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首页> 外文期刊>Contributions to Mineralogy and Petrology >Geochemical and petrological diversity of mafic magmas from Mount St. Helens
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Geochemical and petrological diversity of mafic magmas from Mount St. Helens

机译：圣海伦山MAFIC岩浆地球化学和岩石学多样性

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摘要

Quaternary eruptive products in the Cascade arc include a variety of different basalt types. At Mount St. Helens (MSH), the most active volcano in the Cascades throughout the last 35ka, three different mafic endmembers erupted at the end of the Castle Creek period (1900-1700years B.P.): (1) high-field strength element (HFSE)-rich basalt enriched in K, Ti, P, and incompatible trace elements; (2) low-K olivine tholeiite (LKOT) with lower amounts of incompatible trace elements; and (3) calc-alkaline (arc-type) basaltic andesite with a typical subduction signature, i.e., enrichment in fluid-mobile large ion lithophile elements (LILE) relative to immobile high-field strength elements (HFSE). Each type has compositions projecting backwards to more primitive endmembers in the Cascades. Single units encompassing basaltic-to-basaltic andesitic compositions with intermediate trace-element abundances form two almost continuous trends towards basaltic andesite. These trends are interpreted to result from assimilation of pre-existing, more evolved, calc-alkaline material (and in one case mixing of different mafic magma types) during migration of the magmas through the crust. Most of the erupted basalts are porphyritic (10-30%) with an assemblage dominated by olivine and plagioclase and show disequilibrium textures preventing detailed reconstruction of mantle melting processes. Although typical hydrous arc basalt produced by flux melting in the mantle is absent in the eruptive products of MSH, arc-type basaltic andesite suggests its presence at depth. LKOT magmas are interpreted as decompression melts from the upper mantle, whereas HFSE-rich basalts are likely derived from the water-poor periphery of the main flux melting regime, potentially tapping a trace-element-enriched source. Primitive spinel compositions and whole-rock trace-element variations indicate at least two distinct, relatively fertile lherzolite sources for these two basalt types. Weak crustal zones associated with an old fracture system beneath MSH likely provide conduits for fast and isolated ascent of distinct batches of magma, bypassing the lower crustal mush zone. The eruption of the basalts through the upper crustal magma system and main edifice is consistent with an offset plumbing system suggested by geophysical data.
机译：级联弧形弧形爆发产品包括各种不同的玄武岩类型。在圣海伦山（MSH），在过去35KA中最活跃的火山，三种不同的MAFIC Endmembers在城堡溪期（1900-1700年BP）结束时爆发：（1）高场强元件（ HFSE） - 富集碱基玄武器富含K，Ti，P和不相容的微量元素; （2）低k橄榄石山竹（LKOT），具有较低的不相容痕量元素; （3）钙碱（Arc型）玄武岩，具有典型的俯冲特征，即流体 - 移动大离子型锂尼元件（Lile）相对于固定的高场强度元件（HFSE）。每种类型都具有向后投射到级联中的更原始的终点的组合物。包括中间微量元素丰度的玄武岩至玄武岩和塞米氏菌组合物的单一单元形成了两种几乎持续朝向玄武岩的趋势。这些趋势被解释为在岩浆迁移过程中，解释为同化预先存在，更进化的钙碱材料（以及不同抹布岩浆类型的一个壳体混合）。大多数爆发的沼泽地是卟啉（10-30％），其中由橄榄石和Plagioclase主导的组合，并显示不平衡纹理，防止了披风熔化过程的详细重建。虽然在MSH的爆发产物中缺乏通过焊条中熔化的透气熔化的典型湿弧玄武岩，但Arc型玄武岩亚体缺席表明其在深度的存在。 LKOT Magmas被解释为减压熔体从上部地幔熔化，而HFSE丰富的基础可能来自主要助熔剂制度的水差的外围，可能攻丝富含痕量的富含源。原始尖晶石组合物和全岩体痕量元素变化表明这两种玄武岩类型的至少两个不同的相对肥沃的漂流岩源。与MSH下方的旧骨折系统相关的薄弱的地壳区可能提供导管，用于快速和孤立的岩浆中的不同批次的岩浆，绕过下层地壳糊涂区。通过上层地壳岩浆系统和主大厦的沼泽喷发是与地球物理数据建议的偏移管道系统一致的。

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