Experimental petrology of monotonous intermediate magmas
Experimental petrology of monotonous intermediate magmas
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DOI：
10.1144/sp422.9
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发表时间：
2015-01-01
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影响因子：
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通讯作者：
Blundy, Jon
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作者：
Caricchi, Luca;Blundy, Jon
大体积、高结晶度、化学均质的熔结凝灰岩，被称为“单调的中间体”，为研究地壳岩浆储层的演化提供了独特的机会。本文介绍了科罗拉多（美国）鱼峡谷凝灰岩（FCT）中英安岩的热液实验结果，这是一个典型的单一中间存款的例子，以表征含水英安岩岩浆的化学和物理性质随温度的变化。实验（200 MPa，720-1100 ℃）涵盖了FCT岩浆的推断喷发前条件，并显示在790 +/- 10 ℃或接近水饱和的条件下，提供了与喷发岩浆的化学和物理性质的最佳匹配。结果表明，含水量在控制岩浆的化学和物理演化过程中起着重要作用，水饱和和水欠饱和岩浆的行为也有明显的差异。然而，在这两种情况下，都存在一个宽的温度区间（200摄氏度），在该区间内，晶体分数几乎没有变化。通过重铸这种行为的焓，而不是温度，作为自变量，我们表明，这个间隔对应于结晶度的变化，每单位的能量增加或减少从系统中的最小值，这样的小扰动的热含量的系统（例如，通过冷却或新的岩浆注入）的结果在岩浆性质的变化很小。该区间的晶体含量为55- 65wt%，接近于单调中间体的斑晶含量（40- 55wt%）。我们认为，富含晶体的岩浆往往会在这个“岩石学陷阱”中沉淀下来，随着系统的发展，随着时间的推移，物理和化学性质几乎没有变化。岩石学上的圈闭使大量的中间岩浆在地壳浅层聚集，直到这些富含晶体的岩浆的净浮力足以克服顶板岩石的强度，从而导致潜在的巨大喷发。
Large-volume, high-crystallinity, chemically homogeneous ignimbrites, dubbed 'monotonous intermediates', provide a unique opportunity to investigate the evolution of crustal magmatic reservoirs. We present the results of hydrothermal experiments on a dacite from Fish Canyon Tuff (FCT) in Colorado (USA), a classic example of a monotonous intermediate deposit, in order to characterize the variations in chemical and physical properties of hydrous dacite magmas as a function of temperature. The experiments (200 MPa, 720-1100 degrees C) span the inferred pre-eruptive conditions of FCT magmas, and are shown to provide the best match to the chemical and physical properties of the erupted magmas at 790 +/- 10 degrees C under conditions at or close to water-saturation. The results show the important effect of water content in controlling the chemical and physical evolution of magma, and the contrasted behaviour of water-saturated v. water-undersaturated magmas. In both cases, however, there is a broad interval of temperature (200 degrees C) over which crystal fraction changes little. By recasting this behaviour in terms of enthalpy, rather than temperature, as the independent variable we show that this interval corresponds to a minimum in the change in crystallinity per unit of energy added or subtracted from the system, such that small perturbations to the heat content of the system (e.g. by cooling or new magma injections) results in very little change in magma properties. The crystal content in this interval is 55-65 wt%, which is close to the phenocryst content (40-55 wt%) of monotonous intermediates. We propose that crystal-rich magmas tend to settle in this 'petrological trap', changing little in physical and chemical properties over time as the system grows. Petrological trapping enables very large volumes of intermediate magma to accumulate in the shallow crust until such time as the net buoyancy force of these crystal-rich magma is sufficient to overcome the strength of the roof rocks, leading to a potentially very large eruption.