AUTHOR=Gherardi Fabrizio , Barsanti Mattia , Principe Claudia , Magro Gabriella 

TITLE=Helium isotopes in Plinian and inter-Plinian volcanic products of Vesuvius, Italy

JOURNAL=Frontiers in Earth Science

VOLUME=10

YEAR=2022

URL=https://www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2022.1011203

DOI=10.3389/feart.2022.1011203

ISSN=2296-6463

ABSTRACT=<p>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 <italic>79 AD</italic> Plinian eruption. <inline-formula id="inf1"><mml:math id="m1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>H</mml:mi><mml:mi>e</mml:mi><mml:mo>/</mml:mo><mml:mmultiscripts><mml:mrow><mml:mi>H</mml:mi><mml:mi>e</mml:mi></mml:mrow><mml:mprescripts /><mml:mtext> </mml:mtext><mml:mn>4</mml:mn></mml:mmultiscripts></mml:mrow><mml:mprescripts /><mml:mtext> </mml:mtext><mml:mn>3</mml:mn></mml:mmultiscripts></mml:mrow></mml:math></inline-formula> ratios between 1.5 and 2.7 <inline-formula id="inf2"><mml:math id="m2" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>R</mml:mi><mml:mi>A</mml:mi></mml:msub></mml:mrow></mml:math></inline-formula> 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 <italic>79 AD</italic> pumices were comparable with both lavas and tephra emitted from flank vents and under open-conduit conditions during the <italic>Medieval Period</italic> and <italic>Present Period</italic> of Vesuvius activity, and present-day fumarolic discharges. <inline-formula id="inf3"><mml:math id="m3" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>H</mml:mi><mml:mi>e</mml:mi></mml:mrow><mml:mprescripts /><mml:mtext> </mml:mtext><mml:mn>3</mml:mn></mml:mmultiscripts><mml:mo>/</mml:mo><mml:mmultiscripts><mml:mrow><mml:mi>H</mml:mi><mml:mi>e</mml:mi></mml:mrow><mml:mprescripts /><mml:mtext> </mml:mtext><mml:mn>4</mml:mn></mml:mmultiscripts></mml:mrow></mml:math></inline-formula> values are buffered within an extended, deep-seated reservoir at about 10 <italic>km</italic> filled with magma rising from the mantle. A fact that might potentially limit the accuracy of future eruption forecasting through monitoring of <inline-formula id="inf4"><mml:math id="m4" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>H</mml:mi><mml:mi>e</mml:mi><mml:mo>/</mml:mo><mml:mmultiscripts><mml:mrow><mml:mi>H</mml:mi><mml:mi>e</mml:mi></mml:mrow><mml:mprescripts /><mml:mtext> </mml:mtext><mml:mn>4</mml:mn></mml:mmultiscripts></mml:mrow><mml:mprescripts /><mml:mtext> </mml:mtext><mml:mn>3</mml:mn></mml:mmultiscripts></mml:mrow></mml:math></inline-formula> changes in Vesuvius fumaroles. Ageing and interaction with crustal rocks emerged as possible mechanisms that lowered the <inline-formula id="inf5"><mml:math id="m5" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>H</mml:mi><mml:mi>e</mml:mi><mml:mo>/</mml:mo><mml:mmultiscripts><mml:mrow><mml:mi>H</mml:mi><mml:mi>e</mml:mi></mml:mrow><mml:mprescripts /><mml:mtext> </mml:mtext><mml:mn>4</mml:mn></mml:mmultiscripts></mml:mrow><mml:mprescripts /><mml:mtext> </mml:mtext><mml:mn>3</mml:mn></mml:mmultiscripts></mml:mrow></mml:math></inline-formula> ratio of the melt during its intra-crustal magma chambers stay, with highest values associated with more dynamic conditions.</p>