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首页> 外文期刊>Contributions to Mineralogy and Petrology >Composition and origin of rhyolite melt intersected by drilling in the Krafla geothermal field, Iceland
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Composition and origin of rhyolite melt intersected by drilling in the Krafla geothermal field, Iceland

机译：在冰岛克拉夫拉地热田中钻探相交的流纹岩熔体的成分和成因

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摘要

The Iceland Deep Drilling Project Well 1 was designed as a 4- to 5-km-deep exploration well with the goal of intercepting supercritical hydrothermal fluids in the Krafla geothermal field, Iceland. The well unexpectedly drilled into a high-silica (76.5 % SiO2) rhyolite melt at approximately 2.1 km. Some of the melt vesiculated while extruding into the drill hole, but most of the recovered cuttings are quenched sparsely phyric, vesicle-poor glass. The phenocryst assemblage is comprised of titanomagnetite, plagioclase, augite, and pigeonite. Compositional zoning in plagioclase and exsolution lamellae in augite and pigeonite record changing crystallization conditions as the melt migrated to its present depth of emplacement. The in situ temperature of the melt is estimated to be between 850 and 920 °C based on two-pyroxene geothermometry and modeling of the crystallization sequence. Volatile content of the glass indicated partial degassing at an in situ pressure that is above hydrostatic (~16 MPa) and below lithostatic (~55 MPa). The major element and minor element composition of the melt are consistent with an origin by partial melting of hydrothermally altered basaltic crust at depth, similar to rhyolite erupted within the Krafla Caldera. Chondrite-normalized REE concentrations show strong light REE enrichment and relative flat patterns with negative Eu anomaly. Strontium isotope values (0.70328) are consistent with mantle-derived melt, but oxygen and hydrogen isotope values are depleted (3.1 and −118 ‰, respectively) relative to mantle values. The hydrogen isotope values overlap those of hydrothermal epidote from rocks altered by the meteoric-water-recharged Krafla geothermal system. The rhyolite melt was emplaced into and has reacted with a felsic intrusive suite that has nearly identical composition. The felsite is composed of quartz, alkali feldspar, plagioclase, titanomagnetite, and augite. Emplacement of the rhyolite magma has resulted in partial melting of the felsite, accompanied locally by partial assimilation. The interstitial melt in the felsite has similar normalized SiO2 content as the rhyolite melt but is distinguished by higher K2O and lower CaO and plots near the minimum melt composition in the granite system. Augite in the partially melted felsite has re-equilibrated to more calcic metamorphic compositions. Rare quenched glass fragments containing glomeroporphyritic crystals derived from the felsite show textural evidence for resorption of alkali feldspar and quartz. The glass in these fragments is enriched in SiO2 relative to the rhyolite melt or the interstitial felsite melt, consistent with the textural evidence for quartz dissolution. The quenching of these melts by drilling fluids at in situ conditions preserves details of the melt–wall rock interaction that would not be readily observed in rocks that had completely crystallized. However, these processes may be recognizable by a combination of textural analysis and in situ analytical techniques that document compositional heterogeneity due to partial melting and local assimilation.
机译：冰岛深度钻井项目1井设计为4至5公里深的勘探井，目的是在冰岛克拉夫拉地热田中拦截超临界热液。该井出人意料地在约2.1公里处钻入了高二氧化硅（76.5％SiO2）流纹岩熔体中。一些熔融物在挤出到钻孔中时发生了气泡化，但是大部分回收的切屑是稀疏的，稀疏的，气泡少的玻璃淬火的。面晶集合体由钛磁铁矿，斜长石，辉石和皂石组成。当熔体迁移到其当前的沉积深度时，斜长石和溶解片晶的组成区带（在闪锌矿和鸽皮石中）记录了变化的结晶条件。根据二吡咯地热法和结晶顺序的建模，估计熔体的原位温度在850至920°C之间。玻璃的挥发物含量表明在高于静水压（〜16 MPa）和低于静水压（〜55 MPa）的原位压力下部分脱气。熔体的主要元素和次要元素组成与深部热液蚀变玄武质地壳的部分融化相一致，类似于克拉夫拉火山口内流纹岩的喷发。球粒晶归一化的稀土元素浓度显示出强烈的轻稀土元素富集和相对平坦的模式，负Eu异常。锶同位素值（0.70328）与地幔衍生的熔体一致，但相对于地幔值，氧和氢同位素值却被消耗掉（分别为3.1和-118‰）。氢同位素值与由陨石水补充的克拉夫拉地热系统所改变的岩石中的热液史迪奇值重叠。流纹岩熔体被放入具有几乎相同组成的长英质侵入性套件中并与之反应。该长石由石英，碱金属长石，斜长石，钛磁铁矿和mag石组成。流纹岩岩浆的侵袭导致了长石的部分融化，局部伴有部分同化作用。镁铁矿中的间隙熔体具有与流纹岩熔体相似的归一化SiO2含量，但其特征在于较高的K2O和较低的CaO，并且在花岗岩系统中的熔体组成最小附近分布。部分熔融的铁矿石中的奥铁石重新平衡为更多的钙质变质成分。含有源自镁铁矿的肾小球卟啉晶体的稀有淬火玻璃碎片显示出对长石和石英的吸收的结构证据。这些碎片中的玻璃相对于流纹岩熔体或间隙铁矾土熔体富含SiO2，这与石英溶解的结构证据一致。通过在原位条件下钻井液对这些熔体进行淬火，可以保留熔体与围岩相互作用的细节，而在已经完全结晶的岩石中很难观察到这些细节。但是，这些过程可以通过结合纹理分析和原位分析技术来识别，这些技术记录了由于部分熔化和局部同化引起的成分异质性。

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