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

Abstract

[1] 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 % SiO 2 ). They are porphyritic (35 -58 vol % crystals) and contain phenocrysts of plagioclase (Plag) (An 50 -90 ), orthopyroxene (Opx) (En 52 -60 ), and magnetite (Mt) ($Mt 70 ). Glasses (both interstitial and trapped) are rhyolitic (74 -77 wt % SiO 2 ). 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 % SiO 2 ) 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, ÁNNO = +0.4 -0.8, melt H 2 O content of 5.3-6.3 wt %). New experimental data on a basaltic andesite composition (53 wt % SiO 2 ) from stage 2, at 4 kbar, 950 -1025°C, for melt H 2 O 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 Fe 3+ -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 + Fe T ). They have low temperatures ( 1050°C), high melt H 2 O contents (>5 -6 wt %), and f O2 (ÁNNO 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.INDEX TERMS: 3630 Mineralogy and Petrology: Experimental mineralogy and petrology; 8439 Volcanology: Physics and chemistry of magma bodies; 8414 Volcanology: Eruption mechanisms;

On the conditions of magma mixing and its bearing on andesite production in the crust

“…A lthough a strong case has been made for andesite arc magmas being by-products of basalt crystallization 1,2 , the generation of rocks of intermediate compositions has been also proposed to result from the juxtaposition and largescale mixing of chemically contrasted magmas [3][4][5][6] , such as basalt and rhyolite 3 . Whether the latter is a general 3,4 or a subsidiary 7 process remains, however, debated.…”

Determining magmatic processes from analysis of phenocrysts and gabbroic xenoliths contained in Calbuco andesites

“…They concluded that the lower than expected K 2 0 contents of Calbuco andesites actually reflect abnormally high SiO 2 abundances at a given extent of crystallization as fractional crystallization of hornblende drives the residual liquid in the direction of SiO 2 enrichment (Sisson and Grove, 1993a;Cawthorn and O'Hara, 1976;Pichavant et al, 2002;L6pez-Escobar et al, 1995;Yagi and Takeshita, 1987;Allen and Boettcher, 1983;Foden and Green, 1992). Consistent with this, L6pez-Escobar et al…”

“…from Calbuco, an overall temperature range for the eruptive products is 845-975 C for the andesites and 900-1145 C for basaltic andesites. According to individual phase diagrams, the temperature ranges for phase stability are 845-895 C for andesite, 900-950 C for basaltic andesite , 925-975 C for andesite and 975-1145 C for basaltic andesite (Luhr, 2002), and 935-970 C for andesite and 935+ C for basaltic andesite depending on H 2 0 concentration (Pichavant et al, 2002). Overall, these published temperatures overlap the calculated temperatures, despite the lack of quartz in equilibrium with the hornblende.…”

“…Rutherford and Devine (2003) have used an alternative Al in hornblende geobarometer to calculate pressures at Soufriere Hills without quartz and hornblende existing in equilibrium. They found that the use of the geobarometer without the required mineral stability was justified due to the fact that it returned a pressure which was consistent with the necessary water pressure to produce the H 2 0 observed in melt (Luhr, 2002), and 4 kbars (Pichavant et al, 2002) for andesites, and 2+ kbars , up to 2.1 kbars (Luhr, 2002), and 4 kbars (Pichavant et al, 2002) for basaltic andesite. These pressures correspond to depths of 3.8+ km, 4.5-6.8 km, and 12 km for andesite respectively and 6.7+ km, 1.2-6.3 km, and 12 km for basaltic andesite respectively.…”

“…Little is known about how andesites are actually formed despite the fact that the Earth's crust has an overall andesitic composition. Arguments for andesite formation have included elevated dissolved water contents in the magma Boettcher, 1978 and1983;Moore and Carmichael, 1998;Foden and Green, 1992), crystal fractionation Moore and Carmichael, 1998;Pichavant et al, 2002;Beard, 1986;Grove et al, 1982;L6pez-Escobar et al, 1995;Cawthorn and O'Hara, 1976 and references therein), crystallization of hornblende Boettcher, 1978 and1983;Green and Ringwood, 1967;Cawthorn and O'Hara, 1976;Yagi and Takeshita, 1987;L6pez-Escobar et al, 1995), magnetite crystallization (Cawthorn and O'Hara, 1976 and references therein;Martel et al, 1999), crustal assimilation (Grove et al, 1982;L6pez-Escobar et al, 1995), magma mixing (Grove et al, 1982;L6pez-Escobar et al, 1995), partial melting of subducted oceanic crust (Cawthorn and O'Hara, 1976 and references therein), and direct and partial melting of mantle peridotite (Cawthorn and O'Hara, 1976 and references therein). Since Calbuco andesites have a complex mineral assemblage, each mineral phase is also examined to try to determine the relationship between phases.…”