The May 18, 1980, eruption of Mount St. Helens: 3. Stability and chemistry of amphibole in the magma chamber
The May 18, 1980, eruption of Mount St. Helens: 3. Stability and chemistry of amphibole in the magma chamber
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DOI：
10.1029/jb093ib10p11949
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发表时间：
1988-10
影响因子：
--
通讯作者：
M. Rutherford;J. Devine
中科院分区：
文献类型：
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作者：
M. Rutherford;J. Devine
对1980年5月18日的圣海伦斯英安岩和1980年10月的更镁铁质的英安岩进行了其他实验，以解决5月18日喷发前角闪石稳定性和岩浆室中溶解挥发物的问题。这些实验是在920℃，流体压力220兆帕或320兆帕下进行的，与以前的工作相反，是在NNO和MnO-Mn3O4氧气缓冲液之间的ƒO2下进行的。在这些条件下，熔体中存在铁钛氧化物，当流体中的XH2O大于0.67时，闪石是稳定的。与熔体平衡的斜长石的An含量随着流体中XH2O的减少而减少，在角闪石实验中，在XH2O为0.67时，斜长石达到了天然的斜长石边缘成分(An49)。在这些水不饱和的条件下，实验生产的角闪石、低钙辉石和富钙辉石在组成上相当于5月18日白色浮石中的斑晶。角闪石实验中的熔体(玻璃)的范围从平均斜长石熔体包裹体成分[Rutherford等人，1985]到随着XH2O接近1.0而略微演化的成分。对天然角闪石中的熔体包裹体进行了分析，并结合实验玻璃分析，在SiO_2变化图上绘制了成分图。角闪石熔体包裹体定义了一条从相对原始成分(68wt%SiO_2，无水基础)到更演化的平均斜长石熔体包裹体成分(73wt%SiO_2)的液态下降线。角闪石熔体包裹体(差值法)的挥发分含量为5.0±0.5wt%，与XH2O=0.67时产生的角闪石实验熔体的挥发分含量相当。
Additional experiments have been done with the May 18, 1980, Mount St. Helens dacite and a more mafic, October 1980 sample to resolve questions concerning amphibole stability and dissolved volatiles in the magma chamber prior to the May 18 eruption. The experiments were done at 920°C, at fluid pressures of 220 or 320 MPa, and, in contrast to previous work, at an ƒO2 between the NNO and MnO-Mn3O4 oxygen buffers. Fe-Ti oxides are present in the melt under these conditions, and amphibole is stable when XH2O in the fluid is greater than 0.67. The An content of plagioclase in equilibrium with melt decreases with decreasing XH2O in the fluid and, in the amphibole-bearing experiments, reaches the natural plagioclase rim compositions (An49) at an XH2O of 0.67. Under these H2O-undersaturated conditions the experimentally produced amphibole, low-Ca pyrpxene, and Ca-rich pyroxene are compositionally equivalent to phenocrysts in the May 18 white pumice. The melts (glasses) in amphibole-bearing experiments range from the average plagioclase melt inclusion composition [Rutherford et al., 1985] to slightly less evolved compositions as XH2O approaches 1.0. Melt inclusions in natural amphiboles were analyzed, and the compositions were plotted on SiO2 variation diagrams along with experimental glass analyses. The amphibole melt inclusions define a liquid line of descent for the magina which extends from relatively primitive compositions (68 wt % SiO2, anhydrous basis) to the more evolved average plagioclase melt inclusion composition (73 wt % SiO2). The volatile content of the amphibole melt inclusions (difference method) reaches 5.0±0.5 wt %, which compares favorably with the volatile content of the amphibole-bearing experimental melts produced at XH2O = 0.67.