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Li and B Contents in Silicate Melts as Tracers of Volcanic Degassing

Abstract : Understandingdegassing processesof magmatic systemsis crucial for the prediction and risk assessmentof volcanic eruptions.Lithium and boron are light elements,moderately incompatibleto silicate minerals,and both have two stable isotopes. Over the past two decades, Li and B have been widely used to detect subduction-related processes, but they may also be considered as volcanic geospeedometers.During magmatic decompression, thesolubility of volatile phases(mainly H2O and CO2)decreases, causing volatile exsolution and bubble formation.As Li and B are fluid-mobile elements, their abundance and distribution in the melt will be strongly affected by magma degassing. Therefore, we have measured the concentrationsof the two elements in erupted matrix glassesand olivine-bearing melt inclusionsof several natural samples;namelythe 1944 and AD 79 eruptions of Mt. Vesuvius (Italy), the 2007 eruption and other historic lavas from Piton de la Fournaise(La Réunion), the Minoaneruption of Santorini (Greece),andthe 1902 eruption of Mt.Pelée (Martinique). Major elements have beenmeasured by electron microprobe, while Li and B elemental concentrations were analyzed by LA-ICP-MS.Natural melt inclusions showverylowLi and B values(average contents are 12.3ppm and 7.6ppm, respectively). While the naturally erupted and degassed matrix glasseshave Li and B contents in the range of 20-60ppm. Additionally, sets of decompression and diffusion-couple experimentson synthetic water-bearing rhyolitic glasses are currently being performed inan internally heated pressure vesselat elevated pressures and temperatures. This will allow us to comparethe results of our natural samplesto the experimental data, findingpotential correlationsbetween the decompression rate and the Li and B elemental and isotopic behaviour.Preliminary results of element mapping and profile measurements emphasize the fluid mobilty of Li and B during experimental magma degassing.
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https://hal.uca.fr/hal-02438779
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Submitted on : Tuesday, January 14, 2020 - 12:57:14 PM
Last modification on : Thursday, April 9, 2020 - 1:13:00 AM

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Roberta Spallanzani, Sarah B. Cichy, Max Wilke, M. Oelze, Kenneth Koga, et al.. Li and B Contents in Silicate Melts as Tracers of Volcanic Degassing. Goldschmidt, Aug 2019, Barcelona, Spain. ⟨hal-02438779⟩

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