DataSheet1_Helium isotopes in Plinian and inter-Plinian volcanic products of Vesuvius, Italy.pdf

We investigated helium isotopes on gas extracted by crushing from melt and fluid inclusions in minerals from Plinian and inter-Plinian tephra and lavas of Vesuvius, Italy. Erupted products of different ages were considered, from Avellino eruption (1995 BCE) to the last eruption of 1944, with special focus on the 79 AD Plinian eruption. He/He 4 3 ratios between 1.5 and 2.7 RA were measured, with the highest values associated with rocks representative of the roof and the walls of the magma chambers (cumulates). Lowest values occurred in sanidines representative of magma-skarn interfaces. Noteworthy, the highest measured values of the 79 AD pumices were comparable with both lavas and tephra emitted from flank vents and under open-conduit conditions during the Medieval Period and Present Period of Vesuvius activity, and present-day fumarolic discharges. He 3/He 4 values are buffered within an extended, deep-seated reservoir at about 10 km filled with magma rising from the mantle. A fact that might potentially limit the accuracy of future eruption forecasting through monitoring of He/He 4 3 changes in Vesuvius fumaroles. Ageing and interaction with crustal rocks emerged as possible mechanisms that lowered the He/He 4 3 ratio of the melt during its intra-crustal magma chambers stay, with highest values associated with more dynamic conditions.

本研究针对意大利维苏威火山的普林尼式（Plinian）及普林尼间歇式火山碎屑岩与熔岩中的矿物内熔体和流体包裹体，通过粉碎法提取气体并开展氦同位素分析。研究涵盖了不同喷发期次的火山产物，时间跨度从阿韦利诺喷发（公元前1995年）至1944年的末次喷发，重点聚焦公元79年的普林尼式喷发事件。本次测得的³He/⁴He比值介于1.5 RA至2.7 RA之间（RA为大气³He/⁴He比值），最高值对应代表岩浆房顶板及围岩的堆积岩（cumulates）；最低比值则出现在代表岩浆-矽卡岩（skarn）界面的透长石（sanidine）中。值得注意的是，公元79年浮岩样品测得的最高³He/⁴He比值，与维苏威火山中世纪及现代活动期间，通过侧翼喷口在开放通道条件下喷发的熔岩与火山碎屑岩，以及现今的喷气排放物的比值相当。约10km深度的大型深部储库中，³He/⁴He比值处于缓冲状态，该储库充填有来自地幔的上升岩浆——这一现象或通过监测维苏威火山喷气口的³He/⁴He比值变化，对未来火山喷发预报的准确性造成潜在限制。熔体在壳内岩浆房驻留期间，其³He/⁴He比值降低的可能机制包括老化作用与地壳岩石相互作用，且比值越高对应岩浆房内的动力学条件越活跃。