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Physical conditions, structure and dynamics of a zoned magma chamber : Mount Pelée (Martinique, Lesser Antilles arc).

Abstract : Experimental results and petrologic observations of the eruptive history of Mount Pelée are integrated, and a model for the magma storage system is presented. Recent (stage 3) Plinian and Pelean activity (P1, 650 years B.P.; 1902, 1929) erupted relatively homogeneous andesites (average 62 wt % SiO2). They are porphyritic (35–58 vol % crystals) and contain phenocrysts of plagioclase (Plag) (An50–90), orthopyroxene (Opx) (En52–60), and magnetite (Mt) (∼Mt70). Glasses (both interstitial and trapped) are rhyolitic (74–77 wt % SiO2). Clinopyroxene (Cpx), ilmenite (Ilm), amphibole (Amph) (mostly resorbed pargasitic hornblendes), and olivine (Ol) are present as minor phases. Products of 1902 and 1929 contain mafic enclaves (51–59 wt % SiO2) with compositions similar to basaltic andesite lavas erupted during stage 2 (40,000–19,500 years B.P.). Conditions in the andesitic part of the magma chamber, as determined from experimental phase equilibria, do not differ between the P1, 1902, and 1929 eruptions (875–900°C, 2 ± 0.5 kbar, ΔNNO = +0.4–0.8, melt H2O content of 5.3–6.3 wt %). New experimental data on a basaltic andesite composition (53 wt % SiO2) from stage 2, at 4 kbar, 950–1025°C, for melt H2O concentrations from 8.3 to 2.6 wt %, and fO2 between NNO and NNO + 4 simulate crystallization in the mafic part of the chamber. Liquidus or near-liquidus Ol, An-rich Plag, Al- and Fe3+-rich salite and augite, pargasitic hornblende, and Al- and Mg-rich Mt have compositions close to phenocrysts in mafic products from stages 2 and 3. Experimental liquids range from basaltic andesite to dacite. Application of experimentally derived mineral-melt Al/Si and Fe/Mg partition coefficients to mineral compositions from mafic lavas and cumulates from stage 2 shows that the chamber is fed by relatively evolved parental basaltic liquids (Mg # ∼ 55–60)(Mg # = Mg/(Mg + FeT). They have low temperatures (≤1050°C), high melt H2O contents (>5–6 wt %), and fO2 (ΔNNO mostly between +1 and +2) and crystallize an Ol + Cpx + Mt assemblage followed by Plag + Amph, although Amph may have started to crystallize with Ol and Cpx. Compositions of natural glasses and amphibole in mafic cumulates and lavas record a continuous evolution from basaltic-basaltic andesite to basaltic andesite-dacite liquids. Crystal fractionation of basaltic magmas is the main process controlling the chemical diversity at Mount Pelée. Crystallization in the mafic part produces an andesitic-dacitic residual liquid which subsequently evolves to produce the andesitic part. The present-day situation is typical of low fluxes of mafic magmas in comparison with stage 2.
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Michel Pichavant, Caroline Martel, Jean-Louis Bourdier, Bruno Scaillet. Physical conditions, structure and dynamics of a zoned magma chamber : Mount Pelée (Martinique, Lesser Antilles arc).. Journal of Geophysical Research : Solid Earth, American Geophysical Union, 2002, 107, pp.101029-101055. ⟨10.1029/2001JB000315⟩. ⟨hal-00072954⟩



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