Experimental constraints on syneruptive magma ascent related to the phreatomagmatic phase of the 2000AD eruption of Usu volcano, Japan
Experimental constraints on syneruptive magma ascent related to the phreatomagmatic phase of the 2000AD eruption of Usu volcano, Japan
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与公元 2000 年日本有珠火山喷发的热岩浆阶段相关的协同喷发岩浆上升的实验限制
DOI：
10.1007/s00445-006-0084-3
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发表时间：
2007
影响因子：
3.5
通讯作者：
J. Larsen
中科院分区：
文献类型：
--
作者：
Yuki Suzuki;J. Gardner;J. Larsen
我们通过实验研究了乌苏 2000 年喷发第一次事件中喷出的英安质岩浆，以研究协同喷发岩浆上升的条件。地球物理数据显示，岩浆从山顶下方上升后，到达了西西山下方，即事件火山口的位置。先前对喷射物气泡尺寸分布的研究表明，两个阶段（阶段 1 和阶段 2）具有不同的岩浆上升速率，因为随着第二阶段的开始，岩浆在西西山下方加速。为了模拟第一阶段从主储层（位于西西山下方 4-6 公里（125 兆帕）深度至 2 公里（50 兆帕））的上升，在 900°C 下按照两条路径进行等温减压实验。单步减压 (SSD) 样品快速减压 (0.67 MPa/s) 至最终压力并保持 12 至 144 小时。多步减压 (MSD) 将样品逐步减压至最终压力并立即淬火。在 MSD 中，平均减压速率和总实验持续时间分别在 0.01389 至 0.00015 MPa/s 和 1.5 至 144 小时之间变化。协同爆发结晶仅限于第一阶段，上升条件是通过记录喷射物和实验之间减压诱导结晶的相似性来确定的。实验微晶石的核心成分、数量密度和形状表明，在不到 1.5 小时的时间内上升到 2 公里深度。 SSD 实验中的实验微晶石的体积和数量密度最好地复制了 2 公里的减压率，表明岩浆在喷发前大约 24 小时保持在 2 公里处。根据大地测量结果，2 公里深度的停滞对应于通过堤坝从山顶下方到西西山的水平运输。包括第二阶段在内的总岩浆输送时间尺度为数十小时，比前兆地震活动的时间尺度（3.5天）短，表明喷发的岩浆在前2天内没有移出储层。这与地震数量的时间变化一致，地震数量在 2 天后达到峰值。
We experimentally studied the dacitic magma ejected during the first event in the Usu 2000 eruption to investigate the conditions of syneruptive magmatic ascent. Geophysical data revealed that the magma reached under West Nishiyama, the location of the event’s craters, after rising beneath the summit. Prior study of bubble-size distributions of ejecta shows two stages (stage 1 and stage 2) with different magma ascent rates, as the magma accelerated beneath West Nishiyama with the start of the second stage. To simulate ascent of stage 1 from the main reservoir, which was located at a depth of 4–6 km (125 MPa) to 2 km (50 MPa) beneath West Nishiyama, decompression experiments were conducted isothermally at 900°C following two paths. Single step decompression (SSD) samples were decompressed rapidly (0.67 MPa/s) to their final pressure and held for 12 to 144 hours. Multiple step decompression (MSD) samples were decompressed stepwise to their final pressure and quenched instantly. In MSD, the average decompression rates and total experimental durations varied between 0.01389 to 0.00015 MPa/s and 1.5 to 144 hours, respectively. Syneruptive crystallization was confined to stage 1, and the conditions of ascent were determined by documenting similarities in decompression-induced crystallization between ejecta and experiments. Core compositions, number densities, and shapes of experimental microlites indicate that ascent to 2 km depth occurred in less than 1.5 h. Volumes and number densities of experimental microlites from the SSD experiments that best replicate the decompression rate to 2 km indicate that the magma remained at 2 km for approximately 24 h before the eruption. Stagnation at a depth of 2 km corresponds with horizontal transport through a dike from beneath the summit to West Nishiyama, according to geodetic results. The total magma transport timescale including stage 2 is tens of hours and is shorter than the timescale of precursory seismicity (3.5 days), indicating that the erupted magma did not move out of the reservoir for the first 2 days. This is consistent with the temporal change in numbers of earthquakes, which reached a peak after 2 days.
影响因子：
3.5
作者：
CASHMAN, KV
通讯作者：
CASHMAN, KV