GROUNDMASS CRYSTALLIZATION OF MOUNT ST HELENS DACITE, 1980-1986 - A TOOL FOR INTERPRETING SHALLOW MAGMATIC PROCESSES
GROUNDMASS CRYSTALLIZATION OF MOUNT ST HELENS DACITE, 1980-1986 - A TOOL FOR INTERPRETING SHALLOW MAGMATIC PROCESSES
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DOI：
10.1007/bf00306547
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发表时间：
1992-02-01
影响因子：
3.5
通讯作者：
CASHMAN, KV
中科院分区：
文献类型：
--
作者：
CASHMAN, KV
1980 - 1986年圣海伦斯火山的喷发为在短时间尺度上观察岩浆系统的演化提供了前所未有的机会。基质斜长石的尺寸测量加上测量的基质玻璃，斜长石和铁钛氧化物化学的变化，以记录增加基质结晶度，从而更好地约束提出的物理模型后，1980年5月18日岩浆油藏。对斜长石微晶和微斑晶尺寸的测量表明，在5月18日之后不久，基质斜长石发生了相对较快的生长（约10（-9）cm/s）。相对快速的斜长石生长持续到1980年底，平均速率为3 x 10（-11）cm/s;斜长石生长速率随后下降到1986年<1 x 10（-11）cm/s。基质玻璃和斜长石微斑晶边缘化学的变化反映了基质结晶度的变化，尽管基质玻璃的组成在从5月18日到1980年底的快速组成变化之后，从1981年到1986年似乎保持大致恒定。斜长石微斑晶显示越来越复杂的分区模式随时间的推移，微斑晶核心组成通常与晶体大小呈正相关。这两个观测结果都表明，直到1986年，基质斜长石都在持续生长。根据铁钛氧化物对估算的岩浆温度在1981年大致保持不变，喷发温度约为930 ℃，log f（o2）为-10.8;到1985 - 1986年，氧化物温度降至约870 ℃。化学和结构的变化可以解释为：（1）快速脱气和结晶，以应对岩浆侵入到一个浅（<4.5公里）水库接近1980年5月18日爆发结束;（2）持续结晶在一个大大降低的速度，通过1986年，由于缓慢冷却的浅岩浆水库。增长率（和随之而来的化学变化）似乎在1980年底下降-这是一致的喷发风格的变化，从爆炸性喷发，有时其次是圆顶增长，完全喷出（圆顶建设）事件，可以解释为预期的粘度增加脱气和结晶度增加。浅部储层岩浆两阶段结晶模式与天然气研究结论一致（Casadevall等人，1983年;盖拉赫and Casadevall 1986 a，B），陨石坑变形模式（Chadwick et al. 1988）和1980年后的地震活动（Endo等，1990），虽然它不能解释Hill和Rutherford（1989）关于角闪石反应边生长速率的实验数据。对圣海伦斯英安岩的结构测量也可以用来评估岩浆中的结晶动力学，而这些系统的实验数据几乎不存在。斜长石的生长速率比玄武岩系统中斜长石的估计生长速率慢5 - 10倍，这一结果与更高粘度的熔体相一致。此外，结构变化的模式（平均晶体尺寸和数密度）与Lipman和Banks（1987）在1984年莫纳罗亚火山喷发期间观察到的相似，这表明从玄武岩系统外推所需的斜长石结晶行为的唯一修改是速率的适度降低，因此结晶速率与熔体粘度成比例。
The 1980-1986 eruption of Mount St. Helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale. Groundmass plagioclase size measurements are coupled with measured changes in matrix glass, plagioclase and Fe-Ti oxide chemistry to document increasing groundmass crystallinity, and thus to better constrain proposed physical models of the post-May 18, 1980 magmatic reservoir. Measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth (approximately 10(-9) cm/s) of groundmass plagioclase occurred immediately subsequent to May 18. Relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3 x 10(-11) cm/s; plagioclase growth rates then decreased to < 1 x 10(-11) cm/s through 1986. Changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst-rim chemistry, although the matrix glass composition appears to have remained approximately constant from 1981-1986 after a rapid compositional change from May 18 until the end of 1980. Plagioclase microphenocrysts show increasingly more complex zoning patterns with time; microphenocryst-core compositions are commonly positively correlated with crystal size. Both of these observations indicate continuous groundmass plagioclase growth through 1986. Magmatic temperatures estimated from Fe-Ti oxide pairs are approximately constant through 1981 at eruption temperatures of approximately 930-degrees-C and at log f(o2) of -10.8; by 1985-1986 oxide temperatures decreased to approximately 870-degrees-C. Chemical and textural changes can be explained by: (1) rapid degassing and crystallization in response to the intrusion of magma into a shallow (< 4.5 km) reservoir toward the end of the May 18, 1980 eruption; (2) continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir. Growth rates (and consequent chemical changes) appear to decrease at the end of 1980-this is coincident with the change in eruption style from explosive eruptions, sometimes followed by dome growth, to solely extrusive (dome-building) events, and can be explained by the expected viscosity increase of both degassing and increasing crystallinity. The model of two-stage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies (Casadevall et al. 1983; Gerlach and Casadevall 1986 a, b), patterns of crater deformation (Chadwick et al. 1988) and post-1980 seismicity (Endo et al. 1990), although it does not explain the experimental data of Hill and Rutherford (1989) on the growth rate of amphibole reaction rims. Textural measurements on Mount St. Helens dacite can also be used to evaluate crystallization kinetics in silicic magmas, systems for which experimental data is almost non-existent. Plagioclase growth rates are 5-10 times slower than estimated plagioclase growth rates in basaltic systems, a result consistent with the higher viscosity of a more silicic melt. Furthermore, patterns of textural change (both average crystal size and number density) are similar to those observed during the 1984 Mauna Loa eruption by Lipman and Banks (1987), suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates, such that the rate of crystallization scales with the melt viscosity.