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Highly hydrated magmas stored in a mush at great depths, the source of Dominica’s great pumice eruptions

In the past, the island of Dominica has experienced ignimbritic pumiceous eruptions involving large volumes of magma (4-5 km3 DRE). A study of more than a hundred glassy inclusions has shown that these magmas are among the richest in water ever described and that they are stored at great depth in a transcrustal system (3 - 27 km) down to the Moho.

Highly hydrated magmas stored in a mush at great depths, the source of Dominica’s great pumice eruptions

Publication date: 10/12/2018

Press, Research

Related themes : Natural Hazards

This study, carried out by an international team of researchers from the Institut des Sciences de la Terre de Paris (ISTeP, CNRS/SU), the Institut de Physique du Globe de Paris (IPGP, CNRS/USPC), the Institut des Sciences de la Terre d’Orléans (ISTO/OSUC, CNRS/BRGM/Université d’Orléans) and the Centre de Recherches Pétrographiques et Géochimiques (CRPG, CNRS/Université de Lorraine), also showed that a transcrustal CO2 flow was active.

The island of Dominica (Lesser Antilles arc) is unique in having several active volcanic centres responsible for significant magma production in the central part of the arc. Several field campaigns have been carried out (2011, 2013, 2014) to clarify the chronostratigraphy of the most voluminous ignimbritic pumice eruptions recognised in the Lesser Antilles arc in the last 60 ka (Boudon et al., 2017). One emblematic eruption was known as the “Roseau tuff”, the volume of which was estimated at 58 km3. Recent studies, including our own, show that several eruptions actually occurred in the period 60-24 ka.

Three of them (Layou, Roseau and Grand Fond) emitted volumes of the order of 4-5 km3 (DRE). These ignimbritic eruptions nevertheless involved a volume of magma significantly greater than eruptions on other islands, such as Martinique or Guadeloupe, where the volumes emitted are generally less than 1 km3.

The island of Dominica: (A) the arc of the Lesser Antilles. (B) The island of Dominica with the eruptive centres (Morne aux Diables in the north, Morne Diablotins and Morne Trois Pitons - Micotrin in the centre and Morne Plat Pays in the south) and the ignimbritic eruptions in the central part (Layou, Roseau, Grand Fond). © Hélène Balcone-Boissard

We set out to define the geometry and dynamics of the feeder system beneath Dominica that was responsible for these eruptions, in order to understand how they came about. To do this, we determined the geochemical signature of more than a hundred glassy inclusions of major, trace and volatile elements (H2O, CO2, SO2, F, Cl, Br, S). These magma droplets, trapped by crystals as they grow in magma reservoirs, are key witnesses to the conditions under which magmas are stored at depth. Spot analyses using electron microprobe (CAMPARIS service, France) and ion microprobe (Edinburgh (England) and CRPG, Nancy (France)), following a textural study using scanning electron microscope (ISteP, France) were carried out.

Layou ignimbritic flow deposit (~ 51 ka). © Hélène Balcone-Boissard

The geochemical signature of Dominica’s magmas reveals that they are among the richest in water ever characterised (≤ 8 pds% H2O), which has implications in terms of volcanic hazards.CO2 levels, although generally low, are sometimes very high in some inclusions trapped at low pressure, indicating “CO2 fluxing” through the feeder system, partly responsible for magma dehydration. The trapping depths of the inclusions saturated with volatile elements demonstrate the development of a magmatic feeding system under the Dominica from a depth of 3 to 27 km, i.e. as far as the Moho (located at 28 km).

The magma that fuelled the ignimbritic eruptions is mainly stored between 16 and 20 km according to experimental petrology studies (Solaro et al., in revision).

This geometry, combined with the flow of CO2 through the crust, argues in favour of a transcrustal “mush” system, made up of interconnected lenses of magma, developed even in a subduction context. This type of feeding system architecture is being discussed along the Lesser Antilles arc, as it is a recurring feature of the feeding systems of Montagne Pelée (Martinique), La Soufrière (Guadeloupe) and St Kitts. We now plan to clarify the dynamics of the Dominica feeding system by studying the explosive eruptions involving smaller volumes of magma recognised in the more recent period (< 18 ka).

Glass inclusions and storage conditions. (a) Glassy inclusions in an orthopyroxene crystal from the Roseau eruption (~32 ka). (b) CO2 content (ppm) as a function of H2O content (weight %) of glassy inclusions in orthopyroxenes (solid diamond), plagioclase feldspars (open diamond) and clinopyroxenes (triangle). Morne aux Diables: symbols in grey; Morne Diablotins: symbols in blue (Layou eruption); Morne Trois Pitons - Micotrin; symbols in red (Roseau eruption) and green (Grand Fond eruption). Isobars represent pressure in kbars (Newman and Lowenstern, 2002; VolatileCalc). © Hélène Balcone-Boissard

Ref: Balcone-Boissard H., Boudon G., Blundy J.D., Martel C., Brooker R.A., Deloule E., Solaro C. Matjuschkin V. (2018) Deep pre-eruptive storage of silicic magmas feeding Plinian and dome-forming eruptions of central and northern Dominica (Lesser Antilles) inferred from volatile contents of melt inclusions, Contributions to Mineralogy and Petrology, doi:10.1007/s00410-018-1528-4

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