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

Abstract

Volatiles contribute to magma ascent through the sub-volcanic plumbing system. Here, we investigate melt inclusion compositions in terms of major and trace elements, as well as volatiles (H 2 O, CO 2 , SO 2 , F, Cl, Br, S) for Quaternary Plinian and dome-forming dacite and andesite eruptions in the central and the northern part of Dominica (Lesser Antilles arc). Melt inclusions, hosted in orthopyroxene, clinopyroxene and plagioclase are consistently rhyolitic. Post-entrapment crystallisation effects are limited, and negligible in orthopyroxene-hosted inclusions. Melt inclusions are among the most water-rich yet recorded (≤ 8 wt% H 2 O). CO 2 contents are generally low (< 650 ppm), although in general the highest pressure melt inclusion contain the highest CO 2. Some low-pressure (< 3 kbars) inclusions have elevated CO 2 (up to 1100-1150 ppm), suggestive of fluxing of shallow magmas with CO 2-rich fluids. CO 2-trace element systematics indicate that melts were volatile-saturated at the time of entrapment and can be used for volatile-saturation barometry. The calculated pressure range (0.8-7.5 kbars) indicates that magmas originate from a vertically-extensive (3-27 km depth) storage zone within the crust that may extend to the sub-Dominica Moho (28 km). The vertically-extensive crustal system is consistent with mush models for sub-volcanic arc crust wherein mantle-derived mafic magmas undergo differentiation over a range of crustal depths. The other volatile range of composition for melt inclusions from the central part is F (75-557 ppm), Cl (1525-3137 ppm), Br (6.1-15.4 ppm) and SO 2 (< 140 ppm), and for the northern part it's F (92-798 ppm), Cl (1506-4428 ppm), Br (not determined) and SO 2 (< 569; one value at 1015 ppm). All MIs, regardless of provenance, describe the same Cl/F correlation (8.3 ± 2.7), indicating that the magma source at depth is similar. The high H 2 O content of Dominica magmas has implications for hazard assessment.

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

“…The mineralogy of the magma emitted during the four eruptions is very similar, and similar to that of the ignimbrites (Balcone‐Boissard et al, 2018): In decreasing order of abundance, PLAG, OPX, CPX, and MGT are present as phenocrysts in the magmas. Their composition is given in Table S2.…”

“…Furthermore, some melt inclusions contain high CO 2 concentrations (3,000 to 4,000 ppm) but comparatively low H 2 O concentrations (<2.7 wt%) and therefore very low H 2 O/(H 2 O + CO 2 ) ratios. Balcone‐Boissard et al (2018) also observed such high CO 2 contents in melt inclusions from MTPM ignimbrites. Caricchi et al (2018) attributed this phenomenon to CO 2 flushing from deep levels within the magma plumbing system, driving melt dehydration and crystallization at shallower levels.…”

“…Our study quantifies the volatile degassing budget during the explosive eruptions produced by Morne Trois Pitons‐Micotrin (MTPM) volcanic complex during the last 18 ka and also provides new constraints on the magma plumbing system beneath the central part of Dominica. We have focused our study on a series of plinian eruptions that occurred over the last 20 ka following a series of more voluminous ignimbrites dated to 60–20 ka that have already studied (Balcone‐Boissard et al, 2018; Boudon et al, 2017; Solaro et al, 2019). We conclude by comparing our insights on the magma plumbing system beneath Dominica to other volcanic systems of the Lesser Antilles arc and by comparing halogen and S degassing budgets to those of major explosive eruptions that have occurred worldwide.…”

Evidence from plutonic xenoliths for magma differentiation, mixing and storage in a volatile-rich crystal mush beneath St. Eustatius, Lesser Antilles

“…The high water contents recorded in Statia MIs (0.1–9.1 wt% H 2 O) are consistent with high water contents measured in melt inclusions from Montserrat (Humphreys et al 2009 ; Cassidy et al 2015 ), Bequia (Camejo-Harry et al 2018 ) and Dominica (Balcone-Boissard et al 2018 ) up to 6.2, 7.8 and 8 wt% H 2 O, respectively. In addition, independent estimates of melt water contents using orthopyroxene in andesites from Soufrière Hills Volcano, Montserrat, suggest melt water concentrations of 6–9 wt% H 2 O (Edmonds et al 2016 ).…”

Petrological and experimental constraints on magma storage for large pumiceous eruptions in Dominica island (Lesser Antilles)

“…The phase compositions for the natural samples are given in Table 2. The water contents of the experimental residual glasses were determined following the by-difference method using the EMP analyses and the glass standards in Balcone-Boissard et al (2018). Phase proportions of the experimental charges were obtained by mass-balance calculations and compared to those obtained on natural samples by Boudon et al (2017).…”