TY - JOUR

T1 - Conduit- to localized-scale degassing during Plinian eruptions

T2 - insights from major element and volatile (Cl and H2O) analysis within Vesuvius AD79 pumice

AU - Shea, Thomas

AU - Hellebrand, Eric

AU - Gurioli, Lucia

AU - Tuffen, Hugh

PY - 2014

Y1 - 2014

N2 - Textural investigations of the AD 79 Vesuvius pumice emphasize the complexity of magma degassing and crystallization during the eruption, which emitted two types of pumice (white and gray) associated with different magma bodies of phonolitic and tephriphonolitic compositions respectively. These studies proposed that velocity gradients caused spatial variations in degassing within the ascending magma column at both the conduit and the localized scale. To validate this hypothesis, analyses of volatiles (Cl, H2O) and major elements in pumice glasses and melt inclusions were performed using high spatial resolution tools (microRaman spectrometry and electron microprobe) and combined with major element and volatile concentration profiles and maps. The results indicate that the melt phase differentiated through degassing-induced crystallization of leucite, and that the gray pumice magma was efficiently homogenized prior to degassing. Because Cl diffuses more slowly than H2O during fast ascent, it behaves as an incompatible element and can be used as a tracer of crystallization and H2O degassing. We emphasize the importance of strain localization in generating zones of preferential exsolution and permeable pathways for gases, and establish degassing scenarios that incorporate the effects of shear-zones.

AB - Textural investigations of the AD 79 Vesuvius pumice emphasize the complexity of magma degassing and crystallization during the eruption, which emitted two types of pumice (white and gray) associated with different magma bodies of phonolitic and tephriphonolitic compositions respectively. These studies proposed that velocity gradients caused spatial variations in degassing within the ascending magma column at both the conduit and the localized scale. To validate this hypothesis, analyses of volatiles (Cl, H2O) and major elements in pumice glasses and melt inclusions were performed using high spatial resolution tools (microRaman spectrometry and electron microprobe) and combined with major element and volatile concentration profiles and maps. The results indicate that the melt phase differentiated through degassing-induced crystallization of leucite, and that the gray pumice magma was efficiently homogenized prior to degassing. Because Cl diffuses more slowly than H2O during fast ascent, it behaves as an incompatible element and can be used as a tracer of crystallization and H2O degassing. We emphasize the importance of strain localization in generating zones of preferential exsolution and permeable pathways for gases, and establish degassing scenarios that incorporate the effects of shear-zones.

KW - magma

KW - degassing

KW - crystallization

KW - Vesuvius

KW - AD 79 pumice

KW - volatiles in glass

KW - chlorine

KW - shear-zones

U2 - 10.1093/petrology/egt069

DO - 10.1093/petrology/egt069

M3 - Journal article

VL - 55

SP - 315

EP - 344

JO - Journal of Petrology

JF - Journal of Petrology

SN - 1460-2415

IS - 2

ER -