Department of Earth and Planetary Sciences, MSCO3-2040, 1 University of New Mexico, Albuquerque, New Mexico 87131-0001, USA.
Department of Earth Science, MS-126 Rice University, 6100 Main Street, Houston, Texas 77005, USA.
Nat Commun. 2016 Nov 1;7:13242. doi: 10.1038/ncomms13242.
Mount St Helens is the most active volcano within the Cascade arc; however, its location is unusual because it lies 50 km west of the main axis of arc volcanism. Subduction zone thermal models indicate that the down-going slab is decoupled from the overriding mantle wedge beneath the forearc, resulting in a cold mantle wedge that is unlikely to generate melt. Consequently, the forearc location of Mount St Helens raises questions regarding the extent of the cold mantle wedge and the source region of melts that are responsible for volcanism. Here using, high-resolution active-source seismic data, we show that Mount St Helens sits atop a sharp lateral boundary in Moho reflectivity. Weak-to-absent PmP reflections to the west are attributed to serpentinite in the mantle-wedge, which requires a cold hydrated mantle wedge beneath Mount St Helens (<∼700 °C). These results suggest that the melt source region lies east towards Mount Adams.
圣海伦斯火山是喀斯喀特山脉中最活跃的火山；然而，它的位置很不寻常，因为它位于弧火山活动的主轴线以西 50 公里处。俯冲带热模型表明，向下俯冲的板块与前缘之下的上覆地幔楔体分离，导致冷的地幔楔体不太可能产生熔体。因此，圣海伦斯火山位于前缘，这引发了关于冷地幔楔体的范围以及负责火山活动的熔体源区的问题。在这里，我们使用高分辨率主动源地震数据表明，圣海伦斯火山位于莫霍面反射率的一个明显的横向边界之上。西部较弱至不存在的 PmP 反射归因于地幔楔体中的蛇纹岩，这需要圣海伦斯火山之下有一个冷的含水地幔楔体（<∼700°C）。这些结果表明，熔体源区位于向东靠近亚当斯山的地方。
