D/H ratios and H2O contents record degassing and rehydration history of rhyolitic magma and pyroclasts
D/H ratios and H2O contents record degassing and rehydration history of rhyolitic magma and pyroclasts
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D/H比值和H2O含量记录了流纹质岩浆和火山碎屑的脱气和再水化历史
DOI：
10.1016/j.epsl.2019.115909
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发表时间：
2020
影响因子：
5.3
通讯作者：
Hoxsie, Erin C.
中科院分区：
文献类型：
--
作者：
Giachetti, Thomas;Hudak, Michael R.;Shea, Thomas;Bindeman, Ilya N.;Hoxsie, Erin C.
流纹岩岩浆的火山喷发经常表现出从强大的（火山至普林尼）爆炸事件到形成黑曜石流的粘性熔岩的更温和的渗出的转变。这些喷发的另一个普遍特征是火山碎屑中混杂着火成黑曜石。这种致密的幼年产物在成分上类似于火山灰和黑曜石流，但通常比其流对应物少脱气。火山碎屑黑曜石的形成机制及其提供的有关岩浆脱气作用对流纹岩喷发方式的调节程度的信息目前正受到积极的研究。对1060年美国加州梅迪辛湖火山玻璃山流纹岩喷发中的多孔火山灰、火山碎屑和流纹岩进行了孔隙度、含水量、H2O和氢同位素组成δD分析。火山碎屑中的H2O与δD呈负相关，与δ C呈正相关，表明火山碎屑样品受到了喷发后再水化作用的影响。数值模拟表明，在喷发后的1960年里，这种再水化的平均速率为10−23.5±0.5m2s− 1，导致一些火山碎屑获得了高达1%的大气降水。火山碎屑岩和流纹岩由于其孔隙度很低，不受再水化作用的影响。模拟的脱气岩浆δD-H2O关系与玻璃山样品中测得的值的比较支持这样的观点，即流纹质岩浆在封闭体系中脱气，直到其孔隙度达到约65± 5%，超过该值，在开放体系中脱气，直到淬火。在爆炸阶段，迅速上升的岩浆在变得可渗透后不久就碎裂，产生多孔火山砾和火山灰，这些火山灰在膨胀的气相中继续在开放系统中脱气。最近的研究表明，一些火山灰可能聚集和烧结在不同深度以上的碎裂水平的管道两侧，部分平衡与不断流动的重岩浆蒸汽，解释了广泛的H2O含量和高变化的δD测量的火山碎屑黑曜石。仅使用H2O和δD，不可能排除火成碎屑黑曜石也可能通过可渗透泡沫坍塌形成的可能性，要么在破碎水平以下的管道侧附近同爆，要么在爆炸阶段中散布的更多溢出时期。在喷发的最后阶段，缓慢上升的岩浆在开放系统中脱气，直到到达地表，产生低H2O和δD的流动。这项研究表明，H2O测量高度多孔火山碎屑的几百年或更长的时间不能用来推断同喷发岩浆脱气途径，除非仔细评估喷发后的再水化首先进行。如果他们的形成机制可以更好地理解，在整个爆炸阶段的火山碎屑黑曜石的纹理和挥发分含量的变化的详细分析，可能有助于破译流纹岩喷发通常从爆炸阶段转变为喷涌阶段的原因。
Volcanic eruptions of rhyolitic magma often show shifts from powerful (Vulcanian to Plinian) explosive episodes to a more gentle effusion of viscous lava forming obsidian flows. Another prevailing characteristic of these eruptions is the presence of pyroclastic obsidians intermingled with the explosive tephra. This dense, juvenile product is similar to the tephra and obsidian flow in composition, but is generally less degassed than its flow counterpart. The formation mechanism(s) of pyroclastic obsidians and the information they can provide concerning the extent to which magma degassing modulates the eruptive style of rhyolitic eruptions are currently subject to active research. Porous tephra and pyroclastic and flow obsidians from the 1060CE Glass Mountain rhyolitic eruption at Medicine Lake Volcano (California) were analyzed for their porosity,ϕ, water content, H2O, and hydrogen isotopic composition,δD. H2O in porous pyroclasts is correlated negatively withδD and positively withϕ, indicating that the samples were affected by post-eruptive rehydration. Numerical modeling suggests that this rehydration occurred at an average rate of 10−23.5±0.5m2s−1during the ∼960 years since the eruption, causing some pyroclasts to gain up to 1 wt% of meteoric water. Pyroclastic and flow obsidians were not affected by rehydration due to their very low porosity. Comparison between modeledδD-H2O relationships in degassing magma and values measured in the Glass Mountain samples supports the idea that rhyolitic magma degasses in closed-system until its porosity reaches a value of about 65±5%, beyond which degassing occurs in open-system until quench. During the explosive phase, rapidly ascending magma fragments soon after it becomes permeable, creating porous lapilli and ash that continue to degas in open-system within an expanding gas phase. As suggested by recent studies, some ash may aggregate and sinter on the conduit sides at different depths above the fragmentation level, partly equilibrating with the continuously fluxing heavier magmatic vapor, explaining the wide range of H2O contents and high variability inδD measured in the pyroclastic obsidians. Using only H2O andδD, it is impossible to rule out the possibility that pyroclastic obsidians may also form by permeable foam collapse, either syn-explosively near the conduit sides below the fragmentation level or during more effusive periods interspersed in the explosive phase. During the final effusive phase of the eruption, slowly ascending magma degasses in open-system until it reaches the surface, creating flows with low H2O andδD. This study shows that H2O measured in highly porous pyroclasts of a few hundred years or more cannot be used to infer syn-eruptive magma degassing pathways, unless careful assessment of post-eruptive rehydration is first carried out. If their mechanism of formation can be better understood, detailed analysis of the variations in texture and volatile content of pyroclastic obsidians throughout the explosive phase may help decipher the reasons why rhyolitic eruptions commonly shift from explosive to effusive phases.
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