Explosive volcanism on the ultraslow-spreading Gakkel ridge, Arctic Ocean
Explosive volcanism on the ultraslow-spreading Gakkel ridge, Arctic Ocean
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DOI：
10.1038/nature07075
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发表时间：
2008-06-26
期刊：
影响因子：
64.8
通讯作者：
Soule, Adam
中科院分区：
文献类型：
--
作者：
Sohn, Robert A.;Willis, Claire;Soule, Adam
地球外表面大约60%由大洋中脊火山作用形成的洋壳组成。尽管这片广阔的火山区域只有一小部分经过了目视勘测或取样，但现有证据表明，大洋中脊上的爆发性喷发很罕见，特别是在海水临界点（3000米）以下的深度。在3000米以下的海底从未观察到火山碎屑堆积物，大概是因为大洋中脊玄武岩的挥发物含量通常过低，无法在如此高的静水压力下产生使岩浆破碎所需的气体比例。在国际极地年一次前往东经85°的北冰洋加克尔海脊的探险中，我们使用了新的深潜技术，以获取这片偏远、被冰覆盖的海脊上“零年龄”火山区域的照片和视频图像。在此，我们展示的图像显示，水深4000米的轴谷覆盖着松散的火山碎屑堆积物，包括气泡壁碎片（佩莱之发），覆盖面积很大（>10平方千米）。在这些深度使岩浆破碎至少需要13.5%（重量比）的二氧化碳，这大约是先前在大洋中脊玄武岩中所测到的最高值的十倍。这些观测结果对加克尔海脊在超慢速扩张速率下岩浆挥发物的积聚和排放提出了重要问题，并表明即使在全球大洋中脊火山系统最深的部分也可能存在大规模的火山碎屑活动。
Roughly 60% of the Earth's outer surface is composed of oceanic crust formed by volcanic processes at mid- ocean ridges. Although only a small fraction of this vast volcanic terrain has been visually surveyed or sampled, the available evidence suggests that explosive eruptions are rare on mid- ocean ridges, particularly at depths below the critical point for seawater ( 3,000 m)(1). A pyroclastic deposit has never been observed on the sea floor below 3,000 m, presumably because the volatile content of mid- ocean- ridge basalts is generally too low to produce the gas fractions required for fragmenting a magma at such high hydrostatic pressure. We employed new deep submergence technologies during an International Polar Year expedition to the Gakkel ridge in the Arctic Basin at 856 E, to acquire photographic and video images of 'zero- age' volcanic terrain on this remote, ice- covered ridge. Here we present images revealing that the axial valley at 4,000 m water depth is blanketed with unconsolidated pyroclastic deposits, including bubble wall fragments (limu o Pele)(2), covering a large ( > 10 km(2)) area. At least 13.5 wt% CO2 is necessary to fragment magma at these depths(3), which is about tenfold the highest values previously measured in a mid- ocean- ridge basalt(4). These observations raise important questions about the accumulation and discharge of magmatic volatiles at ultraslow spreading rates on the Gakkel ridge(5) and demonstrate that large- scale pyroclastic activity is possible along even the deepest portions of the global mid- ocean ridge volcanic system.