Evidence for an Active, Transcrustal Magma System in the Last 60 ka and Eruptive Degassing Budget (H₂O, CO₂, S, F, Cl, Br): The Case of Dominica

Abstract

The Morne Trois Pitons‐Micotrin volcanic complex on the island of Dominica (Lesser Antilles) emitted a series of plinian eruptions between 18 and 9 ka BP. We studied it to constrain magma storage conditions and volatile degassing balances, by comparison with the three previous ignimbrites (~60–24 ka BP). Volatile concentrations in glass inclusions and mineral‐melt thermobarometry indicate storage at ≤200 MPa (~6–8 km) and 860–880°C. The magmas feeding these plinian eruptions were stored at a shallower depth than those that older ignimbrites from the same volcanic complex and stored at ~16 km. Close magma composition and similar halogen ratios, however, suggest a common source for the magmas feeding both the plinian eruptions and the ignimbrites. The large eruptive fluxes of F, Cl, and Br to the atmosphere (up to 1.4–2.8 × 10−1 Mt/km3, 1.5–4.0 Mt/km3, and 2–4 × 10−2 Mt/km3, respectively), estimated by the petrological method, support the potentially important role of volcanic halogens in modifying the chemistry of the atmosphere, though Cl is underestimated here because of buffering in a fluid phase. The behavior of S, potentially partitioned in the same fluid phase, prevents here the calculation of an eruptive outgassing budget.

Time-window into the transcrustal plumbing system dynamics of Dominica (Lesser Antilles)

“…Such profiles could be related to fast crystal growth kinetics prior to eruption in the magmatic reservoir, which produce overlapping and complex signals when combined with diffusive relaxation of the initial chemical gradients 44 . Furthermore, 13% of the profiles performed in Goodwill and PPR1-3 (7 for Goodwill, 3 for PPR2 and 4 for PPR3) did not show defined plateaus at the outermost edges of the crystals (Supplementary Figure S4 ), therefore, only 96 out of the 110 profiles analysed by EPMA in the small eruptions were used for diffusion modelling, with a temperature of 890 °C (“ Material and methods ”, T = 886–895 °C for these eruptions 39 ). For Grand Fond, 6 profiles out of 20 were used for diffusion modelling, with a magmatic temperature of 850 °C 12 , 38 , 44 .…”

“…Whole rock compositions of all the studied magmas are andesitic-dacitic, whereas glass compositions are rhyolitic 34 , 38 , 39 , 41 . The pumices possess similar phenocryst abundances of ~ 30 vol% with a dominant proportion of plagioclase (~ 21 vol% for the large eruptions) followed by orthopyroxenes (~ 5% for the large eruptions) and in smaller proportions, clinopyroxenes, Fe-Ti oxides and amphiboles in some units 38 , 39 .…”

“…Furthermore, the storage system beneath Morne Trois Pitons-Micotrin includes two main storage depths 12 , 38 , 39 : at ~ 2–8 km depth (50–200 MPa), based on H 2 O-CO 2 concentrations measured in melt inclusions 39 for the recent small explosive eruptions reservoirs and ~ 12–16 km (300–400 MPa) based on experimental petrology 38 and melt inclusions data 38 for the voluminous pumiceous eruptions reservoirs (Supplementary notes on previous results are available in Supplementary material ).…”

“…( a ) The Lesser Antilles arc 60 – 62 results from the subduction of the northern and southern Atlantic plates beneath the Caribbean plate at a rate of ~ 2 cm/yr 63 . Black: volcanic islands, grey: coral platforms; the map has been modified from a previous study 39 . Bathymetry is in light grey 60 .…”

“…These eruptions were chosen because of their recent ages, a good chronostratigraphic control, well-preserved deposits and homogeneous compositions. This permits an investigation of how the storage system of this volcano changes in time and its dynamics, following a larger explosive eruption associated with deeper magma supply (12–16 km 38 ) and smaller eruptions of magmas stored more shallowly (2–8 km 38 , 39 ). Here, a detailed investigation of orthopyroxene compositions across the last five pumiceous eruptions allows identification of the magma transfer pathways and the existence of disrupting events related to magma remobilisation at depth, by adapting the CSA approach as used for previous large explosive eruptions of Dominica 12 .…”