Magmatic conditions and processes in the storage zone of the 2004-2006 Mount St. Helens dacite
Magmatic conditions and processes in the storage zone of the 2004-2006 Mount St. Helens dacite
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2004-2006 年圣海伦斯山英安岩储藏区的岩浆条件和过程
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发表时间：
2008
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通讯作者：
J. Devine
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作者：
M. Rutherford;J. Devine
2004 年 6 月圣海伦斯火山喷发产生的英安岩在几乎完全结晶的基质中含有 40-50 体积％的斜长石、角闪石、低钙辉石、磁铁矿和钛铁矿斑晶。角闪石和辉石的斑晶长度为 3 至 5 毫米，呈周期性分区，富铁和铝层与富镁和硅层交替一到三个，几乎没有显示区域之间斑晶溶解的迹象。类似尺寸的斜长石斑晶也包含几个范围在 ~An 68 和 An 45-35 之间的循环带。结构证据表明，角闪石、辉石和钛铁矿在富含 An 的斜长石之前开始结晶。磁铁矿和钛铁矿斑晶很小（小于 100 μm），不同颗粒的成分略有不同，并且呈零星分布。磁铁矿-钛铁矿对的平衡温度范围为 820°C 至 890°C，f O2 值为 NNO +1 log 单位。磁铁矿成分表明，2004-6 岩浆是由喷发前不到 5-8 周的岩浆混合形成的，并且岩浆在该温度范围内最后达到平衡。角闪石斑晶分区涉及在一个生长周期中大约等量的压力敏感的 Al-Tschermak 分子取代和温度敏感的伊甸石取代。对天然英安岩进行的水热实验表明，富含铁和铝的角闪石端元的结晶需要在 900°C 的温度下 200-300 MPa 的压力，这一条件接近角闪石稳定性的温度上限。当压力在 900°C 下降至 200 MPa 时，英安质岩浆会结晶出 An 68 斜长石。岩浆必须在此深度冷却，以在岩浆循环回高压之前在角闪石上产生完整的 An 68 -An 40 斜长石带和富镁层，此时会获得新的富铁角闪石层。在小于 200 MPa 的英安岩实验中，角闪石结晶的铝含量低于天然循环分区斑晶中的任何成分。一些 2004-6 角闪石斑晶的外缘似乎是在 100-200 MPa 范围内形成的，1980 年 5 月英安岩浮石中的一些斑晶也是如此。 2004-6 岩浆中 An 35 的斜长石边缘表明，斑晶生长一直持续到压力降至 130 MPa，并且在此深度之前上升缓慢。然后岩浆进入管道，相对快速地上升到地表，如角闪石斑晶上减压引起的非常薄的边缘（小于 5 μm）所示。
The 2004-6 eruption of Mount St. Helens produced dacite that contains 40-50 volume percent phenocrysts of plagioclase, amphibole, low-Ca pyroxene, magnetite, and ilmenite in a groundmass that is nearly totally crystallized. Phenocrysts of amphibole and pyroxene range from 3 to 5 mm long and are cyclically zoned, with one to three alternations of Fe- and Al-rich to Mg- and Si-rich layers showing little indication of phenocryst dissolution between zones. Similar-size plagioclase phenocrysts also contain several cyclic zones ranging between ~An 68 and An 45-35 . Textural evidence indicates that amphibole, pyroxene, and ilmenite began to crystallize before the most An-rich plagioclase. Magnetite and ilmenite phenocrysts are small (less than 100 μm), vary somewhat in composition from grain to grain, and are sporadically zoned. Magnetite-ilmenite pairs yield temperatures of equilibration ranging from 820°C to 890°C and f O2 values of NNO +1 log unit. Magnetite compositions suggest that the 2004-6 magma was formed by mingling of magmas less than 5-8 weeks before eruption and that the magma last equilibrated within this temperature range. The amphibole phenocryst zoning involves approximately equal amounts of a pressure-sensitive Al-Tschermak molecular substitution and a temperature-sensitive edenite substitution in one cycle of growth. Hydrothermal experiments done on the natural dacite show that crystallization of the Fe- and Al-rich amphibole end member requires pressures of 200-300 MPa at temperatures of 900°C, conditions approaching the upper temperature limit of amphibole stability. The dacitic magma crystallizes the An 68 plagioclase when the pressure drops to 200 MPa at 900°C. The magma must cool at this depth to produce a complete An 68 -An 40 plagioclase zone and a Mg-rich layer on the amphiboles before the magma is cycled back to a high pressure, when a new layer of Fe-rich amphibole is acquired. The amphibole crystallizing in the dacite experiments at less than 200 MPa is lower in aluminum than any compositions in the natural cyclically zoned phenocrysts. The outer rim on some 2004-6 amphibole phenocrysts appears to have formed in the 100-200 MPa range, as do some phenocrysts in the May 1980 dacite pumice. Plagioclase rims of An 35 in the 2004-6 magmas indicate that phenocryst growth continued until the pressure decreased to 130 MPa and that ascent was slow until this depth. Magma then entered the conduit for a relatively rapid ascent to the surface as indicated by the very thin (less than 5 μm) decompression-induced rims on the amphibole phenocrysts.