步减压过程中，当低熔体孔隙率为15-21%时，初始泡囊群被擦除。这一现象归因于低熔体粘度有利于囊泡合并。总之，cH2O决定了初始相分离的VND和减压过程中气泡的演化，从而控制了火山喷发的类型。
Supersaturation of H2O during magma ascent leads to degassing of melt by formation and growth of vesicles that may power explosive volcanic eruptions. Here, we present experiments to study the effect of initially dissolved H2O concentration (cH2Oini) on vesicle formation, growth, and coalescence in phonolitic melt. Vesuvius phonolitic melts withcH2Oiniranging between 3.3 and 6.3 wt% were decompressed at rates of 1.7 and 0.17 MPa·s−1and at temperatures ≥ 1323 K. Decompression started from 270 and 200 MPa to final pressures of 150–20 MPa, where samples were quenched isobarically. Optical microscopy and Raman spectroscopic measurements confirm that the glasses obtained were free of microcrystals and Fe-oxide nanolites, implying that the experiments were superliquidus and phase separation of the hydrous melt was homogeneous. A minimum number of the initially formed vesicles, defined by the number density normalized to vesicle-free glass volume (VND), is observed at ~ 5 wt%cH2Oiniwith a logVNDof ~ 5 (in mm−3). The logVNDincreases strongly towards lower and highercH2Oiniby one order of magnitude. Furthermore, an important transition in evolution of vesiculation occurs at ~ 5.6 wt%cH2Oini. At lowercH2Oini, the initialVNDis preserved during further decompression up to melt porosities of 30–50%. At highercH2Oini, the initial vesicle population is erased at low melt porosities of 15–21% during further decompression. This observation is attributed to vesicle coalescence favored by low melt viscosity. In conclusion,cH2Oinidetermines theVNDof initial phase separation and the evolution of vesiculation during decompression that controls the style of volcanic eruptions.
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DOI：
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