II. Kinetics of the magmatic process

II. Kinetics of the magmatic process

MI pic 15
Melt inclusion in olivive. Photo F. Schiavi.

Physical-chemistry of melt inclusions. As a complement of many studies dealing with melt inclusions composition and physical conditions of trapping, in this theme, we develop an original approach consisting in using melt inclusions as natural experimental micro-environments to study the kinetics of dissolution and crystallization of minerals in magmatic liquids. This approach supplies important information concerning the rate and mechanisms of magmatic processes, namely during partial melting and crystallization, as well as during magma transfer thought the crust. Some of the main results obtained with this approach during the period 2010-2015 are:

– Estimate of the rate of water loss in olivine-hosted melt inclusions during adiabatic decompression (Chen et al., 2011)

– Constraints of magma ascent rate based on (i) water loss of melt inclusions (Chen et al., 2012) or (ii) element zonation through olivine-inclusion interface (Le Voyer et al., 2014)

 – Measurement of the trans-crystalline melt migration rate in clinopyroxene under a temperature gradient (Sonzogni et al., 2011).

bubbles
CO2 bubbles in a basaltic magma associated with decompression-induced degassing. Photo: D. Laporte

 

Ascent and degassing of magmas. Determining magma degassing rate during ascent is of paramount importance to constraint eruptive dynamisms and volatile transfer into the atmosphere. Important results have been obtained for the effect of magma ascent rate on the kinetics of nucleation in rhyolitic magmas (Hamada et al., 2010). We are also studying degassing mechanisms of basaltic magmas and the process of volatile diffusive fractionation during bubble growth. In addition to this experimental approach, we study vesiculated natural samples in different geodynamic settings (MORBs, Chavrit et al., 2012; the so-called “floating pumices” of El Hierro eruption, Canary Islands, Sigmarsson et al., 2013)..

 

 

 

 

 

 

Magma chamber dynamics and eruption triggering. Detailed petrological studies (mineral chemistry and microtextures) allow us to understand the main magmatic processes active in magmatic reservoirs, to constrain the pre-eruptive physical conditions (P-T-XH2O), and identify the triggering mechanisms of the main explosive phases. These studies were performed on subduction zone volcanoes (Tungurahua, Ecuador, Samaniego et al., 2011; Ubinas, Peru, Rivera et al., 2014) and hot spot volcanoes (Reunion Island, Welsch et al., 2014). Our work on Andean volcanoes suggests that paroxysms in open-vent volcanoes are triggered by primitive magma recharge into a differentiated reservoir and the subsequent magma mixing. Concerning our work on Reunion Island, detailed textural analyses of olivines in oceanites allow us to propose a new model for olivine growth and crystallization in basaltic magmas. Euhedral olivine crystals are, in fact, skeletal and/or dendritic crystals ripened in an assemblage of crystals and interstitial liquid around the walls of the magma reservoir. Destabilisation of these crystals “mush” during paroxysms produces the textures observed in oceanites.

Pablo
Pre- and syn-eruptive magmatic process in active volcanoes (example of Tungurahua, Ecuador). (c) Mg# profile in clinopyroxene. (b) BSE image of a clinopyroxene showing a resorption boundary and a zoned overgrowth rim. Location of the previous profile is showed (Samaniego et al., 2011).
planisphere_PETRO
Planisphere showing the different volcanoes studied by our team.
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