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首页> 外文期刊>Contributions to Mineralogy and Petrology >Ascent-driven crystallisation of dacite magmas at Mount St Helens, 1980–1986
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Ascent-driven crystallisation of dacite magmas at Mount St Helens, 1980–1986

机译：1980-1986年，圣海伦斯山的达克特岩浆的上升驱动结晶

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摘要

We introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system Qz–Ab–Or–H2O in order to relate variations in volcanic glass chemistry to changing pressure (P) and temperature (T) conditions in the sub-volcanic magma system. By this means an important distinction can be made between ascent-driven and cooling-driven crystallisation under water-saturated or undersaturated conditions. In samples containing feldspar and a silica phase (quartz or tridymite), quantitative P–T estimates of the conditions of last equilibrium between crystals and melt can be made. Formation of highly silicic melts (i.e. >77 wt% SiO2) is a simple consequence of the contraction of the silica phase volume with decreasing pressure, such that high silica glasses can only form by crystallisation at low pressure. Resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation. Groundmass and inclusion glasses in dacites from the 1980–1986 eruption of Mount St Helens volcano (WA) span a wide range in SiO2 (68–80 wt%, anhydrous). The compositions of the least evolved (SiO2-poor) inclusions in amphibole phenocrysts record entrapment of silicic liquids with ≤5.4 wt% water, corresponding to a water saturation pressure of ~200 MPa at 900 °C. The compositions of more evolved (higher SiO2) plagioclase-hosted inclusions and groundmass glasses are consistent with extensive ascent-driven fractional crystallisation of plagioclase, oxide and orthopyroxene phenocrysts and microlites to low pressures. During this polybaric crystallisation, plagioclase phenocrysts trapped melts with a wide range of dissolved water contents (3.5–5.7 wt%). Magmas erupted during the Plinian phase of the 18 May 1980 eruption were derived from a large reservoir at depths of ≥6 km. Subsequent magmas ascended to varying depths within the sub-volcanic system prior to extraction. From glass chemistry and groundmass texture two arrest levels have been identified, at depths of 0.5–1 and 2–4 km. A single dome sample from February 1983 contains groundmass plagioclase, tridymite and quartz, testifying to temperatures of at least 885 °C at 11 MPa. These shallow storage conditions are comparable to those in the cryptodome formed during spring 1980. The corresponding thermal gradient, ≤0.2 °C MPa–1, is consistent with near-adiabatic magma ascent from ~8 km. We argue that the crystallisation history of Mount St Helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation. This challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980–1986 eruption. Because the crystallisation process is both polybaric and fractional, it cannot be simulated directly using isobaric equilibrium crystallisation experiments. However, calculation of the phase proportions in water-saturated 910±15 °C experiments by Rutherford et al. (1985) over the pressure range 220–125 MPa reproduces the crystallisation sequence and phenocryst modes of Mount St Helens dacites from 18 May 1980. By allowing for the effects of fractional versus equilibrium crystallisation, entrained residual source material, and small temperature differences between nature and experiment, phase compositions can also be matched to the natural samples. We conclude that decompression of water-saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres.
机译：我们引入一种新颖的方案，使天然硅玻璃可以投射到合成系统Qz–Ab–Or–H2 O中，以便将火山玻璃化学变化与压力（P）和温度（T）条件联系起来在次火山岩浆系统中。通过这种方式，可以在水饱和或不饱和条件下在上升驱动结晶和冷却驱动结晶之间进行重要区分。在含有长石和二氧化硅相（石英或鳞石英）的样品中，可以对晶体和熔体之间的最后平衡条件进行定量的PT估计。高硅熔体（即> 77 wt％SiO2 ）的形成是二氧化硅相体积随压力降低而收缩的简单结果，因此高二氧化硅玻璃只能通过在低压下结晶来形成。石英晶体的吸收似乎是减压结晶的进一步诊断特征。从1980-1986年圣海伦斯火山（WA）喷发而来的自然界中的地物玻璃和夹杂玻璃的SiO2含量范围很广（68-80 wt％，无水）。角闪石隐晶中演化最少的（SiO2 -poor）夹杂物的组成记录了硅油被≤5.4wt％的水截留，对应于900°C时〜200 MPa的水饱和压力。析出更多（较高的SiO2 ）斜长石夹杂物和磨碎玻璃的组成与斜长石，氧化物和邻苯二酚苯晶和微晶在低压下广泛的上升驱动的分步结晶相一致。在这种多晶体结晶过程中，斜长石的隐晶石捕获的熔体具有广泛的溶解水含量（3.5-5.7 wt％）。 1980年5月18日普林期爆发的岩浆来自≥6 km深度的大型水库。在提取之前，随后的岩浆上升至次火山岩系统内的不同深度。从玻璃化学和地面质量纹理中可以看出，在0.5-1和2-4 km的深度有两个阻滞水平。 1983年2月的一个穹顶样品包含地面斜长石，鳞石英和石英，在11 MPa下的温度至少达到885°C。这些浅层的储存条件与1980年春季形成的隐蔽穹顶的条件相当。相应的热梯度≤0.2°C MPa–1 ，与〜8 km附近的近绝热岩浆上升一致。我们认为，圣海伦斯山粉晶岩浆岩的结晶历史在很大程度上是由于超出水饱和点的热岩浆减压结晶的结果。这挑战了传统观点，即在1980年至1986年喷发之前，在大的岩浆室内通过冷却发生了发晶结晶。由于结晶过程既是多晶的又是分馏的，因此无法使用等压平衡结晶实验直接模拟。然而，Rutherford等人在水饱和的910±15°C实验中计算相比例。 （1985）在220–125 MPa的压力范围内重现了1980年5月18日圣海伦火山dacites的结晶序列和隐晶模式。在实验中，相组成也可以与天然样品匹配。我们得出的结论是，在许多其他硅质火山中心，水饱和岩浆的减压可能是结晶的主要驱动力。

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  • 来源
    《Contributions to Mineralogy and Petrology》 |2001年第6期|631-650|共20页
  • 作者

    Jon Blundy; Kathy Cashman;

  • 作者单位

    CETSEI Department of Earth Sciences University of Bristol Bristol BS8 1RJ UK;

    Department of Geological Sciences University of Oregon Eugene OR 97403-1272 USA;

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