To assess whether magma ascent rates control the style of volcanic eruption, we have studied the petrography, geochemistry and size distribution of microlites of plagioclase and pyroxene from historical eruptions from Tongariro, Ruapehu and Ngauruhoe volcanoes located in the southern Taupo Volcanic Zone, New Zealand. The studied deposits represent glassy andesitic and dacitic tephra shards from the Mangamate, Mangatawai, Tufa Trig and the Ngauruhoe tephra formations, ranging in age from 11,000 years BP to 1996 AD. Covering a range in eruption styles and sizes from Strombolian to Plinian, these samples provide an excellent opportunity to explore fundamental volcanic processes such as pre-eruptive magma ascent processes. Our quantitative petrographic analysis shows that larger microlites (> 30 µm) display complex growth zoning, and only the smallest crystals (< 30 µm) have formed during magma ascent in the conduit. Using a combination of orthopyroxene geothermometry, plagioclase hygrometry, and MELTS modelling, we show that these microlites nucleated at maximum pressures of 550 MPa (c. 16.5 km) from hot andesitic magmas (1010-1130 ˚C) with low H₂O content (0-1.5 wt%). Size distributions of a total of > 60,000 microlites, involving 22 tephras and 99 glass shards, yield concave-up curves, and the slopes of the pyroxene microlite size distributions, in combination with well-constrained orthopyroxene crystal growth rates from one studied tephra, indicate microlite population growth times of ∼3 ± 1 days, irrespective of eruption style. These data imply that microlites form in response to cooling of melts ascending at velocities of < 5 cm s^-1 prior to H₂O exsolution, which only occurs at < 33 MPa. Maximum magma ascent rates in the upper conduit, calculated using the exsolution of water during final decompression, range between 3 to 12 m s^-1, i.e. at least an order of magnitude lower than the hypersonic vent velocities typical of Vulcanian or sub-Plinian eruptions (up to 400 m s^-1). This implies that magma ascent from depths of an average of 4 km occurs in dykes, and that vent velocities at the surface are controlled by a reduction of conduit cross-section towards the surface (e.g. dyke transiting to cylindrical conduit).

中文翻译：

---

缓慢上升的异常高温中间岩浆触发斯特林伯纪爆发至次品系爆发

为了评估岩浆上升速率是否控制着火山喷发的样式，我们研究了位于新西兰陶波火山区南部的汤加里罗，鲁阿佩胡和恩古鲁霍火山的历史喷发带来的斜长石和辉石微岩的岩石学，地球化学和尺寸分布。所研究的沉积物代表了来自Mangamate，Mangatawai，Tufa Trig和Ngauruhoe tephra地层的安第斯山脉和达特科山脉的特菲拉碎片，其年龄从BP的11,000年到公元1996年。这些样本涵盖了从斯特伦伯连纪到普利尼期的一系列喷发样式和大小，为探索基本的火山喷发过程（例如喷发前的岩浆上升过程）提供了绝佳的机会。我们的定量岩相分析表明，较大的微晶岩（> 30 µm）显示出复杂的生长带，并且在岩浆上升过程中，只有最小的晶体（<30 µm）在管道中形成。结合使用邻苯二甲醚地热法，斜长岩湿度法和熔化建模，我们表明，这些microlites在550兆帕（的最大压力核Ç从热安山岩岩浆（1110至30年c）与低小时。16.5公里）₂ O的含量（0-1.5％重量）。总共超过60,000颗微晶的尺寸分布，包括22个特弗拉斯和99个玻璃碎片，产生凹形曲线，以及辉石微晶尺寸分布的斜率，以及来自一个研究的特菲拉的邻苯二茂铬晶体的良好约束生长，表明不论喷发方式如何，微岩种群的生长时间约为3±1天。这些数据表明，微晶岩是在H ₂之前响应于以<5 cm s ^-1的速度上升的熔体冷却而形成的O exsolution，仅在<33 MPa时发生。使用最终减压过程中的水溶解度计算出的上部导管中的最大岩浆上升速率介于3到12 ms ^-1之间，即比典型的Vulcanian或次Plinian喷发的高超声速排放速度低至少一个数量级（高达400毫秒^-1）。这意味着在堤坝中平均从4 km的深度开始岩浆上升，并且表面处的出气口速度通过朝向表面的管道横截面的减小来控制（例如，堤坝过渡到圆柱形管道）。