Chemical Variations in the 1998, 2011, and 2015 Lava Flows From Axial Seamount, Juan de Fuca Ridge: Cooling During Ascent, Lateral Transport, and Flow
Chemical Variations in the 1998, 2011, and 2015 Lava Flows From Axial Seamount, Juan de Fuca Ridge: Cooling During Ascent, Lateral Transport, and Flow
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DOI：
10.1029/2018gc007708
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发表时间：
2018-09-01
影响因子：
3.5
通讯作者：
Fundis, Allison T.
中科院分区：
文献类型：
--
作者：
Clague, David A.;Paduan, Jennifer B.;Fundis, Allison T.
1998年、2011年和2015年在轴海山喷发的熔岩流在化学上是不均匀的，随着距离山顶的距离增加，其玻璃皮中的氧化镁含量减少。这些趋势与火山喷发温度沿裂谷地带下降的趋势是一致的，裂谷地带的温度在火山口20公里范围内类似于0.5摄氏度/千米，在更远的距离类似于0.9摄氏度/千米(仅在2011年喷发时观察到)。岩浆在岩脉中传输过程中的冷却可能是观察到的温度趋势的原因，这与岩墙侵入较少的地区较冷的围岩的影响有关。水流在海底以比夏威夷基拉韦厄火山观测到的3-5倍大的速度前进时也会冷却。1998年和2011年在火山口及其附近喷发的熔岩是略微浓缩的过渡性大洋中脊玄武岩，为玄武岩，玻璃镁含量为7.1-7.6wt%。2015年的熔岩具有类似的富集洋中脊玄武岩典型的不相容元素组成，但喷发在破火山口内和东北缘的熔岩含有7.8-8.3wt%的较高玻璃氧化镁和更丰富的斜长石斑晶，这些斑晶是1290年至1370年期间喷发的正常洋脊玄武岩熔岩的典型特征。2011年至2015年喷发之间的短暂补给期不允许储存在浅层储集层中的岩浆冷却和脱气，这表明最近的补给期比多中心年平均水平要短。胡安德富卡中洋脊轴向海山在1998年、2011年和2015年喷发了多次玄武岩熔岩流。通过对290个熔岩样本以及2015年轻度爆发活动期间产生的单一火山砂样本的分析，确定了流动的成分。熔岩流都是受大洋中脊玄武岩影响的热点，结晶较少。玄武岩以0.5-1摄氏度/公里的速度冷却，在岩脉中从山顶运走时结晶了15%-20%的晶体。当它们离开喷发的裂缝时，它们以类似于3摄氏度/公里的速度冷却。顶区熔体成分随温度和氧化镁含量的不同而不同。最热的熔岩，温度为1201摄氏度，是在休止期4年后于2015年在山顶喷发的熔岩，而较冷的玄武岩，在休止期13年后，于2011年在山顶喷发，温度接近1186摄氏度。1998年熔岩的最高氧化镁含量和温度与2011年喷发的熔岩相似，因此可能有类似的休止期，与过去350年喷发活动的平均休止期一致。2015年的顶峰熔岩是过去350年来轴海山上喷发的最热的熔岩。
Lava flows erupted at Axial Seamount in 1998, 2011, and 2015 are chemically heterogeneous and display decreases in MgO content in their glass rinds with increasing distance from the summit. The trends are consistent with eruption temperature decreases down the rift zones of similar to 0.5 degrees C/km within 20 km of the caldera and similar to 0.9 degrees C/km at greater distance (only observed in the 2011 eruption). Cooling during magma transport in dikes is the likely cause of the temperature trends observed, related to the effects of cooler wall rocks in areas with less frequent dike intrusions. Flows also cooled as they advanced on the seafloor at rates 3-5 times greater than observed at Kilauea volcano in Hawaii for subaerial tube-fed pahoehoe flows. Lavas erupted in and near the caldera in 1998 and 2011 are slightly enriched transitional mid-ocean ridge basalt that are aphyric and have glass MgO content of 7.1-7.6 wt%. The 2015 lavas have similarly enriched incompatible element compositions typical of transitional mid-ocean ridge basalt, but those erupted inside and on the northeast rim of the caldera contain higher glass MgO of 7.8-8.3 wt% and more abundant plagioclase phenocrysts typical of the normal mid-ocean ridge basaltic lavas erupted between 1290 and 1370 CE. The brief recharge period between the 2011 and 2015 eruptions did not allow magma stored in the shallow reservoir to cool and degas as much as between prior eruptions since 1650 CE, suggesting that the most recent recharge period was shorter than the multicentennial average.Plain Language Summary Axial Seamount on the Juan de Fuca mid-ocean ridge erupted multiple lava flows of basaltic lava in 1998, 2011, and 2015. The composition of the flows was determined by analysis of 290 lava samples as well as a single sample of volcanic sand produced during mildly explosive eruptive activity in 2015. The lava flows are all hot spot influenced mid-ocean ridge basalt containing few crystals. Basalts cooled at rates of 0.5-1 degrees C/km and crystallized 15-20% crystals as they were transported away from the summit in dikes. They cooled at rates of similar to 3 degrees C/km as they flowed away from the eruptive fissures. Melt composition from the summit region varies in MgO content and temperature. The hottest lavas, at 1201 degrees C, were those erupted at the summit in 2015 after a 4-year repose period, whereas cooler basalts, at similar to 1186 degrees C, erupted at the summit in 2011 after a 13-year repose period. The 1998 lavas had a similar maximum MgO content and temperature to those erupted in 2011 and so may have had a similar repose period, consistent with the average repose period during the last 350 years of eruptive activity. The 2015 summit lavas were the hottest to erupt at Axial Seamount in the last 350 years.