Non-volatile vs volatile behaviours of halogens during the AD 79 plinian eruption of Mt. Vesuvius, Italy
Pre-eruptive conditions and degassing processes of the AD 79 plinian eruption of Mt. Vesuvius are constrained by systematic F and Cl measurements in melt inclusions and matrix glass of pumice clasts from a complete sequence of the pumice-fallout deposits. The entire ‘white pumice’ (WP) magma and the...
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Published in | Earth and planetary science letters Vol. 269; no. 1; pp. 66 - 79 |
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Main Authors | , , , |
Format | Journal Article |
Language | English |
Published |
Elsevier B.V
15.05.2008
Elsevier |
Subjects | |
Online Access | Get full text |
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Summary: | Pre-eruptive conditions and degassing processes of the AD 79 plinian eruption of Mt. Vesuvius are constrained by systematic F and Cl measurements in melt inclusions and matrix glass of pumice clasts from a complete sequence of the pumice-fallout deposits. The entire ‘white pumice’ (WP) magma and the upper part of the ‘grey pumice’ (GP) magma were saturated relative to sub-critical fluids (a Cl-rich H
2O vapour phase and a brine), with a Cl melt content buffered at ~
5300 ppm, and a mean H
2O content of ~
5%. The majority of the GP magma was not fluid-saturated. From these results it can be estimated that the WP magma chamber had a low vertical extent (<
500 m) and was located at a depth of ~
7.5 km while the GP magma reservoir was located just beneath the WP one, but its vertical extent cannot be constrained. This is approximately two times deeper than previous estimates. H
2O degassing during the WP eruption followed a typical closed-system evolution, whereas GP clasts followed a more complex degassing path. Contrary to H
2O, Cl was not efficiently degassed during the plinian phase of the eruption.
This study shows that F and Cl behave as incompatible elements in fluid-undersaturated phonolitic melts. H
2O saturation is necessary for a significant partitioning of Cl into the fluid phase. However, Cl cannot be extracted in significant quantity from phonolitic melts during rapid H
2O degassing, e.g. during plinian eruptions, due to kinetics effects. Halogen contents are better preserved in volcanic glass (melt inclusions or matrix glass) than H
2O, therefore the combined analysis of both volatile species is required for reliable determination of pre-eruptive conditions and syn-eruptive degassing processes in magmas stored at shallow depths. |
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ISSN: | 0012-821X 1385-013X |
DOI: | 10.1016/j.epsl.2008.02.003 |