Isotopic insights into the degassing and secondary hydration of volcanic glass from the 1980 eruptions of Mount St. Helens
Isotopic insights into the degassing and secondary hydration of volcanic glass from the 1980 eruptions of Mount St. Helens
复制标题
对 1980 年圣海伦斯火山喷发的火山玻璃脱气和二次水化的同位素见解
作者：
A. Seligman;I. Bindeman;A. Eaton;R. Hoblitt
新喷发火山玻璃的残余挥发分记录了其岩浆脱气历史。然而，这一历史随后被沉积后（二次）水合作用所覆盖，其速率和起源尚未得到充分的限制。在这里，我们提出了一个自然实验的结果，使用1980年爆发的圣海伦斯火山的产品。我们测量了1980年干燥夏季采集的样品的水浓度、δ D玻璃和δ 18 OBSG（块状硅酸盐玻璃的δ 18 O），并将其与2015年从相同矿床重新采样的材料进行了比较。从气体逸出管道附近的地下采集的样品显示出较高的水浓度（接近2.0 wt%），这与δ D glass（− 110至− 130‰）和δ 18 OBSG（6.0至6.6‰）值低于1980年的玻璃（分别为− 70至− 100‰和6.8至6.9‰）有关。2015年从1980年夏季沉积物的表面到10 cm次表面采集的样品，平均含水量略有增加，为0.1-0.2 wt%，但与1980年玻璃值相比，δ 18 OBSG值（6.8-6.9‰）相似。然而，这些样品的δDglass值高出15‰;与大气降水的交换预计会产生较低的δDglass值。我们将2015年收集的1980年沉积物上部较高的δ D玻璃值归因于较高δD沃茨的再水化作用，这些水从热的下伏沉积物中脱气数月至一年，使上覆沉积物与相对较高的δD气体水合。我们的数据还有助于富含晶体的火山的岩浆脱气。使用1980年的样品，我们重建的英安岩圣海伦斯山沉积流纹质基块的δD-H2O趋势产生一个趋势，与结晶贫乏的流纹质火山喷发的脱气趋势重叠以前在其他地方研究，这表明类似的行为挥发物从岩浆出溶。此外，我们的数据支持以前的研究提出，出溶挥发物被困在一个快速上升的岩浆，并开始脱气只有在1980年的火山爆发在较浅的深度。
The magmatic degassing history of newly erupted volcanic glass is recorded in its remaining volatile content. However, this history is subsequently overprinted by post-depositional (secondary) hydration, the rates and origins of which are not yet adequately constrained. Here, we present the results of a natural experiment using products of the 1980 eruptions of Mount St. Helens. We measured water concentration, δDglass, and δ18OBSG (δ18O of the bulk silicate glass) of samples collected during the dry summer months of 1980 and compared them with material resampled in 2015 from the same deposits. Samples collected from the subsurface near gas escape pipes show elevated water concentrations (near 2.0 wt%), and these are associated with lower δDglass (− 110 to − 130‰) and δ18OBSG (6.0 to 6.6‰) values than the 1980 glass (− 70 to − 100‰ and 6.8 to 6.9‰, respectively). Samples collected in 2015 from the surface to 10-cm subsurface of the 1980 summer deposits have a small increase in average water contents of 0.1–0.2 wt% but similar δ18OBSG (6.8–6.9‰) values compared to the 1980 glass values. These samples, however, show 15‰ higher δDglass values; exchange with meteoric water is expected to yield lower δDglass values. We attribute higher δDglass values in the upper portion of the 1980 deposits collected in 2015 to rehydration by higher δD waters that were degassed for several months to a year from the hot underlying deposits, which hydrated the overlying deposits with relatively high δD gases. Our data also contribute to magmatic degassing of crystal-rich volcanoes. Using the 1980 samples, our reconstructed δD-H2O trends for the dacitic Mount St. Helens deposits with rhyolitic groundmass yield a trend that overlaps with the degassing trend for crystal-poor rhyolitic eruptions studied previously elsewhere, suggesting similar behavior of volatiles upon exsolution from magma. Furthermore, our data support previous studies proposing that exsolved volatiles were trapped within a rapidly rising magma and started degassing only at shallow depths during the 1980 eruptions.
登录
查看更多内容
影响因子：
5
作者：
Giachetti, Thomas;Gonnermann, Helge M.;Gouldstone, Andrew
通讯作者：
Gouldstone, Andrew
影响因子：
3.5
作者：
D. Holwell;R. Keays;I. McDonald;Megan R. Williams
通讯作者：
D. Holwell;R. Keays;I. McDonald;Megan R. Williams
影响因子：
5.3
作者：
Gardner, James E.;Llewellin, Edward W.;Befus, Kenneth S.
通讯作者：
Befus, Kenneth S.
影响因子：
3.5
作者：
Myhill, Robert;Ojwang, Dickson O.;Miyajima, Nobuyoshi
通讯作者：
Miyajima, Nobuyoshi