Physical conditions, structure, and dynamics of a zoned magma chamber: Mount Pelee (Martinique, Lesser Antilles Arc)

[1] Experimental results and petrologic observations of the eruptive history of Mount Pelee 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) (An(50-90)), orthopyroxene (Opx) (En(52-60)), and magnetite (Mt) (similar toMt(70)). 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 (875-900degreesC, 2 +/- 0.5 kbar, DeltaNNO = +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, 950-1025degreesC, for melt H2O concentrations from 8.3 to 2.6 wt %, and f(O2) 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 # similar to 55-60)(Mg # = Mg/(Mg + Fe-T). They have low temperatures (less than or equal to1050degreesC), high melt H2O contents (>5-6 wt %), and f(O2) (DeltaNNO 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 Pelee. 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.