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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年在圣海伦斯山的达克特岩浆的上升驱动结晶

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We introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system Qz-Ab-Or-H_2O 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 percent SiO_2) 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 SiO_2 (68-80 wt percent, anhydrous). The compositions of the least evolved (SiO_2-poor) inclusions in amphibole phenocrysts record entrapment of silicic liquids with <= 5.4 wt percent water, corresponding to a water saturation pressure of approx 200 MPa at 900 deg C. The compositions of more evolved (higher SiO_2) 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 percent). 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 deg 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 deg C MPa~(-1), is consistent with near-adiabatic magma ascent from approx8 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 deg 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：我们引入了一种新颖的方案，该方案能够将天然硅玻璃投射到合成系统Qz-Ab-Or-H_2O中，以便将火山玻璃化学变化与次火山中不断变化的压力（P）和温度（T）条件联系起来岩浆系统。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P-T估计。高硅熔体（即> 77 wt％的SiO_2）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才能形成。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SiO_2含量范围很广（68％至80％（重量），无水）。角闪石隐晶中演化最少的（SiO_2-贫）夹杂物的成分记录了硅油被<= 5.4 wt％的水截留的情况，对应于900摄氏度时约200 MPa的水饱和压力。 SiO_2）斜长石所包裹的夹杂物和磨碎玻璃与斜长石，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，斜长石隐晶石捕获的熔体具有广泛的溶解水含量（3.5-5.7重量％）。 1980年5月18日普林期爆发的岩浆来自一个深度大于等于6 km的大型水库。在提取之前，随后的岩浆上升到亚火山系统内的不同深度。根据玻璃化学性质和地基质地，已经确定了两个阻滞水平，深度为0.5-1和2-4 km。 1983年2月的一个圆顶样品包含地面斜长石，鳞石英和石英，证明在11 MPa下的温度至少为885摄氏度。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℃MPa〜（-1），与从约8 km起的近绝热岩浆上升相一致。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1980-1986年喷发之前，通过在大型岩浆室内进行冷却，发生了发晶结晶。因为结晶过程既是多晶的又是分馏的，所以不能使用等压平衡结晶实验直接模拟。但是，Rutherford等人在水饱和的910 + -15℃实验中计算相比例。 （1985年）在220-125 MPa的压力范围内，再现了1980年5月18日开始的圣海伦火山dacites的结晶序列和晶态模式。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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