Vesiculation and Microlite Crystallization Induced by Decompression: a Case Study of the 1991–1995 Mt Unzen Eruption (Japan)
Vesiculation and Microlite Crystallization Induced by Decompression: a Case Study of the 1991–1995 Mt Unzen Eruption (Japan)
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减压引起的囊泡化和微晶石结晶：1991-1995 年云仙山喷发案例研究（日本）
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发表时间：
2011
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通讯作者：
H. Behrens
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作者：
S. Cichy;R. Botcharnikov;F. Holtz;H. Behrens
在云仙火山进行了等温减压实验，模拟岩浆从岩浆库深处上升到地壳浅层，对应的压力从300到50 MPa。部分结晶的合成流纹英质岩浆（代表8508 ℃和300 MPa的平衡条件）被用作起始材料;它的成分与1991 ^1995年喷发的云仙岩的基块相同。减压速率为0·0002 ~ 20 MPa·s。减压速率为0·1 MPa·s-1的实验采用连续减压法，减压速率为0·1 MPa·s-1的实验采用多级减压法。实验是流体饱和的，或者只含有水作为流体组分（含H2O），或者含有水和二氧化碳的混合物（H2O = CO2;流体中H2O的初始摩尔分数为0·6）。含H2O实验的实验产物包括角闪石、辉石、氧化物和玻璃。斜长石微晶只在减压速率最低的两个实验（0·0005和0·0002 MPa·s）中成核长大。这些斜长石的长度可达200 ~ 250 mm，与天然样品中斜长石微晶的尺寸一致，实验产物由辉石、氧化物、玻璃和斜长石组成。斜长石微晶在H2O-CO2-系统中已经存在于起始组合中，并生长到最大尺寸为80 mm。在最终压力为50 MPa的残余玻璃中水的平衡浓度达到减压速率为1 MPa s-1的H2O-CO2轴承系统和0·1 MPa s-1的H2O轴承系统。含H_2O体系的气泡数密度（BND）为10 ~ 10 mm 3，含H_2O和CO_2体系的BND为104.6 ~ 10 mm 3。在两种体系中，BND值随减压速率的减小而减小，随减压速率的减小而增大，分别反映了气泡的生长和成核，从残余熔体的化学成分变化可以看出，结晶的开始发生在减压速率为50·1 MPa s时。在最低的减压速率（0·0002 MPa·s）下，含H_2O-CO_2体系中残余熔体的化学组成与天然基质玻璃的化学组成相似。微晶数密度（MND）值作为减压速率的函数没有显著变化。仅斜长石的MND值范围为10至10 mm，而其他相的MND值范围为10至10 mm。我们的实验MNDPl值在天然样品的范围内（10^10 mm）。我们表明，在减压过程中微晶（在我们的实验数据集中为斜长石）的成核和结晶的大小是有用的，以限制岩浆上升速率在相应阶段的结晶开始。根据斜长石微晶的大小和残余熔体的成分，云前岩浆在200 - 50 MPa压力范围内的平均岩浆上升速率估计为10^50 m h。
Isothermal decompression experiments were performed to simulate magma ascent at Unzen volcano from the depths of magma storage to shallow crustal levels, corresponding to pressure decrease from 300 to 50MPa. A partially crystallized synthetic rhyodacitic magma (representing equilibrium conditions at 8508C and 300MPa) was used as a starting material; this has a composition identical to the groundmass of Unzen rocks erupted in 1991^1995. Decompression rates were varied from 0·0002 to 20MPa s . Experiments conducted with decompression rates 0·1MPa s 1 were decompressed continuously; a multi-step decompression approach was used at decompression rates 0·1MPa s . The experiments were fluid-saturated, either containing only water as a fluid component (H2O-bearing) or containing a water and carbon dioxide mixture (H2OþCO2; initial mole fraction of H2O in the fluid 0·6).The experimental products of the H2O-bearing experiments consist of amphibole, pyroxenes, oxides and glass. Plagioclase microlites nucleated and grew only in experiments with the two lowest decompression rates of 0·0005 and 0·0002MPa s . The length of those plagioclases is up to 200^250 mm, which is consistent with the size of plagioclase microlites observed in the natural samples.The experimental products of the H2OþCO2-bearing system are composed of pyroxenes, oxides, glass and plagioclase. Plagioclase microlites in the H2OþCO2-system were already present in the starting assemblage and grew to a maximum size of 80 mm. Equilibrium concentrations of water in the residual glasses at the final pressure of 50MPa are reached at decompression rates 1MPa s 1 for the H2OþCO2-bearing system and 0·1MPa s 1 for the H2O-bearing system. The bubble number density (BND) values range from 10 to 10 mm 3 in the H2O-bearing system and from 10 4·6 to 10 mm 3 in the H2OþCO2-bearing systems. In both systems, BND values decrease with decreasing decompression rate from 20 to 0·01MPa s , and increase with decreasing decompression rates50·01MPa s , which is interpreted to reflect predominant bubble growth and bubble nucleation, respectively.The onset of crystallization, observed from changes in the chemical composition of the residual melt, occurs at decompression rates 50·1MPa s . At the lowest decompression rate (0·0002MPa s ) the chemical composition of the residual melt in the H2OþCO2-bearing system becomes similar to the natural matrix glass composition. There is no significant variation of the microlite number density (MND) value as a function of the decompression rate. The MND values for plagioclases-only range from 10 to 10 mm , whereas the MND values for the other phases range from 10 to 10 mm . Our experimental MNDPl values are in the range of those from natural samples (10^10 mm ).We show that the size of microlites nucleating and crystallizing during decompression (plagioclase in our experimental dataset) is useful to constrain magma ascent rates at the onset of the crystallization of the corresponding phase. Based on the size of plagioclase microlites and on the composition of the residual melts, the average magma ascent rates of Unzen magmas in the pressure range 200 to 50MPa is estimated to be 10^50 m h .